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Genetics
Chapter 8
Genetics Chapter Outline Genetic variability Chapter 8.1 DNA, chromosomes, genes, genotypes, and phenotypes Chromosomes and Karyotype Heredity traits Dominant and recessive Alleles Genetic mutations Molecular clock
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Chapter 8.1.1 Feline genetics: Mendel’s laws
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Genetic variability In the last few decades, the assistance given by the field of genetics to the taxonomy of the Felidae has been of primary importance. In this family of carnivorous mammals, where many species have a notable degree of affinity, not only phylogenetic but also phenotypic, it has not always been easy to distinguish, not even through deep morpho-anatomical analyses, between some taxa such as the oncillas, leopard cats, some small wildcats, and clouded leopards, which only recently have been recognized as a species thanks to their DNA analysis. Not only that but through techniques based on molecular analysis, the complex phylogenetic relationships between the various groups (lineages) of the family have finally been clarified. In fact, even the paleontological findings that should help us to reconstruct the evolutionary history of the family are not always easy to interpret, and sometimes it is not easy to determine some extinct species through fossils, which may consist of only individual teeth or portions of the skull; Furthermore, given the great similarity of the various species at the skeletal level, the findings did not always help us to clarify the phylogenetic history of the current species of cats. The causes of the fossils of felids being extremely partial and also rather rare are numerous; many species that lived in tropical forests have left very few remains as this habitat is not suitable for fossilization. Felines, being a carnivorous species at the highest level of the food pyramid, have and have certainly had
Felines of the World. DOI: https://doi.org/10.1016/B978-0-12-816503-4.00008-8 © 2019 Elsevier Inc. All rights reserved.
Melanism in felines Melanism in the leopard (Mendel’s first and second laws) Melanism in the Jaguar
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Chapter 8.1.2 Genetics of some colors of the domestic cat Spontaneous mutations in the color of the hair: albinism The white lions of Timbavati and Kruger
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Chapter 8.1.3 Crossing between the species of great felines (Pantherinae)
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in the past, populations much less numerous than their prey, so the probability of finding fossils of felids is lower than that of finding, for example, Equidae or elephants, species with high density and living in large groups; the Rancho la Brea Tar Pits, in California, which have rendered many felid fossils, are an exception occurring in exceptional circumstances, and in any case the species of felids that have been found at this site are a large quantity of skeletons of a single species of saber-toothed cat, Smilodon fatalis, and a few specimens of four other fossil species: some Miracinonyx, Panthera atrox, Panthera onca augusta, and Homotherium serum. From a taxonomic point of view it is not easy to assign to the same species populations of isolated felids, which live in very different habitats (mountainous, humid plains, or pampas) and have very different phenotypes: striated, maculated, and unicolor; which is what happened to the colocola or pampas cat (Leopardus colocola). The different populations were considered by almost all experts as three distinct species and so were classified as L. colocola, Leopardus pajeros, and Leopardus braccatus, until the analysis of their DNA revealed that it was a single polymorphic species. Studies based on genetic analysis have revealed important phenomena concerning the past hybridization between related species. Especially through the analysis of mitochondrial DNA, transmitted only by maternal inheritance, it has been possible to verify that different degrees of hybridization have occurred between various species and that interbreeding between related species has
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stimulated the evolution of new species (speciation). For example, some Eurasian and African species of genus Felis (F. silvestris, libyca, and catus), closely related to each other, present in their DNA traces evidence of some ancient hybridizations between them and that they are all found in the domestic cat (Driscoll et al., 2007). Studying the genome of the genus Leopardus in South America it has been shown that the specific diversification (speciation) of the northern oncilla (L. tigrinus) has been favored by an ancient hybridization with another South American species, the pampas cat or colocola (L. colocola) (Trigo et al., 2013); moreover, there are currently hybridization episodes between the southern populations of southern oncilla (L. guttulus) with the sympatric populations of Geoffroy’s cat (L. geoffroyi). What is interesting is that the northern and southern oncillas, considered before this study a single species, do not reveal in their DNA any trace of hybridization between
them. Also, Geoffroy’s cat and colocola, sympatric species in many regions, did not reveal any degree of hybridization. One of the reasons for hybridization between different species has been discovered by studying interbreeding between bobcat (Lynx rufus) and Canadian lynx (Lynx canadensis) on the border between Canada and the United States. Although the level of hybridization between the two species was low, all the hybrid individuals came from areas where L. canadensis was very rare, suggesting that individuals in low-density populations had difficulty finding conspecifics with which to mate for which they seek specimens of the species most similar to their own available in the area, in this case the bobcat. Hence the population density and their distribution in areas with related species and sympatric, together with particular ethological aspects are very important factors to keep in mind in conservation and reintroduction projects (Macdonald and Loveridge, 2010).
Being very similar and belonging to the same genus Lynx, these two species of North American lynx can hybridize where they coexist in the same region (sympatric). https://www.shutterstock.com/it/image-photo/canada-lynx-427454902?src 5 PiYrOXNCqyuZV0cqcXnVEA-4-25; https://www.shutterstock.com/it/image-photo/prowling-lynx-bobcat-on-hunt-499237948?src 5 PiYrOXNCqyuZV0cqcXnVEA-2-88
Chapter 8.1 DNA, chromosomes, genes, genotypes, and phenotypes Chromosomes and Karyotype Like all living things, felids present in the cell nucleus a certain number of chromosomes composed of DNA (deoxyribonucleic acid) which bear the hereditary traits (genes) transmitted from parents to offspring. The chromosomes are present in the cells in pairs; each pair consists of one chromosome inherited from the father and one inherited from the mother. In the mitochondria, organelles are present in the cytoplasm of the cell, chemical reactions occur that have various functions including respiration and cell metabolism. Within the mitochondria
there is a certain amount of DNA (mitochondrial DNA) that is inherited only through the mother; in fact the mitochondria are present in the cytoplasm of the female gametes (ovules), but not in the male gametes (spermatozoa). For this reason all the cells that are formed by the fusion of the maternal ovum and the paternal spermatozoon will contain only mitochondria and mitochondrial DNA of maternal origin. Each species has a fixed number of pairs of nuclear chromosomes, called karyotypes, and all felids have 19, therefore they have 38 chromosomes (2n 5 38), except for the South American small wildcats of the lineage Ocelot, belonging to genus Leopardus (ocelot, margay, oncilla, etc.) that have only 18 pairs, giving 36 chromosomes (2n 5 36) instead of 38.
(A)
(B)
(A) A chromosome and the nitrogenous bases that make up DNA or deoxyribonucleic acid: Adenine, Thymine, Cytosine, Guanine and their chemical structure. (B) The double-helix or spiral structure of the DNA molecule G G
To the left. A graphic representation of RNA and DNA To the right. The molecules that make up the DNA are illustrated. The two branches of the helico formed by a sugar called deoxyribose, and a phosphate group, are held together by chemical bonds (hydrogen bond) between Thymine3Adenine Cytosine3Guanine
https://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/DNA_chemical_structure.svg/878px-DNA_chemical_structure.svg.png; https://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/ Du_chromosome_%C3%A0_l%27ADN_porteur_d%27une_information_%28sans_l%C3%A9gende%29.svg/2000px-Du_chromosome_%C3%A0_l%27ADN_porteur_d%27une_information_%28sans_l %C3%A9gende%29.svg.png; https://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Difference_DNA_RNA-EN.svg/1371px-Difference_DNA_RNA-EN.svg.png; https://upload.wikimedia.org/ wikipedia/commons/f/f5/DNA_Overview_it.png
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This image shows the karyotype or chromosomes of a tiger male where we see the shape, length, and number of chromosomes. Each pair, numbered from 1 to 18, is composed of a paternal chromosome and a maternal one that are similar because they carry the same number of genes. There are 18 pairs of chromosomes (autosomes) plus a pair of sex chromosomes, the nineteenth couple, composed of the so-called “heterosomes” because they are different from each other; it is the X chromosome and the Y chromosome whose presence determines the sex of the individual. In mammals, if there are two X chromosomes (XX) in the karyotype, we will have a female individual; if there is an X chromosome and a Y chromosome (XY), we will have a male. As can also be seen from the image, X and Y are different and the Y chromosome is small and carries few genes, but is fundamental in the determination of sex. Conversely, in birds, two X chromosomes (XX) is the male, and XY female. https://upload.wikimedia.org/wikipedia/commons/9/94/Karyotype_of_Siberian_tiger.png
Heredity traits By simplifying, we can say that every pair of chromosomes carries a certain number of genes. The set of all the genes of all the couples of chromosomes is called the “genotype” and corresponds to the typical traits, inherited from the parents, and that “distinguish” the species at the genetic or chromosomal level, making it unique. In the chromosomes of felids there are about 30,000 different genes. Hereditary traits manifest themselves in the individual through “somatic characteristics” such as: colors (fur, eyes, or flowers in plants, etc.), anatomical, and/or physiological prerogatives, etc.; the set of these characteristics, highlighted outwardly by the genotype, is called the “phenotype” (from the Greek phainein, which means “to appear,” and ty´pos, which means “footprint”). Each gene, a regulator of the physical-somatic characteristics of the phenotype, is composed of two alleles: one carried from the maternal chromosome and one from the paternal chromosome.
Dominant and recessive Alleles The genes that regulate the characteristics of the individual are revealed in the phenotype in various ways
according to the type of alleles that compose them. There are genes composed of two alleles that carry the same character and are called homozygotes, and alleles that carry two different characters called heterozygotes. Some alleles can mix and reveal themselves in the phenotype with intermediate characteristics (codominant or semidominant alleles); in man the genes that determine the blood groups or those that determine the color of the skin are codominant; simplifying the second case we can have an allele that determines light skin and dark skin (heterozygous), the phenotype is revealed with a skin of intermediate color. For the gene of color of some flowers, an allele that gives the color white and an allele that gives the color red is revealed in the phenotype with flowers of intermediate color, that is pink, or with white flowers and red flowers on the same plant. Other genes instead have alleles that do not mix their characteristics; thus only one of the two alleles will be revealed in the phenotype, while the other remains latent. Some alleles, called dominant, are those that are revealed in the phenotype of an individual (dominant character), the recessive alleles are not revealed (recessive character) if on the other chromosome there is an allele with a dominant character, but only if the same allele is present at the chromosome; they remain
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however present in the genotype and can be inherited by future generations. In humans the allele that determines “blue eyes” is recessive with respect to the allele that determines brown eyes. Only those who have inherited two “blue eyes” alleles (homozygous for blue eyes) will have blue eyes; while a person with brown eyes may have inherited: >G two “brown eyes” alleles (homozygous for brown eyes); >G or a “brown eyes” allele and a “blue eyes” allele (heterozygous), and therefore only the first is dominant and manifested in the phenotype
Blue Brown Blue
Genotype alleles / Blue / Brown
Homozygote Homozygote
Blue eyes Brown eyes
/
Heterozygote
Brown eyes
Brown
Phenotype
Genetic mutations During the cell divisions that lead to the formation of a new individual the chromosomes and the DNA they contain are duplicated over and over again, forming new cells. During these phases some stretches of DNA may undergo changes at the biochemical level that change the original structure of the DNA and are called “genetic mutations.” By genetic mutation we mean any stable and inheritable modification of the genetic material (especially of DNA) due to external agents or chance. A mutation then modifies the genotype of an individual and can eventually modify the phenotype according to its characteristics and interactions with the environment. It is with mutations that evolutionary processes are favored, such as speciation, adaptation to the environment, etc. These mutations determine so-called “genetic variability,” or the condition for which organisms, even within the same species, are not all identical, but differ from one another for one or more characteristics. On this variability, through the genetic recombination that favors the emergence of new alleles with different characteristics, operates natural selection, which promotes favorable mutations and eliminates unfavorable ones.
Molecular clock The molecular clock is a technique used to estimate the time that has elapsed since the separation between two
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species, starting from the study of the differences existing in DNA or in the sequences in the amino acids of some proteins. In genetics, the most recent common ancestor (“most or lost recent common ancestor”) of any set of organisms (species or specimens), represents the progenitor from which all the organisms of the group are direct descendants. To calculate the time elapsed since a common ancestor separated by forming two different taxa, the molecular clock technique is used. This technique is based on the hypothesis that “random mutations, with which the genes evolve, occur with frequencies that are almost constant over time.” Considering this assumption valid, it becomes possible to estimate the time elapsed from the moment in which the divergence between two species descended from the same common ancestor (lost common ancestor or coancestor) used most frequently when it comes to coalescence between species; or you also use the most recent common ancestor or MRCA term that is most commonly used when it comes to a common ancestor at the level of specimens. The results are obtained “simply” by evaluating the number of differences, present in related DNA sequences or in the corresponding proteins. Phylogenetics or phylogeny studies the origin and evolution, up to the temporal succession of the processes of speciation, of a set of organisms. This is an essential task of systematics that also deals with determining ancestral relationships between known species (extant and/or extinct). The molecular clock technique is an important tool for studying the phylogeny of various taxa using the information of molecular genetics to determine a correct scientific classification of organisms (taxonomy) or to study differences in the selective forces. The knowledge of almost constant rates of molecular evolution in specific groups of lines facilitates the recognition of the duration of phylogenetic events, such as the divergence between living taxa and in the construction of the phylogenetic tree. The molecular clock technique can calculate times between one evolutionary event and another, such as the birth of a new species or the divergence between two groups of taxa, but cannot date these events and give a precise moment within the time scale (eras, periods, etc.). To obtain datable times, for example, in “Mya ago,” one must compare on one side the chronological periods calculated with the molecular clock and on the other the times obtained with the dating of the fossils of the taxa that is being analyzed, often calculating the age through the decay rate of radiocarbon or carbon 14 (C14). In this way, having wellclassified and correctly dated fossils enables more precise and safer phylogenetic trees to be obtained.
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Using all these techniques with computerized mathematical models such as BEAST “Bayesian evolutionary analysis sampling trees” previous generations of geneticist zoologists have obtained phylogenetic trees that have revolutionized the taxonomy of the family Felidae, discovering unsuspected phylogenetic kinships, or even new species (to date totaling 41) and modifying the temporal hypotheses on the evolutionary history, thanks also to the last important fossil records found and dated (e.g., Tseng et al., 2013, Panthera blytheae). The researchers examined the DNA of species, from all the chromosomes, together with the X and Y sexual chromosomes (heterosomes) of maternal and paternal origin, and DNA of matrilinear origin present in the mitochondria. The most accredited phylogenetic trees of the Felidae family were obtained at the beginning of the 21st century by J.S., O’Brien, W.E Johnson, C.A. Driscoll, A.C. Kitchener, N. Yamaguchi, D.W. Macdonald et al.; more recently (2016) other phylogenetic trees have been proposed, with some variations in the chronology and the phylogenetic kinship, based on the latest research that
takes into account hypotheses of ancient hybridizations between related species (which are still happening today), these are illustrated in the elaborate graphs by G. Li, B. W. Davis, E. Eizirik, W.J. Murphy, et al. (see Chapter 2: Family Felidae).
Chapter 8.1.1 Feline genetics: Mendel’s laws Gregor Johann Mendel (1822 84), a biologist and mathematician from the Czech Republic, but German-speaking, is considered the precursor of modern genetics for his observations on hereditary traits (inheritance). He first introduced the innovative concept for the era of “hereditary,” stating that the basis of inheritance is specific agents obtained from the parents. Applying the laws of statistics and the calculation of probabilities, the study of biological inheritance came to formulate three laws, according to which the inherited characteristics are transmitted from one generation to another.
Black jaguar (Panthera onca), the melanistic form of jaguar. Photo by M. Korinek.
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Melanism in felines Melanism is a mutation of the characteristic that regulates the color of the hair or skin, and which can appear in many species of felines. This mutation is given by an allele (but it seems four alleles are involved) that increases the amount of dark pigments called melanins (eumelanine) in the phenotype of the individuals affected, so that the specimens appears completely black, instead of, for example, spotted on a light background (in fact they are spotted on a dark background). In the leopard, melanism is a recessive allele with respect to the allele that makes the hair light and speckled, which instead is the dominant color. In addition to the leopard there are other 11 species of felids in which it is quite common to find melanic individuals (serval, guigna, African and Asian golden cat, Geoffroy’s cat, etc.).
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In some species such as the jaguar it is easy to find black specimens because, unlike in the leopard, melanism in these species is a dominant character. In others, like the jaguarundi, melanism involves the MC1R allele (called the melanocortin receptor). Depending on the molecules that bind to this allele, there are different pigment products: black eumelanine or reddish phaeomelanine. For this reason, in the jaguarundi the color of the hair is variable with specimens that can be black, brown, or red; individuals heterozygous for MC1R are almost black, homozygous individuals are fawn-reddish. Based on the pigments that MC1R can produce, there is a great variety of colorations within the same species ranging from black to brown grizzled agouti, to tawnyreddish.
Jaguarundi furs of natural different colors. https://upload.wikimedia.org/wikipedia/commons/b/b8/Felis_%28herpailurus%29_yaguarorundi_% 28Jaguarondi%29_fur_skin_2.jpg
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Color variations in the fur of the jaguarundi: from black to red depending on the types of allele and whether it is homozygous or heterozygous. https:// https:// www.shutterstock.com/it/image-photo/jaguarundi-puma-yagouaroundi-resting-on-stump-298592792?src 5 SLXvS62sEdz31w6YPOp5Iw-1-6; www.shutterstock.com/it/image-photo/jaguarundi-wild-1212931666?src 5 SLXvS62sEdz31w6YPOp5Iw-1-3
Melanism in the leopard (Mendel’s first and second laws) The mutation of melanism causing black fur is particularly common in the Asian leopard, among which we find the so-called black panthers, which are very common especially on Java island, where the local leopard subspecies (Panthera pardus melas), isolated on the island for about 800,000 years, has a majority affected by melanism. Melanism is less common, but is also present, in leopards
Genotype 1
1 Homozygote spotted
in the African and Indonesian forests. In the forest areas this mutation is maintained as it is cryptic in the depths of vegetation, whereas in the more luminous open habitats black leopards are very visible and therefore more prone to natural selection. Since the color black is a recessive character, compared to the color of fur we can have only two leopard phenotypes: spotted or black, but belonging to three different genotypes.
Genotype 2
2
Heterozygote spotted/black
Spot./Spot.
Spot./black
< ____________________________________________> Spotted phenotype
Genotype 3
3 Homozygote black
Black./black
< ____________________> Black phenotype
(1) Spotted phenotype with pure homozygote genotype with two equal Spotted-Spotted alleles (Spot./Spot.). Both dominant. (2) Spotted phenotype with heterozygote genotype (Spot./black), where there are two different alleles: one dominant Spotted (Spot.) and one, melanic black (black.), recessive. (3) Black phenotype (black panther). This phenotype is possible only if the leopard has a homozygote genotype with two recessive alleles that are both black melanistic (black./black.).
For Mendel’s first law of or law of dominance (or law of homogeneity of phenotype), specimens born from the intersection of two homozygous parents, who differ for an allelic pair, will have the phenotype determined by the dominant allele. With a broader meaning than Mendel’s work, it can be enunciated as the law of “uniformity of first generation hybrids” (all of which are equal to each other and to the parent with dominant alleles). If we cross two individuals homozygous for dominant (spotted) and for a recessive (black melanic) character in the first generation we will have 100% spotted leopards but bearers of the black recessive allele.
Mendel’s second law is important because for the first time the hereditary traits were mentioned and it was noted that each gene carries two alleles. For Mendel’s second law or the law of segregation, alleles are separated at the moment of the formation of gametes. With reproduction female and male gametes with alleles equal or different come together to form homozygous (with alleles alike) or heterozygous genes (with different alleles) with this probability: 25% of individuals homozygous for the dominant allele; 50% of individuals heterozygous for both alleles and with a phenotype similar to the dominant homozygote; 25% of individuals homozygous for the recessive allele. Therefore if you cross two maculated leopards but both heterozygous and carriers of the black allele (Spot./black) they will have: 25% probability of generating homozygous spotted offspring (Spot./Spot.); 50% probability of generating heterozygous spotted offspring (Spot./black); 25% chance of generating offspring with homozygous black fur (black/black).
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Melanism in the Jaguar Melanism in the jaguar follows the same laws of Mendel’s inheritance, but in contrast to the leopard, melanism in the jaguar is a dominant character,
whereby only the homozygous specimens carrying both the “spot” alleles 5 light-spotted hair (spot./spot.) have spotted fur, while those carrying the “Black” allele 5 black hair (spot./Black or Black/Black) have black fur.
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Chapter 8.1.2 Genetics of some colors of the domestic cat All species of domestic farm animals (cattle, sheep, goats, pigs, horses) have in their chromosomes the genes that determine the coloration typical of the wild species from which they come, and this coloring is a dominant characteristic (sometimes with more alleles). If a group of domestic animals is returned to the wild, after some generations, the specimens no longer subject to human selection but to that of nature, will present a phenotype very similar to that of the original wild species from which they derive. Even among dogs it is so; wild dogs tend to resume the phenotype of the Wolf. The gray coloration with dark vertical stripes of the domestic cat is a characteristic inherited from the genes of the wild species (Felis libyca libyca) from which the tabby is derived and which in nature has this color; all wild cats after generations in the wild resume this coloring.
The striped gray coat of the domestic cat (mackerel tabby) is a dominant character and is due to an allele called Mackerel tabby pattern (TaM) and in the past all cats had this quality, as can be seen from paintings by the ancient Egyptians and from the mosaics found in the cities of the ancient Roman Empire and also later. It is not until the Middle Ages that we see the first representations of blotched cats (recessive allele blotched tabby pattern Tab), and of various colors, due, after centuries of “domestication” to natural alleles that have remained latent or always in the recessive phase, due to rare mutations arising accidentally, or to characteristics obtained with crossings specifically selected with mutant specimens. In doing so, coupling a gray Mackerel tabby pattern (TaM) cat with a blotched recessive allele blotched tabby pattern Tab cat, we will have generations that follow the Mendel laws.
A cat (female) with the typical fur with black and red spots, called tortoiseshell. https://upload.wikimedia.org/wikipedia/commons/c/c5/Short-haired_tortoiseshell_cat.jpg
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Tortoiseshell fur in cats is a mixture of black and red fur spots. Alleles that give fur of these colors have the same opportunity to reveal themselves in the phenotype (codominant alleles). Not having color alleles on the sexual chromosome Y, the male (XY) may have only red (R) or black fur (B).
The females, having two X chromosomes, both with the possibility of having red and black fur alleles, have three opportunities to have the genotype RR (fur red), BB (fur black), or RB (fur tortoiseshell). Therefore all cats with tortoiseshell fur can only be females.
The table shows the possible genotypes in female and male domestic cats with red and black fur alleles; alleles are taken from sex chromosome X but not from sex chromosome Y. Tortoiseshell is present only with the RB genome, therefore it is impossible in males (only R- or only B-).
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Spontaneous mutations in the color of the hair: albinism Albinism is not one of the traits that appears on a gene and determines hair color, but is a random and spontaneous mutation (a type of error) that can sometimes appear in genes and that inhibits the formation of any colored pigment, by which specimens affected by complete
albinism are pure white. Even the irises of the eyes lack the pigment that makes them colored and therefore they are transparent, meaning they appear reddish due to blood vessels. Often those animals with albinism also have problems with “photophobia” in daytime vision and can find daylight uncomfortable.
Withe tigers cub and adult; they are blue eyes and black stripes. The light color of these tigers is not due to the genetic mutation called “albinism” but to a recessive gene called “leucism” that inhibits the formation of the yellow-orange pigment that gives the background color to tiger fur. The white tigers keep the dark stripes due to a pigment called melanin the formation of which is not inhibited by leucism; they also have eyes with blue-colored irises. Normally tigers have green eyes because the green color is created by yellow and blue pigments, however, not having yellow pigments due to leucism, white tigers have blue eyes (left photo). The leucism gene is present in the genome of tigers but is rare in nature. Among the tigers it appears only in the Indian royal tiger or Bengal tiger (Panthera tigris tigris) and particularly in the Indian district of “Rewa,” where they have been selected and bred by a local maharaja since 1948 after the capture of a wild specimen. However, there are also almost white tigers that present with very few dark stripes only on the head and tail but these are not affected by true total albinism. (Right) Photo by V. Martegani.
The white lions of Timbavati and Kruger Occasionally there is a white lion in the wild, in some South Africa reserves, particularly in the Timbavati Nature Reserve and in the Kruger National Park. This coloring penalizes lions in the wild because they are easily sighted by their prey, which makes hunting more difficult. A white lion is therefore often condemned to
death from starvation. These lions are not completely white but blond; the cause is to be found in a gene called chinchilla or color inhibitor, and it is also a form of leucism that inhibits the formation of the yellow-red pigment. Being a recessive gene only homozygous lions for this gene will be present with light blond, almost white, hair.
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An almost white lioness (pale fawn) with her three white cubs. Photo by K. Rudlof.
Chapter 8.1.3 Crossing between the species of great felines (Pantherinae) The fact that we can cross species of different felids and obtain partially fertile hybrids demonstrates the low genetic variability within this family of closely related species. Many zoological parks have attempted to cross the various species of big cats: tigers, lions, leopards, and
jaguars, especially. These crossings have proved to be possible because all these species have great genetic affinity, all belonging to the genus Panthera, and having the same number and type of chromosomes. Normally, however, the results of these crossings present sterile male specimens, while the females are often fertile. Naturally, this type of interbreeding is not found in the wild.
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(Left) A male liger specimen in a zoo. This is an interbreeding (crossbreeding) between a male lion and a female tiger. (Right) Another liger born from interbreeding between a lion and a Siberian tiger at the “Siberian park Harbin” in China. https://www.shutterstock. com/it/image-photo/white-liger-walk-zoo-aviary-ligr-1008744487?src 5 -h_eHABB_fgCzqS0dsKzmQ-1-0 https://www.shutterstock.com/it/image-photo/ liger-siberian-tiger-park-harbin-china-521992762?src 5 Ty8Ak3A-JpuJpna3n4h4Lw-1-1
A taxidermized specimen of a leopon, a crossbreeding between a lioness and a leopard displayed in a natural history museum. https://upload.wikimedia.org/wikipedia/commons/d/d5/Leopon_leokichi.JPG
Genetics Chapter Outline Genetic variability Chapter 8.1 DNA, chromosomes, genes, genotypes, and phenotypes Chromosomes and Karyotype Heredity traits Dominant and recessive Alleles Genetic mutations Molecular clock
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Chapter 8.1.1 Feline genetics: Mendel’s laws
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Genetic variability In the last few decades, the assistance given by the field of genetics to the taxonomy of the Felidae has been of primary importance. In this family of carnivorous mammals, where many species have a notable degree of affinity, not only phylogenetic but also phenotypic, it has not always been easy to distinguish, not even through deep morpho-anatomical analyses, between some taxa such as the oncillas, leopard cats, some small wildcats, and clouded leopards, which only recently have been recognized as a species thanks to their DNA analysis. Not only that but through techniques based on molecular analysis, the complex phylogenetic relationships between the various groups (lineages) of the family have finally been clarified. In fact, even the paleontological findings that should help us to reconstruct the evolutionary history of the family are not always easy to interpret, and sometimes it is not easy to determine some extinct species through fossils, which may consist of only individual teeth or portions of the skull; Furthermore, given the great similarity of the various species at the skeletal level, the findings did not always help us to clarify the phylogenetic history of the current species of cats. The causes of the fossils of felids being extremely partial and also rather rare are numerous; many species that lived in tropical forests have left very few remains as this habitat is not suitable for fossilization. Felines, being a carnivorous species at the highest level of the food pyramid, have and have certainly had
Felines of the World. DOI: https://doi.org/10.1016/B978-0-12-816503-4.00008-8 © 2019 Elsevier Inc. All rights reserved.
Melanism in felines Melanism in the leopard (Mendel’s first and second laws) Melanism in the Jaguar
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Chapter 8.1.2 Genetics of some colors of the domestic cat Spontaneous mutations in the color of the hair: albinism The white lions of Timbavati and Kruger
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Chapter 8.1.3 Crossing between the species of great felines (Pantherinae)
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in the past, populations much less numerous than their prey, so the probability of finding fossils of felids is lower than that of finding, for example, Equidae or elephants, species with high density and living in large groups; the Rancho la Brea Tar Pits, in California, which have rendered many felid fossils, are an exception occurring in exceptional circumstances, and in any case the species of felids that have been found at this site are a large quantity of skeletons of a single species of saber-toothed cat, Smilodon fatalis, and a few specimens of four other fossil species: some Miracinonyx, Panthera atrox, Panthera onca augusta, and Homotherium serum. From a taxonomic point of view it is not easy to assign to the same species populations of isolated felids, which live in very different habitats (mountainous, humid plains, or pampas) and have very different phenotypes: striated, maculated, and unicolor; which is what happened to the colocola or pampas cat (Leopardus colocola). The different populations were considered by almost all experts as three distinct species and so were classified as L. colocola, Leopardus pajeros, and Leopardus braccatus, until the analysis of their DNA revealed that it was a single polymorphic species. Studies based on genetic analysis have revealed important phenomena concerning the past hybridization between related species. Especially through the analysis of mitochondrial DNA, transmitted only by maternal inheritance, it has been possible to verify that different degrees of hybridization have occurred between various species and that interbreeding between related species has
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stimulated the evolution of new species (speciation). For example, some Eurasian and African species of genus Felis (F. silvestris, libyca, and catus), closely related to each other, present in their DNA traces evidence of some ancient hybridizations between them and that they are all found in the domestic cat (Driscoll et al., 2007). Studying the genome of the genus Leopardus in South America it has been shown that the specific diversification (speciation) of the northern oncilla (L. tigrinus) has been favored by an ancient hybridization with another South American species, the pampas cat or colocola (L. colocola) (Trigo et al., 2013); moreover, there are currently hybridization episodes between the southern populations of southern oncilla (L. guttulus) with the sympatric populations of Geoffroy’s cat (L. geoffroyi). What is interesting is that the northern and southern oncillas, considered before this study a single species, do not reveal in their DNA any trace of hybridization between
them. Also, Geoffroy’s cat and colocola, sympatric species in many regions, did not reveal any degree of hybridization. One of the reasons for hybridization between different species has been discovered by studying interbreeding between bobcat (Lynx rufus) and Canadian lynx (Lynx canadensis) on the border between Canada and the United States. Although the level of hybridization between the two species was low, all the hybrid individuals came from areas where L. canadensis was very rare, suggesting that individuals in low-density populations had difficulty finding conspecifics with which to mate for which they seek specimens of the species most similar to their own available in the area, in this case the bobcat. Hence the population density and their distribution in areas with related species and sympatric, together with particular ethological aspects are very important factors to keep in mind in conservation and reintroduction projects (Macdonald and Loveridge, 2010).
Being very similar and belonging to the same genus Lynx, these two species of North American lynx can hybridize where they coexist in the same region (sympatric). https://www.shutterstock.com/it/image-photo/canada-lynx-427454902?src 5 PiYrOXNCqyuZV0cqcXnVEA-4-25; https://www.shutterstock.com/it/image-photo/prowling-lynx-bobcat-on-hunt-499237948?src 5 PiYrOXNCqyuZV0cqcXnVEA-2-88
Chapter 8.1 DNA, chromosomes, genes, genotypes, and phenotypes Chromosomes and Karyotype Like all living things, felids present in the cell nucleus a certain number of chromosomes composed of DNA (deoxyribonucleic acid) which bear the hereditary traits (genes) transmitted from parents to offspring. The chromosomes are present in the cells in pairs; each pair consists of one chromosome inherited from the father and one inherited from the mother. In the mitochondria, organelles are present in the cytoplasm of the cell, chemical reactions occur that have various functions including respiration and cell metabolism. Within the mitochondria
there is a certain amount of DNA (mitochondrial DNA) that is inherited only through the mother; in fact the mitochondria are present in the cytoplasm of the female gametes (ovules), but not in the male gametes (spermatozoa). For this reason all the cells that are formed by the fusion of the maternal ovum and the paternal spermatozoon will contain only mitochondria and mitochondrial DNA of maternal origin. Each species has a fixed number of pairs of nuclear chromosomes, called karyotypes, and all felids have 19, therefore they have 38 chromosomes (2n 5 38), except for the South American small wildcats of the lineage Ocelot, belonging to genus Leopardus (ocelot, margay, oncilla, etc.) that have only 18 pairs, giving 36 chromosomes (2n 5 36) instead of 38.
(A)
(B)
(A) A chromosome and the nitrogenous bases that make up DNA or deoxyribonucleic acid: Adenine, Thymine, Cytosine, Guanine and their chemical structure. (B) The double-helix or spiral structure of the DNA molecule G G
To the left. A graphic representation of RNA and DNA To the right. The molecules that make up the DNA are illustrated. The two branches of the helico formed by a sugar called deoxyribose, and a phosphate group, are held together by chemical bonds (hydrogen bond) between Thymine3Adenine Cytosine3Guanine
https://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/DNA_chemical_structure.svg/878px-DNA_chemical_structure.svg.png; https://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/ Du_chromosome_%C3%A0_l%27ADN_porteur_d%27une_information_%28sans_l%C3%A9gende%29.svg/2000px-Du_chromosome_%C3%A0_l%27ADN_porteur_d%27une_information_%28sans_l %C3%A9gende%29.svg.png; https://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Difference_DNA_RNA-EN.svg/1371px-Difference_DNA_RNA-EN.svg.png; https://upload.wikimedia.org/ wikipedia/commons/f/f5/DNA_Overview_it.png
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This image shows the karyotype or chromosomes of a tiger male where we see the shape, length, and number of chromosomes. Each pair, numbered from 1 to 18, is composed of a paternal chromosome and a maternal one that are similar because they carry the same number of genes. There are 18 pairs of chromosomes (autosomes) plus a pair of sex chromosomes, the nineteenth couple, composed of the so-called “heterosomes” because they are different from each other; it is the X chromosome and the Y chromosome whose presence determines the sex of the individual. In mammals, if there are two X chromosomes (XX) in the karyotype, we will have a female individual; if there is an X chromosome and a Y chromosome (XY), we will have a male. As can also be seen from the image, X and Y are different and the Y chromosome is small and carries few genes, but is fundamental in the determination of sex. Conversely, in birds, two X chromosomes (XX) is the male, and XY female. https://upload.wikimedia.org/wikipedia/commons/9/94/Karyotype_of_Siberian_tiger.png
Heredity traits By simplifying, we can say that every pair of chromosomes carries a certain number of genes. The set of all the genes of all the couples of chromosomes is called the “genotype” and corresponds to the typical traits, inherited from the parents, and that “distinguish” the species at the genetic or chromosomal level, making it unique. In the chromosomes of felids there are about 30,000 different genes. Hereditary traits manifest themselves in the individual through “somatic characteristics” such as: colors (fur, eyes, or flowers in plants, etc.), anatomical, and/or physiological prerogatives, etc.; the set of these characteristics, highlighted outwardly by the genotype, is called the “phenotype” (from the Greek phainein, which means “to appear,” and ty´pos, which means “footprint”). Each gene, a regulator of the physical-somatic characteristics of the phenotype, is composed of two alleles: one carried from the maternal chromosome and one from the paternal chromosome.
Dominant and recessive Alleles The genes that regulate the characteristics of the individual are revealed in the phenotype in various ways
according to the type of alleles that compose them. There are genes composed of two alleles that carry the same character and are called homozygotes, and alleles that carry two different characters called heterozygotes. Some alleles can mix and reveal themselves in the phenotype with intermediate characteristics (codominant or semidominant alleles); in man the genes that determine the blood groups or those that determine the color of the skin are codominant; simplifying the second case we can have an allele that determines light skin and dark skin (heterozygous), the phenotype is revealed with a skin of intermediate color. For the gene of color of some flowers, an allele that gives the color white and an allele that gives the color red is revealed in the phenotype with flowers of intermediate color, that is pink, or with white flowers and red flowers on the same plant. Other genes instead have alleles that do not mix their characteristics; thus only one of the two alleles will be revealed in the phenotype, while the other remains latent. Some alleles, called dominant, are those that are revealed in the phenotype of an individual (dominant character), the recessive alleles are not revealed (recessive character) if on the other chromosome there is an allele with a dominant character, but only if the same allele is present at the chromosome; they remain
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however present in the genotype and can be inherited by future generations. In humans the allele that determines “blue eyes” is recessive with respect to the allele that determines brown eyes. Only those who have inherited two “blue eyes” alleles (homozygous for blue eyes) will have blue eyes; while a person with brown eyes may have inherited: >G two “brown eyes” alleles (homozygous for brown eyes); >G or a “brown eyes” allele and a “blue eyes” allele (heterozygous), and therefore only the first is dominant and manifested in the phenotype
Blue Brown Blue
Genotype alleles / Blue / Brown
Homozygote Homozygote
Blue eyes Brown eyes
/
Heterozygote
Brown eyes
Brown
Phenotype
Genetic mutations During the cell divisions that lead to the formation of a new individual the chromosomes and the DNA they contain are duplicated over and over again, forming new cells. During these phases some stretches of DNA may undergo changes at the biochemical level that change the original structure of the DNA and are called “genetic mutations.” By genetic mutation we mean any stable and inheritable modification of the genetic material (especially of DNA) due to external agents or chance. A mutation then modifies the genotype of an individual and can eventually modify the phenotype according to its characteristics and interactions with the environment. It is with mutations that evolutionary processes are favored, such as speciation, adaptation to the environment, etc. These mutations determine so-called “genetic variability,” or the condition for which organisms, even within the same species, are not all identical, but differ from one another for one or more characteristics. On this variability, through the genetic recombination that favors the emergence of new alleles with different characteristics, operates natural selection, which promotes favorable mutations and eliminates unfavorable ones.
Molecular clock The molecular clock is a technique used to estimate the time that has elapsed since the separation between two
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species, starting from the study of the differences existing in DNA or in the sequences in the amino acids of some proteins. In genetics, the most recent common ancestor (“most or lost recent common ancestor”) of any set of organisms (species or specimens), represents the progenitor from which all the organisms of the group are direct descendants. To calculate the time elapsed since a common ancestor separated by forming two different taxa, the molecular clock technique is used. This technique is based on the hypothesis that “random mutations, with which the genes evolve, occur with frequencies that are almost constant over time.” Considering this assumption valid, it becomes possible to estimate the time elapsed from the moment in which the divergence between two species descended from the same common ancestor (lost common ancestor or coancestor) used most frequently when it comes to coalescence between species; or you also use the most recent common ancestor or MRCA term that is most commonly used when it comes to a common ancestor at the level of specimens. The results are obtained “simply” by evaluating the number of differences, present in related DNA sequences or in the corresponding proteins. Phylogenetics or phylogeny studies the origin and evolution, up to the temporal succession of the processes of speciation, of a set of organisms. This is an essential task of systematics that also deals with determining ancestral relationships between known species (extant and/or extinct). The molecular clock technique is an important tool for studying the phylogeny of various taxa using the information of molecular genetics to determine a correct scientific classification of organisms (taxonomy) or to study differences in the selective forces. The knowledge of almost constant rates of molecular evolution in specific groups of lines facilitates the recognition of the duration of phylogenetic events, such as the divergence between living taxa and in the construction of the phylogenetic tree. The molecular clock technique can calculate times between one evolutionary event and another, such as the birth of a new species or the divergence between two groups of taxa, but cannot date these events and give a precise moment within the time scale (eras, periods, etc.). To obtain datable times, for example, in “Mya ago,” one must compare on one side the chronological periods calculated with the molecular clock and on the other the times obtained with the dating of the fossils of the taxa that is being analyzed, often calculating the age through the decay rate of radiocarbon or carbon 14 (C14). In this way, having wellclassified and correctly dated fossils enables more precise and safer phylogenetic trees to be obtained.
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Using all these techniques with computerized mathematical models such as BEAST “Bayesian evolutionary analysis sampling trees” previous generations of geneticist zoologists have obtained phylogenetic trees that have revolutionized the taxonomy of the family Felidae, discovering unsuspected phylogenetic kinships, or even new species (to date totaling 41) and modifying the temporal hypotheses on the evolutionary history, thanks also to the last important fossil records found and dated (e.g., Tseng et al., 2013, Panthera blytheae). The researchers examined the DNA of species, from all the chromosomes, together with the X and Y sexual chromosomes (heterosomes) of maternal and paternal origin, and DNA of matrilinear origin present in the mitochondria. The most accredited phylogenetic trees of the Felidae family were obtained at the beginning of the 21st century by J.S., O’Brien, W.E Johnson, C.A. Driscoll, A.C. Kitchener, N. Yamaguchi, D.W. Macdonald et al.; more recently (2016) other phylogenetic trees have been proposed, with some variations in the chronology and the phylogenetic kinship, based on the latest research that
takes into account hypotheses of ancient hybridizations between related species (which are still happening today), these are illustrated in the elaborate graphs by G. Li, B. W. Davis, E. Eizirik, W.J. Murphy, et al. (see Chapter 2: Family Felidae).
Chapter 8.1.1 Feline genetics: Mendel’s laws Gregor Johann Mendel (1822 84), a biologist and mathematician from the Czech Republic, but German-speaking, is considered the precursor of modern genetics for his observations on hereditary traits (inheritance). He first introduced the innovative concept for the era of “hereditary,” stating that the basis of inheritance is specific agents obtained from the parents. Applying the laws of statistics and the calculation of probabilities, the study of biological inheritance came to formulate three laws, according to which the inherited characteristics are transmitted from one generation to another.
Black jaguar (Panthera onca), the melanistic form of jaguar. Photo by M. Korinek.
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Melanism in felines Melanism is a mutation of the characteristic that regulates the color of the hair or skin, and which can appear in many species of felines. This mutation is given by an allele (but it seems four alleles are involved) that increases the amount of dark pigments called melanins (eumelanine) in the phenotype of the individuals affected, so that the specimens appears completely black, instead of, for example, spotted on a light background (in fact they are spotted on a dark background). In the leopard, melanism is a recessive allele with respect to the allele that makes the hair light and speckled, which instead is the dominant color. In addition to the leopard there are other 11 species of felids in which it is quite common to find melanic individuals (serval, guigna, African and Asian golden cat, Geoffroy’s cat, etc.).
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In some species such as the jaguar it is easy to find black specimens because, unlike in the leopard, melanism in these species is a dominant character. In others, like the jaguarundi, melanism involves the MC1R allele (called the melanocortin receptor). Depending on the molecules that bind to this allele, there are different pigment products: black eumelanine or reddish phaeomelanine. For this reason, in the jaguarundi the color of the hair is variable with specimens that can be black, brown, or red; individuals heterozygous for MC1R are almost black, homozygous individuals are fawn-reddish. Based on the pigments that MC1R can produce, there is a great variety of colorations within the same species ranging from black to brown grizzled agouti, to tawnyreddish.
Jaguarundi furs of natural different colors. https://upload.wikimedia.org/wikipedia/commons/b/b8/Felis_%28herpailurus%29_yaguarorundi_% 28Jaguarondi%29_fur_skin_2.jpg
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Color variations in the fur of the jaguarundi: from black to red depending on the types of allele and whether it is homozygous or heterozygous. https:// https:// www.shutterstock.com/it/image-photo/jaguarundi-puma-yagouaroundi-resting-on-stump-298592792?src 5 SLXvS62sEdz31w6YPOp5Iw-1-6; www.shutterstock.com/it/image-photo/jaguarundi-wild-1212931666?src 5 SLXvS62sEdz31w6YPOp5Iw-1-3
Melanism in the leopard (Mendel’s first and second laws) The mutation of melanism causing black fur is particularly common in the Asian leopard, among which we find the so-called black panthers, which are very common especially on Java island, where the local leopard subspecies (Panthera pardus melas), isolated on the island for about 800,000 years, has a majority affected by melanism. Melanism is less common, but is also present, in leopards
Genotype 1
1 Homozygote spotted
in the African and Indonesian forests. In the forest areas this mutation is maintained as it is cryptic in the depths of vegetation, whereas in the more luminous open habitats black leopards are very visible and therefore more prone to natural selection. Since the color black is a recessive character, compared to the color of fur we can have only two leopard phenotypes: spotted or black, but belonging to three different genotypes.
Genotype 2
2
Heterozygote spotted/black
Spot./Spot.
Spot./black
< ____________________________________________> Spotted phenotype
Genotype 3
3 Homozygote black
Black./black
< ____________________> Black phenotype
(1) Spotted phenotype with pure homozygote genotype with two equal Spotted-Spotted alleles (Spot./Spot.). Both dominant. (2) Spotted phenotype with heterozygote genotype (Spot./black), where there are two different alleles: one dominant Spotted (Spot.) and one, melanic black (black.), recessive. (3) Black phenotype (black panther). This phenotype is possible only if the leopard has a homozygote genotype with two recessive alleles that are both black melanistic (black./black.).
For Mendel’s first law of or law of dominance (or law of homogeneity of phenotype), specimens born from the intersection of two homozygous parents, who differ for an allelic pair, will have the phenotype determined by the dominant allele. With a broader meaning than Mendel’s work, it can be enunciated as the law of “uniformity of first generation hybrids” (all of which are equal to each other and to the parent with dominant alleles). If we cross two individuals homozygous for dominant (spotted) and for a recessive (black melanic) character in the first generation we will have 100% spotted leopards but bearers of the black recessive allele.
Mendel’s second law is important because for the first time the hereditary traits were mentioned and it was noted that each gene carries two alleles. For Mendel’s second law or the law of segregation, alleles are separated at the moment of the formation of gametes. With reproduction female and male gametes with alleles equal or different come together to form homozygous (with alleles alike) or heterozygous genes (with different alleles) with this probability: 25% of individuals homozygous for the dominant allele; 50% of individuals heterozygous for both alleles and with a phenotype similar to the dominant homozygote; 25% of individuals homozygous for the recessive allele. Therefore if you cross two maculated leopards but both heterozygous and carriers of the black allele (Spot./black) they will have: 25% probability of generating homozygous spotted offspring (Spot./Spot.); 50% probability of generating heterozygous spotted offspring (Spot./black); 25% chance of generating offspring with homozygous black fur (black/black).
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Melanism in the Jaguar Melanism in the jaguar follows the same laws of Mendel’s inheritance, but in contrast to the leopard, melanism in the jaguar is a dominant character,
whereby only the homozygous specimens carrying both the “spot” alleles 5 light-spotted hair (spot./spot.) have spotted fur, while those carrying the “Black” allele 5 black hair (spot./Black or Black/Black) have black fur.
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Chapter 8.1.2 Genetics of some colors of the domestic cat All species of domestic farm animals (cattle, sheep, goats, pigs, horses) have in their chromosomes the genes that determine the coloration typical of the wild species from which they come, and this coloring is a dominant characteristic (sometimes with more alleles). If a group of domestic animals is returned to the wild, after some generations, the specimens no longer subject to human selection but to that of nature, will present a phenotype very similar to that of the original wild species from which they derive. Even among dogs it is so; wild dogs tend to resume the phenotype of the Wolf. The gray coloration with dark vertical stripes of the domestic cat is a characteristic inherited from the genes of the wild species (Felis libyca libyca) from which the tabby is derived and which in nature has this color; all wild cats after generations in the wild resume this coloring.
The striped gray coat of the domestic cat (mackerel tabby) is a dominant character and is due to an allele called Mackerel tabby pattern (TaM) and in the past all cats had this quality, as can be seen from paintings by the ancient Egyptians and from the mosaics found in the cities of the ancient Roman Empire and also later. It is not until the Middle Ages that we see the first representations of blotched cats (recessive allele blotched tabby pattern Tab), and of various colors, due, after centuries of “domestication” to natural alleles that have remained latent or always in the recessive phase, due to rare mutations arising accidentally, or to characteristics obtained with crossings specifically selected with mutant specimens. In doing so, coupling a gray Mackerel tabby pattern (TaM) cat with a blotched recessive allele blotched tabby pattern Tab cat, we will have generations that follow the Mendel laws.
A cat (female) with the typical fur with black and red spots, called tortoiseshell. https://upload.wikimedia.org/wikipedia/commons/c/c5/Short-haired_tortoiseshell_cat.jpg
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Tortoiseshell fur in cats is a mixture of black and red fur spots. Alleles that give fur of these colors have the same opportunity to reveal themselves in the phenotype (codominant alleles). Not having color alleles on the sexual chromosome Y, the male (XY) may have only red (R) or black fur (B).
The females, having two X chromosomes, both with the possibility of having red and black fur alleles, have three opportunities to have the genotype RR (fur red), BB (fur black), or RB (fur tortoiseshell). Therefore all cats with tortoiseshell fur can only be females.
The table shows the possible genotypes in female and male domestic cats with red and black fur alleles; alleles are taken from sex chromosome X but not from sex chromosome Y. Tortoiseshell is present only with the RB genome, therefore it is impossible in males (only R- or only B-).
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Spontaneous mutations in the color of the hair: albinism Albinism is not one of the traits that appears on a gene and determines hair color, but is a random and spontaneous mutation (a type of error) that can sometimes appear in genes and that inhibits the formation of any colored pigment, by which specimens affected by complete
albinism are pure white. Even the irises of the eyes lack the pigment that makes them colored and therefore they are transparent, meaning they appear reddish due to blood vessels. Often those animals with albinism also have problems with “photophobia” in daytime vision and can find daylight uncomfortable.
Withe tigers cub and adult; they are blue eyes and black stripes. The light color of these tigers is not due to the genetic mutation called “albinism” but to a recessive gene called “leucism” that inhibits the formation of the yellow-orange pigment that gives the background color to tiger fur. The white tigers keep the dark stripes due to a pigment called melanin the formation of which is not inhibited by leucism; they also have eyes with blue-colored irises. Normally tigers have green eyes because the green color is created by yellow and blue pigments, however, not having yellow pigments due to leucism, white tigers have blue eyes (left photo). The leucism gene is present in the genome of tigers but is rare in nature. Among the tigers it appears only in the Indian royal tiger or Bengal tiger (Panthera tigris tigris) and particularly in the Indian district of “Rewa,” where they have been selected and bred by a local maharaja since 1948 after the capture of a wild specimen. However, there are also almost white tigers that present with very few dark stripes only on the head and tail but these are not affected by true total albinism. (Right) Photo by V. Martegani.
The white lions of Timbavati and Kruger Occasionally there is a white lion in the wild, in some South Africa reserves, particularly in the Timbavati Nature Reserve and in the Kruger National Park. This coloring penalizes lions in the wild because they are easily sighted by their prey, which makes hunting more difficult. A white lion is therefore often condemned to
death from starvation. These lions are not completely white but blond; the cause is to be found in a gene called chinchilla or color inhibitor, and it is also a form of leucism that inhibits the formation of the yellow-red pigment. Being a recessive gene only homozygous lions for this gene will be present with light blond, almost white, hair.
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An almost white lioness (pale fawn) with her three white cubs. Photo by K. Rudlof.
Chapter 8.1.3 Crossing between the species of great felines (Pantherinae) The fact that we can cross species of different felids and obtain partially fertile hybrids demonstrates the low genetic variability within this family of closely related species. Many zoological parks have attempted to cross the various species of big cats: tigers, lions, leopards, and
jaguars, especially. These crossings have proved to be possible because all these species have great genetic affinity, all belonging to the genus Panthera, and having the same number and type of chromosomes. Normally, however, the results of these crossings present sterile male specimens, while the females are often fertile. Naturally, this type of interbreeding is not found in the wild.
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(Left) A male liger specimen in a zoo. This is an interbreeding (crossbreeding) between a male lion and a female tiger. (Right) Another liger born from interbreeding between a lion and a Siberian tiger at the “Siberian park Harbin” in China. https://www.shutterstock. com/it/image-photo/white-liger-walk-zoo-aviary-ligr-1008744487?src 5 -h_eHABB_fgCzqS0dsKzmQ-1-0 https://www.shutterstock.com/it/image-photo/ liger-siberian-tiger-park-harbin-china-521992762?src 5 Ty8Ak3A-JpuJpna3n4h4Lw-1-1
A taxidermized specimen of a leopon, a crossbreeding between a lioness and a leopard displayed in a natural history museum. https://upload.wikimedia.org/wikipedia/commons/d/d5/Leopon_leokichi.JPG