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Eukaryotes, Origin of
1428 Evolutionary Ecology | Eukaryotes, Origin of
Eukaryotes, Origin of B DeRennaux, University of Tennessee, Knoxville, TN, USA ª 2008 Elsevier B.V. All rights reserved.
Introduction Origin of Eukaryotes
Endosymbionts Further Reading
Introduction
in low abundance before the rise in oxygen while others believe they originated shortly after. Scientists do agree however that the rise in oxygen is likely responsible for their increase in abundance and rise to multicellularity. It is important to note that much of the theories regarding the origins of eukaryotes are speculative and it is unlikely that any one hypothesis will ever be championed above all others. However, as we sequence more genomes (particularly those of proposed early eukaryotes, and complete genome sequences of organisms from all domains of life) and develop more sophisticated phylogenetic techniques, it will be possible to narrow the likely possibilities.
Eukaryotes are cells commonly identified by the presence of a nucleus. Eukaryota is one of the three domains of life (bacteria and archea are the others and collectively referred to as prokaryotes) and encompasses single-celled organisms as well as all multicellular life. The evolution of eukaryotes is rather unique because it is substantially non-Darwinian; where Darwinian evolution largely concerns itself with beneficial genes arising by mutation and then spreading through differential reproductive success, the story of eukaryotes is one of significant horizontal gene flow (the exchange of genes across species) and of distinct organisms fusing into entirely new organisms. Eukaryotes contain a variety of structures that distinguish them from prokaryotes (archea and bacteria), most notably the nucleus and a host of organelles (endoplasmic reticulum, Golgi bodies, peroxisomes, mitochondria, chloroplasts, etc.). They also possess flagella of unique structure (containing microtubules) and cytoskeletal structures made of microtubules and microfilaments (neither of which is found in prokaryotes). Eukaryotes also often organize their DNA around histones (proteins that act like spools) and package it into chromosomes. Some eukaryotes also participate in some form of sexual reproduction though many retain the ability to reproduce asexually. Timing of the emergence of earliest eukaryotes is greatly debated. The first evidence of eukaryotes puts their origin at about 2.3–1.6 billion years ago. This range is derived primarily from phylogenetic analyses. Multicellular algae fossils are dated at 1.2 billion years, thus providing an absolute lower bound while fossils of single-celled organisms resembling algae are dated to around 2.2 billion years old, though scientists debate whether these represent eukaryotes or are merely similar in appearance. Most calculations place the oldest eukaryotes close to the first global glaciation approximately 2.3 billion years ago. It is believed that a rise in the number in photosynthetic prokaryotes led to an increase in oxygen, which resulted in global cooling. This increase in oxygen is believed to have decimated populations of anaerobic prokaryotes. Some scientists believe that eukaryotes existed
Eukaryotes as Chimera Eukaryotes are chimeric in that their DNA is the fusion of more than one genome. Phylogenetic analyses of eukaryotic genes result in many conflicting results, some eukaryotic genes appear most recently derived from archea, while others from bacteria. If these conflicts were infrequent, horizontal gene flow could be an adequate explanation. However, these conflicts are frequent and appear to fall into distinct categories. Genes involved in the maintenance and manipulation of the genetic code appear to be most recently derived from archea, whereas genes that are involved in metabolic processes are most similar to those of bacteria. Thus, it appears that at least two organisms are responsible for the major contributions to the eukaryotic genome and thus any theory on the evolution of eukaryotes must be one that can account for this pattern. Curiously, genes believed to be of bacterial origin are sometimes grouped most closely with Gram-negative bacteria (like Rickettsia), while others are grouped most closely with Gram-positive bacteria (suggesting at least three major contributors). Debate exists however concerning whether these two groups are each monophyletic (consisting of a common ancestor and its descendents). The Archezoa were (as many of these organisms are now being placed elsewhere) a group containing various eukaryotes that lacked mitochondria (amitochondriates) and often lacked or possessed ‘primitive’ versions of some eukaryotic traits. They are largely parasitic, inhabiting
Evolutionary Ecology | Eukaryotes, Origin of
low oxygen environments, and were believed to descend from an ancient eukaryote before the acquisition of mitochondria. Recent phylogenetic analyses however have shown that many of these species are relatively recent with highly simplified structures resulting from their parasitism and anoxic environments. Structures similar to mitochondria have been found in many (though they may lack DNA) as well as nuclear genes normally associated with mitochondrial DNA. Before the demise of Archezoa scientists had a rather gradual interpretation of eukaryotic evolution from simple cells to more complicated eukaryotes gradually with eukaryotic lineages diverging along the way acting as living fossils of earlier versions. With recent discoveries however, scientists are now of the opinion that most of the major features of eukaryotes (nucleus, cytoskeleton, ability to endocytose, and mitochondria) evolved very rapidly. It is now believed that if any early diverging lineages existed, they are now extinct and did not contribute to modern lineages. No matter the theories concerning the origin of eukaryotes, there are hypotheses that can account for their chimeric nature. Woese has proposed that early in the evolution of eukaryotes horizontal gene flow was much more common, and as genomes became more complex and integrated these rates decreased. Doolittle posits that the advent of the cytoskeleton and ability to phagocytoze gave eukaryotes the potential to incorporate DNA from ingested organisms. Over time, numerous organisms contributed to the genome of early eukaryotes. While it is difficult to test either of these hypotheses, if future comprehensive phylogenetic analyses across a great many organisms point to ‘few’ donors for the origin of eukaryotes, then these ideas would be disproved.
Origin of Eukaryotes An important feature of eukaryotes when discussing their evolution is that they are believed to have originally lacked cell walls or a rigid membrane as found in most prokaryotes. These early eukaryotes would have needed some kind of cytoskeleton to maintain integrity and shape. Cytoskeletal elements are also involved in eukaryotic reproduction, mobility, and the ability to endocytoze. Curiously, no solid evidence exists for the evolution of the cytoskeleton from some prokaryotic precursor (though actin and tubulin have weak similarity to archeon proteins). Thus, it seems likely that the cytoskeleton evolved largely de novo within the early eukaryote and also that many of the structural and functional characteristics of modern eukaryotes were possibly present in some of the earliest eukaryotes. Reasons for the loss of a cell wall are unclear though it may have been in response to bacteria-secreted antibiotics that often target cell walls.
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Probably the simplest theory for the origin of eukaryotes is that the proto-eukaryote diverged from an archeon and that it later derived its bacterial characteristics from its proto-mitochondrial symbiote; however, another popular theory holds that a bacteria and archea genomes fused prior to the acquisition of the mitochondrion ancestor. In these scenarios it is unclear whether the early eukaryote possessed a nucleus before the mitochondrial event or whether it developed afterwards. Some hypothesize that the nucleus must have been in place prior to prevent harmful genetic interactions with the proto-mitochondria genome, others that the nucleus evolved in response to the acquisition. Other theories hold that the nucleus is the result of a host bacteria fusing with an archeon or virus (something resembling mimiviruses which have rather large genomes and are less dependent on their hosts cellular machinery; referred to as viral eukaryogenesis). In some versions the bacterial host is RNA-based and the genes controlling the manipulation of genetic code were replaced by those of the DNA-based archeon or virus, leaving the other cellular machinery bacterial. Figure 1 shows the four major competing theories for the origin of early eukaryotes resulting from ambiguity of two major events. First, have they diverged from an archeon or are they the result of a fusion between an archeon and a bacterium, and second, did eukaryotes exist in an amitochondriate phase during their evolution? The ‘hydrogen hypothesis’ proposed by Martin and Muller holds that a methanogen archeon (which metabolizes hydrogen and carbon dioxide and releases methane) endocytozed the future mitochondria and that it provided the host cell with hydrogen and carbon dioxide as by-products of anaerobic respiration. This idea was inspired by hydrogenosomes which are simplified mitochondria often found in anaerobic eukaryotes and are believed to have evolved independently multiple times. Lynn Margulis has proposed that eukaryotes originated when an archeon developed an ectosymbiotic relationship with a spirochete (a types of bacteria) and eventually their genomes fused; this places an exogenous origin for the flagellum and occurs before acquiring a bacterial endosymbiont. This idea was inspired when observing that some eukaryotes rely on symbiotic spirochetes for their motility (parabasalids within the termite gut). This theory has many of bacteria-like genes originating from the spirochete and not the later mitochondria. However given that there is little similarity between spirochetes and eukaryotic flagella, this theory is currently rather unpopular.
Endosymbionts Two of the more observable organelles in eukaryotes are mitochondria and plastids (plastids which contain
1430 Evolutionary Ecology | Eukaryotes, Origin of (a)
(b)
M EK
E
C
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N
N
AR
MAN AZ
FLA
E
C
N
N
CH
AR
AR
CH
BA
AR
a
FLE
CH
f
AR
a
BA
f
Figure 1 The main competing theories of eukaryotic origin. Schematic diagrams describing the Archezoa (a) and anti-Archezoa (b) hypotheses, and their archaeal (a) and fusion (f) versions as envisioned from genomic and biochemical perspectives. AR, archaeon; BA, bacterium; CH, chimeric prokaryote; AZ, archezoon; EK, eukaryote; MAN, mitochondrial ancestor; FLA, free-living -proteobacterium; RLE, rickettsia-like endosymbiont; N, nucleus with multiple chromosomes; E, endomembrane system; C, cytoskeleton; M, mitochondria. From Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599–1618.
chlorophyll a and b are chloroplasts). Mitochondria are often referred to as cellular ‘power plants’ and are responsible for oxidative phosphorylation; this enables aerobic respiration (which yields 15 times as much ATP from glucose than does anaerobic respiration alone), while chloroplasts allow for photosynthesis. The acquisition of these organelles would clearly have been an energetic boon to the eukaryotes possessing them. Unlike theories on the origins of the eukaryotes, the ‘endosymbiotic theory’ has remained relatively unchanged for decades, has a host of strong evidence, and is almost universally accepted. The endosymbiotic theory dates back to 1883 when Andreas Schimper observed that chloroplasts divided similar to cyanobacteria (blue-green algae which are bacteria despite their common name). In 1905 Konstantin Mereschkowsky suggested that plastids were originally endosymbionts
(beneficial organisms living within another organism), and in the 1920s, Ivan Wallin suggested the same for mitochondria. These ideas were based on visual similarity when viewed under light microscopy and were largely ridiculed until the later half of the 1960s, after which Lynn Margulis helped popularized the ‘endosymbiotic theory’ in her book Origin of Eukaryotic Cells (1970). The ‘endosymbiotic theory’ states that mitochondria and plastids evolved from an engulfed bacteria and cyanobacteria, respectively. It is believed that a host cell engulfed an anaerobic bacterium (perhaps as prey or a parasite) and that over time a symbiosis arose, eventually becoming an obligate symbiosis (a mutually beneficial interaction required by both participants) before evolving into mitochondria. Later a mitochondria-harboring host engulfed a cyanobacterium, which similarly evolved into plastids. The mitochondrial event is believed to have
Evolutionary Ecology | Eukaryotes, Origin of
occurred near the origin of eukaryotes themselves, and the plastid event about a half billion years later. Evidence Much of the evidence for the endosymbiotic theory comes from the structure and handling of these organelles’ genetic codes. Both mitochondria and plastids have DNA sequences in circles as that of bacteria. Their DNA also lacks histones (proteins that the DNA is wrapped around) which are present in eukaryotes and some archea. In addition, mitochondria and plastid transcription begin with the amino acid fMet (formylmethionine) as in bacteria, not Met (methionine) as in eukaryotes. Ribosome sizes are questionable evidence for the endosymbiotic theory. Bacteria usually have ribosomes of 70s (Svedberg units) and eukaryotes usually have around 80s in their cytoplasm. While the mitochondrial and plastid ribosomes are usually of around 70s, they do in fact vary among species from around 60s to 80s, thus overlapping both bacterial and cytoplasmic eukaryote ribosome sizes. Other evidence for the endosymbiotic theory comes from the two membranes usually surrounding these organelles. The inner membrane belongs to that of the original bacteria and outer membrane presumably a result from the original engulfment. The outer membrane has approximately a 1:1 protein–lipid ratio by dry weight, similar to many eukaryotic cytoplasmic membranes, while the inner membrane (which is made of two layers) has approximately a 3:1, similar to many bacteria. These organelles and bacteria also both utilize electron transport enzymes lacking elsewhere in eukaryotes. Some of the best evidence for the endosymbiotic theory however comes from bioinformatics. Phylogenetic analyses of various bacteria, mitochondria from various hosts from various kingdoms, and nuclear DNA from those hosts usually place mitochondria as most related to a group of bacteria known as proteobacteria, often placed closest to Rickettsia and other -proteobacteria. The -proteobacteria as a group are almost entirely symbiotic or parasitic which may have predisposed the mitochondrial ancestor to an existence within its host. Chloroplasts are most often placed next to cyanobacteria and both contain thylakoids and chlorophyll a; cyanobacteria are also involved in a number of symbioses including lichens and corals. Problems Some inconsistencies do exist between bacteria and plastids/mitochondria. Mitochondria possess the ability to fuse with other mitochondria (this is believed to be the result of simplification that has occurred within the mitochondrial membranes over time) and the genetic
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information within a mitochondrion or plastid also differs in some ways to that of bacteria. These organelles possess higher concentrations of introns (DNA that will be spliced out prior to translation) than bacteria and intron types not found in bacteria. The genomes of these organelles are also much smaller than those of bacteria. Gene flow between the nucleus and the organelles is believed to be responsible for the genetic discrepancies. Gene flow from the nucleus to an organelle first requires a copy of a segment of DNA from the organelle to insert into the nuclear DNA and gain expression. Loss of function can then occur in the original gene. A similar process can explain migration from the nucleus to the organelles. While organelle DNA fragments within nuclear DNA are rather common, nuclear DNA fragments within organelle DNA are rare (the exception being plant mitochondria which has experienced a rather large increase in genome size over time). Such genetic transfers operate with a frequency similar to that of mutation. The introns within the organelles and the increase in the plant mitochondria genome over time are thought to be due to a transfer from the nucleus to the organelles. The size discrepancy between the genome sizes of chloroplasts and mitochondria is believed to be a result of selection for genes originally possessed by the organelles to reside in the nucleus. Selection for the migration of genes to the nucleus comes in many possible forms. First, it is believed to be physically more difficult for DNA to enter these organelles as compared to the nucleus. Population genetics also tells us that sexual reproduction (which many eukaryotes posses) enables the more rapid spread of advantageous genes, and overcomes Muller’s ratchet (the idea that asexual reproduction lacks an effective means of ridding the genome of deleterious mutations that gradually build up). Also, chloroplasts and mitochondria perform redox (reduction–oxidation) reactions, which are prone to producing deleterious mutations. Finally, it has been proposed that smaller organelle genomes would replicate faster than those with redundant genes and thus be selected for. The reasons for maintenance of organelle DNA are not as clear. Some reasons might include, that the remaining genes code for proteins that cannot be easily transported into the organelle (possibly membranespanning or strongly hydrophobic proteins) or that these genes are harmful if expressed in the cytoplasm. Another potential reason may be the slight differences in organelle versus nuclear transcription/translation. Secondary Endocytosis of Plastids Green algae and plants are believed to be descendents of the original plastid-harboring eukaryote. However, some protists contain plastids that are believed to be from other
1432 Ecological Processes | Evapotranspiration
eukaryotes. These protists have within their endoplasmic reticulum an endosymbiont resembling a eukaryote. This endosymbiont possesses its own plasma membrane and ribosomes (which are both similar to those of eukaryotes) as well as the remnant of a nucleus containing its own DNA (a nucleomorph), and finally plastids. Thus, these organisms have four sets of DNA: mitochondrial, plastid, nuclear, and nucleomorph. Some protists may even contain plastids from tertiary endocytosis.
also the result of symbiosis (see the section entitled ‘Origin of eukaryotes’). However, this fell largely out of favor when flagella were found not to possess DNA, and subsequently that spirochetes are rather dissimilar to eukaryotic flagella. See also: Evolutionary Ecology: Overview; Fitness.
Further Reading Other Possible Symbiotic Events Christian de Duve proposed that peroxisomes (which contain enzymes for oxidative reactions) may also have originated as endosymbionts; however, because of their single layer membranes, lack of DNA, and the recent observation that peroxisomes can be formed from the endoplasmic reticulum, it is now generally accepted that they arose endogenously. Lynn Margulis proposed in 1967 that along with mitochondria and plastids, the eukaryotic flagella was
Brown JR and Doolittle WF (1997) Archea and the prokaryote-toeukaryote transition. Microbiology and Molecular Biology Reviews 61: 456–502. Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599–1618. Gupta RS and Golding GB (1996) The origin of the eukaryotic cell. Trends in Biochemical Sciences 21: 166–171. Katz LA (1998) Changing perspectives on the origin of eukaryotes. Trends in Ecology and Evolution 13: 493–497. Katz LA (1999) The tangled web: Gene genealogies and the origin of eukaryotes. American Naturalist 154: S137–S145. Roger AJ (1999) Reconstructing early events in eukaryotic evolution. American Naturalist 154: S146–S153.
Evapotranspiration S Irmak, University of Nebraska–Lincoln, Lincoln, NE, USA ª 2008 Elsevier B.V. All rights reserved.
Introduction The Hydrologic Cycle and ET ET Terminology
Crop Coefficient Concept Further Reading
Introduction
is irrigated and contributes more than one-third of the total world food production. In the United States, about 12% of the cropped area is irrigated and contributes about 25% of the total value of the United States crops. In the United States and around the world, irrigated agriculture uses most of the water withdrawals from the surface and groundwater supplies. Thus, accurate quantification of plant water use (evapotranspiration) is crucial for better management and allocation of water resources. The process known as evapotranspiration (ET) is of great importance in many disciplines. Accurate quantification of ET in agroecosystems is critical for better planning, managing, and efficient use of water resources, especially in arid or semiarid environments where lack of precipitation usually limits plant growth and yield and negatively affects ecological balances. Quantification of ET is also crucial in water allocation, irrigation management, evaluating the effects of changing land use on water
Water is one of the most important limited natural resources. Declining water resources and water quality problems have resulted in dramatic increase in the need for water-conserving methodologies on a field, watershed, and regional scale and this makes efficient use of freshwater resources an obligation of each user. During the 30-year period from 1950 to 1980, the actual level of per capita water supply decreased significantly in many countries due to population increases. It has been projected that in early year 2000 considerably low water availability per capita is anticipated in many regions of the world. As water becomes increasingly scarce and the need becomes more pressing, newer and more complete methods of measuring and evaluating techniques of handling water resources are necessary. In terms of agricultural production, approximately 17% of the cropped area of the world
Eukaryotes, Origin of B DeRennaux, University of Tennessee, Knoxville, TN, USA ª 2008 Elsevier B.V. All rights reserved.
Introduction Origin of Eukaryotes
Endosymbionts Further Reading
Introduction
in low abundance before the rise in oxygen while others believe they originated shortly after. Scientists do agree however that the rise in oxygen is likely responsible for their increase in abundance and rise to multicellularity. It is important to note that much of the theories regarding the origins of eukaryotes are speculative and it is unlikely that any one hypothesis will ever be championed above all others. However, as we sequence more genomes (particularly those of proposed early eukaryotes, and complete genome sequences of organisms from all domains of life) and develop more sophisticated phylogenetic techniques, it will be possible to narrow the likely possibilities.
Eukaryotes are cells commonly identified by the presence of a nucleus. Eukaryota is one of the three domains of life (bacteria and archea are the others and collectively referred to as prokaryotes) and encompasses single-celled organisms as well as all multicellular life. The evolution of eukaryotes is rather unique because it is substantially non-Darwinian; where Darwinian evolution largely concerns itself with beneficial genes arising by mutation and then spreading through differential reproductive success, the story of eukaryotes is one of significant horizontal gene flow (the exchange of genes across species) and of distinct organisms fusing into entirely new organisms. Eukaryotes contain a variety of structures that distinguish them from prokaryotes (archea and bacteria), most notably the nucleus and a host of organelles (endoplasmic reticulum, Golgi bodies, peroxisomes, mitochondria, chloroplasts, etc.). They also possess flagella of unique structure (containing microtubules) and cytoskeletal structures made of microtubules and microfilaments (neither of which is found in prokaryotes). Eukaryotes also often organize their DNA around histones (proteins that act like spools) and package it into chromosomes. Some eukaryotes also participate in some form of sexual reproduction though many retain the ability to reproduce asexually. Timing of the emergence of earliest eukaryotes is greatly debated. The first evidence of eukaryotes puts their origin at about 2.3–1.6 billion years ago. This range is derived primarily from phylogenetic analyses. Multicellular algae fossils are dated at 1.2 billion years, thus providing an absolute lower bound while fossils of single-celled organisms resembling algae are dated to around 2.2 billion years old, though scientists debate whether these represent eukaryotes or are merely similar in appearance. Most calculations place the oldest eukaryotes close to the first global glaciation approximately 2.3 billion years ago. It is believed that a rise in the number in photosynthetic prokaryotes led to an increase in oxygen, which resulted in global cooling. This increase in oxygen is believed to have decimated populations of anaerobic prokaryotes. Some scientists believe that eukaryotes existed
Eukaryotes as Chimera Eukaryotes are chimeric in that their DNA is the fusion of more than one genome. Phylogenetic analyses of eukaryotic genes result in many conflicting results, some eukaryotic genes appear most recently derived from archea, while others from bacteria. If these conflicts were infrequent, horizontal gene flow could be an adequate explanation. However, these conflicts are frequent and appear to fall into distinct categories. Genes involved in the maintenance and manipulation of the genetic code appear to be most recently derived from archea, whereas genes that are involved in metabolic processes are most similar to those of bacteria. Thus, it appears that at least two organisms are responsible for the major contributions to the eukaryotic genome and thus any theory on the evolution of eukaryotes must be one that can account for this pattern. Curiously, genes believed to be of bacterial origin are sometimes grouped most closely with Gram-negative bacteria (like Rickettsia), while others are grouped most closely with Gram-positive bacteria (suggesting at least three major contributors). Debate exists however concerning whether these two groups are each monophyletic (consisting of a common ancestor and its descendents). The Archezoa were (as many of these organisms are now being placed elsewhere) a group containing various eukaryotes that lacked mitochondria (amitochondriates) and often lacked or possessed ‘primitive’ versions of some eukaryotic traits. They are largely parasitic, inhabiting
Evolutionary Ecology | Eukaryotes, Origin of
low oxygen environments, and were believed to descend from an ancient eukaryote before the acquisition of mitochondria. Recent phylogenetic analyses however have shown that many of these species are relatively recent with highly simplified structures resulting from their parasitism and anoxic environments. Structures similar to mitochondria have been found in many (though they may lack DNA) as well as nuclear genes normally associated with mitochondrial DNA. Before the demise of Archezoa scientists had a rather gradual interpretation of eukaryotic evolution from simple cells to more complicated eukaryotes gradually with eukaryotic lineages diverging along the way acting as living fossils of earlier versions. With recent discoveries however, scientists are now of the opinion that most of the major features of eukaryotes (nucleus, cytoskeleton, ability to endocytose, and mitochondria) evolved very rapidly. It is now believed that if any early diverging lineages existed, they are now extinct and did not contribute to modern lineages. No matter the theories concerning the origin of eukaryotes, there are hypotheses that can account for their chimeric nature. Woese has proposed that early in the evolution of eukaryotes horizontal gene flow was much more common, and as genomes became more complex and integrated these rates decreased. Doolittle posits that the advent of the cytoskeleton and ability to phagocytoze gave eukaryotes the potential to incorporate DNA from ingested organisms. Over time, numerous organisms contributed to the genome of early eukaryotes. While it is difficult to test either of these hypotheses, if future comprehensive phylogenetic analyses across a great many organisms point to ‘few’ donors for the origin of eukaryotes, then these ideas would be disproved.
Origin of Eukaryotes An important feature of eukaryotes when discussing their evolution is that they are believed to have originally lacked cell walls or a rigid membrane as found in most prokaryotes. These early eukaryotes would have needed some kind of cytoskeleton to maintain integrity and shape. Cytoskeletal elements are also involved in eukaryotic reproduction, mobility, and the ability to endocytoze. Curiously, no solid evidence exists for the evolution of the cytoskeleton from some prokaryotic precursor (though actin and tubulin have weak similarity to archeon proteins). Thus, it seems likely that the cytoskeleton evolved largely de novo within the early eukaryote and also that many of the structural and functional characteristics of modern eukaryotes were possibly present in some of the earliest eukaryotes. Reasons for the loss of a cell wall are unclear though it may have been in response to bacteria-secreted antibiotics that often target cell walls.
1429
Probably the simplest theory for the origin of eukaryotes is that the proto-eukaryote diverged from an archeon and that it later derived its bacterial characteristics from its proto-mitochondrial symbiote; however, another popular theory holds that a bacteria and archea genomes fused prior to the acquisition of the mitochondrion ancestor. In these scenarios it is unclear whether the early eukaryote possessed a nucleus before the mitochondrial event or whether it developed afterwards. Some hypothesize that the nucleus must have been in place prior to prevent harmful genetic interactions with the proto-mitochondria genome, others that the nucleus evolved in response to the acquisition. Other theories hold that the nucleus is the result of a host bacteria fusing with an archeon or virus (something resembling mimiviruses which have rather large genomes and are less dependent on their hosts cellular machinery; referred to as viral eukaryogenesis). In some versions the bacterial host is RNA-based and the genes controlling the manipulation of genetic code were replaced by those of the DNA-based archeon or virus, leaving the other cellular machinery bacterial. Figure 1 shows the four major competing theories for the origin of early eukaryotes resulting from ambiguity of two major events. First, have they diverged from an archeon or are they the result of a fusion between an archeon and a bacterium, and second, did eukaryotes exist in an amitochondriate phase during their evolution? The ‘hydrogen hypothesis’ proposed by Martin and Muller holds that a methanogen archeon (which metabolizes hydrogen and carbon dioxide and releases methane) endocytozed the future mitochondria and that it provided the host cell with hydrogen and carbon dioxide as by-products of anaerobic respiration. This idea was inspired by hydrogenosomes which are simplified mitochondria often found in anaerobic eukaryotes and are believed to have evolved independently multiple times. Lynn Margulis has proposed that eukaryotes originated when an archeon developed an ectosymbiotic relationship with a spirochete (a types of bacteria) and eventually their genomes fused; this places an exogenous origin for the flagellum and occurs before acquiring a bacterial endosymbiont. This idea was inspired when observing that some eukaryotes rely on symbiotic spirochetes for their motility (parabasalids within the termite gut). This theory has many of bacteria-like genes originating from the spirochete and not the later mitochondria. However given that there is little similarity between spirochetes and eukaryotic flagella, this theory is currently rather unpopular.
Endosymbionts Two of the more observable organelles in eukaryotes are mitochondria and plastids (plastids which contain
1430 Evolutionary Ecology | Eukaryotes, Origin of (a)
(b)
M EK
E
C
N
N
N
AR
MAN AZ
FLA
E
C
N
N
CH
AR
AR
CH
BA
AR
a
FLE
CH
f
AR
a
BA
f
Figure 1 The main competing theories of eukaryotic origin. Schematic diagrams describing the Archezoa (a) and anti-Archezoa (b) hypotheses, and their archaeal (a) and fusion (f) versions as envisioned from genomic and biochemical perspectives. AR, archaeon; BA, bacterium; CH, chimeric prokaryote; AZ, archezoon; EK, eukaryote; MAN, mitochondrial ancestor; FLA, free-living -proteobacterium; RLE, rickettsia-like endosymbiont; N, nucleus with multiple chromosomes; E, endomembrane system; C, cytoskeleton; M, mitochondria. From Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599–1618.
chlorophyll a and b are chloroplasts). Mitochondria are often referred to as cellular ‘power plants’ and are responsible for oxidative phosphorylation; this enables aerobic respiration (which yields 15 times as much ATP from glucose than does anaerobic respiration alone), while chloroplasts allow for photosynthesis. The acquisition of these organelles would clearly have been an energetic boon to the eukaryotes possessing them. Unlike theories on the origins of the eukaryotes, the ‘endosymbiotic theory’ has remained relatively unchanged for decades, has a host of strong evidence, and is almost universally accepted. The endosymbiotic theory dates back to 1883 when Andreas Schimper observed that chloroplasts divided similar to cyanobacteria (blue-green algae which are bacteria despite their common name). In 1905 Konstantin Mereschkowsky suggested that plastids were originally endosymbionts
(beneficial organisms living within another organism), and in the 1920s, Ivan Wallin suggested the same for mitochondria. These ideas were based on visual similarity when viewed under light microscopy and were largely ridiculed until the later half of the 1960s, after which Lynn Margulis helped popularized the ‘endosymbiotic theory’ in her book Origin of Eukaryotic Cells (1970). The ‘endosymbiotic theory’ states that mitochondria and plastids evolved from an engulfed bacteria and cyanobacteria, respectively. It is believed that a host cell engulfed an anaerobic bacterium (perhaps as prey or a parasite) and that over time a symbiosis arose, eventually becoming an obligate symbiosis (a mutually beneficial interaction required by both participants) before evolving into mitochondria. Later a mitochondria-harboring host engulfed a cyanobacterium, which similarly evolved into plastids. The mitochondrial event is believed to have
Evolutionary Ecology | Eukaryotes, Origin of
occurred near the origin of eukaryotes themselves, and the plastid event about a half billion years later. Evidence Much of the evidence for the endosymbiotic theory comes from the structure and handling of these organelles’ genetic codes. Both mitochondria and plastids have DNA sequences in circles as that of bacteria. Their DNA also lacks histones (proteins that the DNA is wrapped around) which are present in eukaryotes and some archea. In addition, mitochondria and plastid transcription begin with the amino acid fMet (formylmethionine) as in bacteria, not Met (methionine) as in eukaryotes. Ribosome sizes are questionable evidence for the endosymbiotic theory. Bacteria usually have ribosomes of 70s (Svedberg units) and eukaryotes usually have around 80s in their cytoplasm. While the mitochondrial and plastid ribosomes are usually of around 70s, they do in fact vary among species from around 60s to 80s, thus overlapping both bacterial and cytoplasmic eukaryote ribosome sizes. Other evidence for the endosymbiotic theory comes from the two membranes usually surrounding these organelles. The inner membrane belongs to that of the original bacteria and outer membrane presumably a result from the original engulfment. The outer membrane has approximately a 1:1 protein–lipid ratio by dry weight, similar to many eukaryotic cytoplasmic membranes, while the inner membrane (which is made of two layers) has approximately a 3:1, similar to many bacteria. These organelles and bacteria also both utilize electron transport enzymes lacking elsewhere in eukaryotes. Some of the best evidence for the endosymbiotic theory however comes from bioinformatics. Phylogenetic analyses of various bacteria, mitochondria from various hosts from various kingdoms, and nuclear DNA from those hosts usually place mitochondria as most related to a group of bacteria known as proteobacteria, often placed closest to Rickettsia and other -proteobacteria. The -proteobacteria as a group are almost entirely symbiotic or parasitic which may have predisposed the mitochondrial ancestor to an existence within its host. Chloroplasts are most often placed next to cyanobacteria and both contain thylakoids and chlorophyll a; cyanobacteria are also involved in a number of symbioses including lichens and corals. Problems Some inconsistencies do exist between bacteria and plastids/mitochondria. Mitochondria possess the ability to fuse with other mitochondria (this is believed to be the result of simplification that has occurred within the mitochondrial membranes over time) and the genetic
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information within a mitochondrion or plastid also differs in some ways to that of bacteria. These organelles possess higher concentrations of introns (DNA that will be spliced out prior to translation) than bacteria and intron types not found in bacteria. The genomes of these organelles are also much smaller than those of bacteria. Gene flow between the nucleus and the organelles is believed to be responsible for the genetic discrepancies. Gene flow from the nucleus to an organelle first requires a copy of a segment of DNA from the organelle to insert into the nuclear DNA and gain expression. Loss of function can then occur in the original gene. A similar process can explain migration from the nucleus to the organelles. While organelle DNA fragments within nuclear DNA are rather common, nuclear DNA fragments within organelle DNA are rare (the exception being plant mitochondria which has experienced a rather large increase in genome size over time). Such genetic transfers operate with a frequency similar to that of mutation. The introns within the organelles and the increase in the plant mitochondria genome over time are thought to be due to a transfer from the nucleus to the organelles. The size discrepancy between the genome sizes of chloroplasts and mitochondria is believed to be a result of selection for genes originally possessed by the organelles to reside in the nucleus. Selection for the migration of genes to the nucleus comes in many possible forms. First, it is believed to be physically more difficult for DNA to enter these organelles as compared to the nucleus. Population genetics also tells us that sexual reproduction (which many eukaryotes posses) enables the more rapid spread of advantageous genes, and overcomes Muller’s ratchet (the idea that asexual reproduction lacks an effective means of ridding the genome of deleterious mutations that gradually build up). Also, chloroplasts and mitochondria perform redox (reduction–oxidation) reactions, which are prone to producing deleterious mutations. Finally, it has been proposed that smaller organelle genomes would replicate faster than those with redundant genes and thus be selected for. The reasons for maintenance of organelle DNA are not as clear. Some reasons might include, that the remaining genes code for proteins that cannot be easily transported into the organelle (possibly membranespanning or strongly hydrophobic proteins) or that these genes are harmful if expressed in the cytoplasm. Another potential reason may be the slight differences in organelle versus nuclear transcription/translation. Secondary Endocytosis of Plastids Green algae and plants are believed to be descendents of the original plastid-harboring eukaryote. However, some protists contain plastids that are believed to be from other
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eukaryotes. These protists have within their endoplasmic reticulum an endosymbiont resembling a eukaryote. This endosymbiont possesses its own plasma membrane and ribosomes (which are both similar to those of eukaryotes) as well as the remnant of a nucleus containing its own DNA (a nucleomorph), and finally plastids. Thus, these organisms have four sets of DNA: mitochondrial, plastid, nuclear, and nucleomorph. Some protists may even contain plastids from tertiary endocytosis.
also the result of symbiosis (see the section entitled ‘Origin of eukaryotes’). However, this fell largely out of favor when flagella were found not to possess DNA, and subsequently that spirochetes are rather dissimilar to eukaryotic flagella. See also: Evolutionary Ecology: Overview; Fitness.
Further Reading Other Possible Symbiotic Events Christian de Duve proposed that peroxisomes (which contain enzymes for oxidative reactions) may also have originated as endosymbionts; however, because of their single layer membranes, lack of DNA, and the recent observation that peroxisomes can be formed from the endoplasmic reticulum, it is now generally accepted that they arose endogenously. Lynn Margulis proposed in 1967 that along with mitochondria and plastids, the eukaryotic flagella was
Brown JR and Doolittle WF (1997) Archea and the prokaryote-toeukaryote transition. Microbiology and Molecular Biology Reviews 61: 456–502. Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell. European Journal of Biochemistry 270(8): 1599–1618. Gupta RS and Golding GB (1996) The origin of the eukaryotic cell. Trends in Biochemical Sciences 21: 166–171. Katz LA (1998) Changing perspectives on the origin of eukaryotes. Trends in Ecology and Evolution 13: 493–497. Katz LA (1999) The tangled web: Gene genealogies and the origin of eukaryotes. American Naturalist 154: S137–S145. Roger AJ (1999) Reconstructing early events in eukaryotic evolution. American Naturalist 154: S146–S153.
Evapotranspiration S Irmak, University of Nebraska–Lincoln, Lincoln, NE, USA ª 2008 Elsevier B.V. All rights reserved.
Introduction The Hydrologic Cycle and ET ET Terminology
Crop Coefficient Concept Further Reading
Introduction
is irrigated and contributes more than one-third of the total world food production. In the United States, about 12% of the cropped area is irrigated and contributes about 25% of the total value of the United States crops. In the United States and around the world, irrigated agriculture uses most of the water withdrawals from the surface and groundwater supplies. Thus, accurate quantification of plant water use (evapotranspiration) is crucial for better management and allocation of water resources. The process known as evapotranspiration (ET) is of great importance in many disciplines. Accurate quantification of ET in agroecosystems is critical for better planning, managing, and efficient use of water resources, especially in arid or semiarid environments where lack of precipitation usually limits plant growth and yield and negatively affects ecological balances. Quantification of ET is also crucial in water allocation, irrigation management, evaluating the effects of changing land use on water
Water is one of the most important limited natural resources. Declining water resources and water quality problems have resulted in dramatic increase in the need for water-conserving methodologies on a field, watershed, and regional scale and this makes efficient use of freshwater resources an obligation of each user. During the 30-year period from 1950 to 1980, the actual level of per capita water supply decreased significantly in many countries due to population increases. It has been projected that in early year 2000 considerably low water availability per capita is anticipated in many regions of the world. As water becomes increasingly scarce and the need becomes more pressing, newer and more complete methods of measuring and evaluating techniques of handling water resources are necessary. In terms of agricultural production, approximately 17% of the cropped area of the world