A hypothesis on the action of chromosomal genes

J. theor. Biol. (1984) 108, 645-661

A Hypothesis on the Action of Chromosomal Genes LI ZHEN-GANG AND WU QIU-YING

Department of Biology, University of Science and Technology of China, Hefei, Anhui, Peoples Republic of China (Received 5 May 1983 and in final form 9 January 1984) According to the analysis of the chromosomal genes which act in development, we advance a model of gene action in development. In this model, the development is regarded as the programming action of different gene groups. The action of each and every gene group will activate the next gene group and make a self-inhibition. Therefore, the activation and inhibition of genes is a chain reaction during development; it is unlikely that the action of genes is entirely from inhibition to respective activation (Morgan model, 1934) or entirely from activation to respective inhibition (Caplan-Ordhai model, 1978). We use this model to explain some biological problems, such as development and differentiation, senescence and the cancer. According to our model, development and differentiation are not the same thing, senescence is the self-inhibition of the last gene group in somatic cells and the oncogene is the connexion gene in the programming action of gene groups.

Introduction I f we regar d c h r o m o s o m a l genes as a blueprint o f development, h o w strange a blueprint it is! According to this blueprint, d e v e l o p m e n t is not merely a building process o f adult characters, but passes alternately through a series of construction and demolishing processes, until finally the adult b o d y is completed. For example, in an animal that m e t a m o r p h o s e s the larval organs and tissues will collapse completely and the adult b o d y is reconstructed on the ruins o f larva. Even in the d e v e l o p m e n t of an animal that does not m e t a m o r p h o s e , there are m a n y characters which have nothing to do with the adult body, such as the gill splits, tail and caecum in h u m a n embryo. This p h e n o m e n o n suggests that d e v e l o p m e n t should be a series o f gene groups which act sequentially, but independently, of each other. The c h r o m o s o m e s are a muster o f genes. We have only been concerned about the action of individual genes but have rarely thought o f the relation between the gene's action and chromosomes. In c h r o m o s o m e s , D N A content is affected by the presence in most genomes o f large quantities o f repetitive D N A which are not generally transcribed at all. In addition to this, in the 645 0022--5193/84/120645 + 17 $03.00/0

© 1984 Academic Press Inc. (London) Ltd.

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eukaryotes most genes contain introns which are transcribed, but the material of which is degraded from the messenger RNA precusor in the maturation of the message. On average, about 70% of DNA is in the portion of genome which codes for gene products, and of this 70%, 90% consists of introns, in which there are many repetitive sequences. If the gene lost its introns it would become a non-active gene, a pseudogene. A particularly interesting example is mouse o~-globin pseudogene, in which the two introns sequences have been removed precisely (Nishioka, Leder & Leder, 1980). Moreover, recent studies indicates that even satellite DNA can be transcribed (Diaz et al., 1981). Triturus histone gene clusters are interspersed among large blocks of a major satellite DNA (Stephenson, Erba & Gall, 1981). Transcription of the histone genes occurs on specific lampbrush loops, and involves the production of large RNAs containing both histone and satellite DNA sequences. This phenomenon may provide a model for the transcription of genes in heterochromatin. Therefore, we must not take the chromosomes only as a carrier of genes as they also have some functions of genetic control in development. However the universal occurrence and the gross interspersion pattern of repetitive sequences in chromosomes suggest that it is not of fundamental importance to processes controlling individual gene expression, but either the lampbrush loops or the globin gene clusters are transcribed and regulated within a particular developmental stage. Therefore, the repetitive sequences appear to play a role in the expression of gene groups during development.

Behavior of Chromosomal Genes in Development

There are some aspects of the behavior of chromosomal genes in development that enable us to draw some inferences on the action of chromosomal genes.

(A) H E T E R O C H R O M A T I Z A T I O N

Many researchers point out that the heterochromatization of chromosomes is a gradual process. The DNA adjacent to the centromere almost amounts to 5-10% of total chromosomal DNA. It is euchromatic early in development and becomes permanently heteromatic as soon as the cleavage divisions cease. Recently, recombinant methods have also been used to show that high repetitive DNA sequences are located in the long heteromorphic arm of Triturus chromosome 1 transcribed during the lampbrush stage of oogenesis (Varley, Macgregor & Erba 1980). Thus they may contain genes programmed to function in the early embryonic stage.

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In somatic cells of an adult, the Y chromosome is heterochromatic and unactive; but in spermatocytes it is active. For example, the Y chromosome of Drosophila hydei shows five different loops, each loop probably one fertility gene (Watson, 1976). Another form is the differential heterochromatization of homologous chromosomes. The most widely known example is the mammalian X chromosomes (Catanach, 1975). However, both X chromosomes are active throughout the cleavage stage. Hereafter, only one X chromosome is active in a female and the other is inactive by heterochromatization. However, a number of observations have indicated that even the heterochromatic X chromosome is not permanent or static. There are reactivation events that can occur throughout the development process. The first, in mammals is that although one X chromosome is heterochromatic in the female, it is not equal to the X0 female. It is common knowledge that an X0 woman is a Turner's syndrome patient and her development is abnormal in many aspects. Secondly, in some tissues, for example, in the muscle of Macropus parryi, the PGK (phosphoglycerate kinase) alleles are active in both X chromosomes (Vandenberg, Cooper & Sharman, 1973). In the case of G6PD (glucose-6-phosphate dehydrogenase), which in marsupials is probably a dimer or tetramer; In electropherogram, the hybrid band of the interaction product was seen (Cooper et al., 1975). All these strongly suggest that both alleles must be active in the same cell. The similar events are also well documented in the Coccids (Brown & Nur, 1964). In the cleavage stage the chromosomes of all embryonic cells appears to be euchromatic; thereafter at blastula half of the chromosomes become heterochromatized, in these embryos destined to become male. In the female embryo none of the chromosomes are heterochromatized; all of them remain in the euchromatic state. There is evidence that the male receives a set of chromosomes from his mother which remains euchromatic and genetically active and is transmitted in his sperm. The set which he receives from his father becomes heterochromatic and genetically inert, and is discarded rather than transmitted. As the X chromosome, heterochromatization of the paternal chromosome set in the male is not permanent or static following development. Here are four lines of evidence. First a heterochromatic paternal set is not seen in many tissues, although it is evident in all cells earlier in development. Second, the treatment of males with high doses of radiation is lethal not only for their daughters but also their sons. Third, males as well as females from interspecific crosses are lethal. Fourth a heterochromatic set from one species cannot be substituted for that of another. If the heterochromatic set of a species were completely inert, it should be possible to replace it with a heterochromatic set of comparable.

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However, the differential heterochromatization is not a simple problem of chromosomal haploid. It is a progressive interaction of two homologous chromosomes in development. Probably, one is from active to progressive inhibition and another from inhibitive to progressive action. As a result of this interaction the different gene groups act in a proper order. For example, the X chromosome inactivation does not occur along the entire chromosome at the same time; rather, it occurs gradually and spreads along the chromosome. Martin et al. (1978) reported that the reduction of G-6-PDH activity as a result of X chromosome within mouse tetratocarcinoma cells always preceded the reduction of another X-linked enzyme-hypoxanthine phosphoribosyl transferase. All these observations indicate that the regular heterochromatization is a form of chromosomal control on genes action during development. (B) PUFFS

Extensive investigation have been made of puffs during the development stages of many Dipteran flies. It has been observed that some puffs do not change at all during development, other are present only during the molting period, while still other puffs are restricted to the pupal molt (Berendes, 1965). Kiknadze (1972) pointed out that there are four types of stage-specific puffs in salivary glands during Chironomus thummi development: moulting puffs, which significantly enlarge or diminish during moulting; larvae puffs; metamorphosis puffs, which appear periodically during moulting; and methamorphosis puffs, which appear only during metamorphosis. For example, at the end of the last phase of the fourth stage and directly at the moment of pupamation, besides a number of moulting pulis and larvae puffs, eleven new metamorphosis puffs appear, and four disappear; in the red pupae stage two methamorphosis puffs and some moulting puffs demonstrate a noticeable size increase; in the grey pupae stage, the main portion of puffs disappear, only one methamorphosis puff and some small active center of puffs remain. Stage-specific puffs were also found in D. melanogaster. For example, puffing was found in 66 regions of the X chromosome. In 52 of these puffing is constant, and in 14 it is stage-specific. The maximum in puffing activities occurs during the transformation from larval to pupal stages (Belyaeva et al., 1974; Ashburner, 1967). The stage-specific puffs suggest that the sequential action of gene groups is a developmental event and not a differentiative event. Because the cells of salivary glands are already differentiative tissues, we cannot tell that there are any correlations between the functions of the salivary gland and the changes of puffing picture.

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In addition, it has been proved that the puffs are induced by the ecdysone. As is generally known, different hormones act sequentially in the development process and it has been proved that the hormones are the inducer of gene expression. Therefore, the puffing can be regarded as a cytological indicator of the action of chromosomal genes. (C) T H E

EXPRESSION

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On the cytological level, we find the existence of gene only by the mutation. Similarly, we find the action of gene groups in development only by some gross changes of chromosomes in developmental process. If there is no chromosomal heterochromatization, puffing or elimination, will the gene groups act sequentially throughout development? The isolation of globin gene clusters by molecular cloning provides a positive answer. Maniatis et al. (1980) point out that globin genes in clusters are arranged in order of their expression during development, and in the same orientation as in transcription. Thus the human and chicken a-like globin genes are arranged in the order 5'-embryonic-adult-Y (Engel & Dodgson, 1980). The human (Maniatis et al., 1980) and rabbit a-like globin genes are arranged in the order 5'-embryonic-foetal-adult-Y (Hardison et al., 1979). In humans, the a-like and/3-like subunits of hemoglobin are a small group of genes that are expressed sequentially during development. The earliest embryonic hemoglobin tetramer, Gower 1, consists of e(/3-1ike) and ~(a-like) polypeptide chains (Gale, Clegg & Huehns, 1979). Beginning at approximately eight weeks of gestation the embryonic chains are gradually replaced by the adult a-globin chain and two different fetal/3-like chains, designated GT and AT. During the transition period between embryonic and fetal development, Hb Gower 2(a2e2) and Hb Portland (~2T2) are detected. Then the Hb F(a2y2) eventually becomes the predominant Hb tetramer throughout the remainder of foetal life. Beginning just prior to birth, the T-globin chains are gradually replaced by the adult/3-and 8-globin polypeptides. Six months after birth 97-98% of the hemoglobin is Hb A(a2/32), while Hb A2(a282) accounts for approximately 2%. The globin gene clusters are often associated with repeated sequences (both high and middle repetitive) which exist in introns and flanking sequences of genes. Recent studies pointed out: that the globin gene clusters cannot encode functional globin polypeptides because the reading frame of code are changed by small deletions or insertions (Cleary et al., 1980); if the introns are removed, the gene will be become a pseudogene (e.g. mouse a-globin pseudogene; Nishioka et al., 1980). Therefore, the globin genes express sequentially in development, which may be relative to the repetitive sequences in an unknown mechanism. The universal occurrence

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of repetitive sequences in eukaryotic DNA and their interspersion with single-copy sequences have led us to the proposal that repetitive sequences play a more important role in the expression of the majority of genes in development than the chromosomal loss, heterochromatization or puffing. (D) T H E C H R O M O S O M A L G E N E G R O U P IS A R E A L I T Y

Dose the chromosomal gene group is a abstract concept of set theory or a reality? Elston & Glassman (1967) demonstrated that genes which are closely related functionally are usually on the same chromosome and clustered in one region of the chromosome. For example, in D. melanogaster, about 40% of wing mutations are located on the second chromosome and about 26% of the wing mutations are located on the third chromosome. The eye mutations (for shape, structure and color) are 40% on the third chromosome and 29% on the second chromosome (Golubovsky, 1972). In D. virilis Alexander (1976) has demonstrated that in the X chromosome there are two groups of genes which control wing development. The first group consist of 72.0 (we), 74.9 (to), and 78.1 (dy) while the second group consist of 94.5 (Bx), 96.0 (br), 100.4 (Ds), 102.9 (N), 103.3 (Ax) and 104.5 (T). Two small groups of genes are on the fifth chromosome and one of these determines chaeta formation: 44.2 (rr), 44.5 (tt), and 44.5 (sb), the other group affects the eyes' shape and color: 150.0 (po) and 151-5 (Ds). Another spectacular case is the bithorax cluster in D. melanogaster, where all five genes (a, b, c, d, e) are closely linked and are located in the region of 58.8 on the third chromosome. It has been suggested that these genes control the thorax development of the fly. The bithorax model of Lewis (1963, 1964) deals with a series of bithorax pseudoallelic mutations in D. melanogaster. The bithorax mutation is a homeotic mutation which induces the transformation of one body part into another. The a (bithorax) mutation causes the transformation of the anterior metathorax into the anterior mesothorax. As a result of the transformation of the anterior part of the halter disc into the anterior part of the wing, such flies have two normal and two defective wings. The b (contrabithorax) mutation is a transformation of the posterior mesothorax into the posterior metathorax. The c (ultrabithorax) mutation is similar to the a mutation and is generally lethal in the homozygous condition. The d (bithoraxoid) is characterized by the transformation of the first abdominal segment into the posterior mesothorax; such flies have eight instead of six legs. The e (postbithorax) mutation induces the transformation of the posterior metathorax into the posterior mesothorax. There is a complete transformation of the metathorax into the mesothorax when a fly has both the e mutation and the a mutation. Such

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flies have a double mesothorax with four wings. All these five genes are closely linked. If there is at least one wild-type allele, the mutation alleles will not affect the phenotype. In addition, there is a well_defined polarity effect in the bithorax cluster: the transheterozygote d + / + e heterozygote shows the e transformation but not d, and the c + / + d heterozygote shows the type d transformation. Thus, each locus influences the loci to the right of it. Therefore, the bithorax cluster is an example of a group of genes which are not only closely linked but also interrelated in a certain stage of development. Kauffman et al. 1(975) showed that deficiences on the left part of the X chromosome caused an early lethal effect, while deficiences on the'right part became lethal at later stages of ontogenesis. Hochman (1973) investigated the lethal effect of fourth chromosome deletions on development in D. melanogaster. Deficiences lethal during the embryonic period are on the left, deficiences lethal in the larvae are in the middle, and deficiences lethal in the pupae are on the right. In general, adult cells contain a complete set of chromosomes of embryonic cells, but a small number of animals have a regular loss of some chromosomes within soma cells in the course of development. Boveri (1887) first observed the loss of part of the chromosomes in the cleaving eggs of Ascaris megalocephala. In the Oligarces paradoxus, the germ cells show 66 chromosomes while the soma cells contain only 10 such elements, and the Mayetiole destructer has 40 chromosomes while during the fifth cleavage division 32 chromosomes are eliminated from soma cells. Chromosomal loss shows clearly that there really is a group of genes which only acts in the early stage of life cycle (i.e. the gametogenesis and early embryonic stage). In fact, five closely linked genes which control successive development stages during meiosis and the first 5 h of embryogenesis have been found. These genes are located on the X chromosome of 19. melanogaster near the scutella locus. All these facts suggest that the gene group is not an abstract concept of set theory but a reality. It is possible that the gene groups, which act progressively in development, may be packed by one or several chromosomal regions and not scattered randomly on every chromosome, except for some kinds of gene which are necessary to every development stage. Even the 5S rRNA genes in Xenopus can be also divided into two types; the somatic type and oocyte-specific type (Ford & Mathieson, 1976). In X. lavis oocytes synthesize predominantly oocyte-type 5s rRNA (over 50%), but about 10% of the somatic type is synthesized. Somatic cells normally synthesize predominantly only the somatic type of 5S rRNA. Ford & Mathieson (1976) report that oocyte-type 5S rRNA is synthesized in transformed somatic

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cells in which oocyte-type 5S rRNA genes groups are translocated to a site adjacent to the nucleolus. It was suggested that oocyte-type genes may be repressed in somatic cells by being located in some chromosomal regions (e.g. heterochromatic) while such genes are translocated into a euchromatic region which may cause their activation. A Model of Chromosomal Genes

As mentioned above, according to the occurrence of regular chromosomal elimination, heteromatization, puffs and the expression o f globin clusters or others in development, we can infer that the chromosomes contain a number of gene groups which act in a proper order throughout development. Here we advance a model of the action o f chromosomal genes. (1) The chromosomes are not only the carders o f genes but also the programming controller of gene action. An entire set of chromosomes contains a number of gene groups which act in a proper order throughout development (Fig. 1).

FIG. 1. A diagram of the action model of chromosomal genes in development. We suppose it contains four gene groups A, B, C, D.

(2) The action of gene groups are sequential. As a result of the action o f one gene group, the next gene group is activated and make a self-inhibition. (3) The gene groups are relatively independent of each other. When the action o f one gene group has been accomplished, it may be eliminated or heterochromatization. The chromosomal loss and heteromatization often occurr in the early development stage. It indicates that the developmental

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process can be divided into three stages at leastmthe early stage, embryonic and post-embryonic stage. As described above, the early stage development is an important stage which is controled by a big gene group. Each stage may be further divided into several stages. Every stage is controlled by a gene group. (4) Viewed from the angle of phylogeny, the first gene group A is a conservative group which controls the early stage of the life cycle. In high organisms it may be started by a sexual (gametogenesis) or asexual (e.g. sporogony, budding or parthenogenesis) process and accomplished in early embryonic stage. It will probably be divided into several smaller stages. The other stages are epigenetic. Every epigenetic gene group connected with the first gene group A at a certain time of phylogenetic process as a last gene group. However, when it has not been the last gene group and as one middle link of the development chain afterwards, it shall activate the next epigenetic gene group and the connexion with A is weakened but never cut. Under a particular condition, the epigenetic gene group may not enter the next epigenetic stage and resume the connexion to the first gene group A. For example, the Axolotl is the neotenous larva of some salamanders in the family of Ambystomidae. Because they have been living in some Mexican lakes which are deficient in iodide, so the epigenetic gene group which controls the larval development cannot activate the next epigenetic gene group (adult gene group) but resumes the connexion with the first gene group A, so-called poedogenesis. Therefore, in Fig. 1, the connexion between B (or C) and A is shown by a dotted line and the normal development route is shown by solid line. (5). The mechanism of the action of gene groups remains. Probably, the hormones and the repetitive sequences are two important factors. Many facts allow us to consider the hormones as a genetic inductor. As is commonly known, at a development stage certain hormones appear which bind with the receptor and transport into the nucleus, in which they associate with the chromatin and activate a group of genes. The hormonal system affects the action of genes from the very beginning of development. For example, amphibian oocytes cannot induce DNA synthesis in nuclei transplanted into them before ovulation. Such an ability is possible only after stimulation with hypophyseal hormones and the destruction of the germinal vesicle. According to Gurdon (1967) this ability plays a crucial role in the initiation of development. Besides the hormonal role in the regulation of metamorphosis in amphibians, the transition from one larval stage to another and from larval stage to pupal stage is controlled by the ratio of concentrations of juvenile hormone and ecdysone, which are well known. That the

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level of hormones affects the development of the genital system is also known. There are crucial periods when the developing genitals are sensitive to hormonal action. In the rabbit foetus, this period occurs on the day 19 or 20 of development and continues for 2-3 days. Genital development is relatively independent of hormonal influence before and after this crucial period (Jost, 1961). The universal occurrence of repetitive sequences in eukaryotic DNA and their interspersion with single-copy sequences have led to the proposal that repetitive sequences are involved in the expression of genes. The isolation of globin gene clusters by molecular cloning provides an opportunity to study repeat sequences in relations to well-defined sets of developmentally regulated genes. Globin gene clusters are often with repetitive sequences. Sequences of both high and middle repetitive frequency are found within introns or both flanks of genes. Such repetitive sequence elements have been mapped in the human (Fritsch, Lawn & Maniatis, 1980), rabbit (Shen & Maniatis 1980) and chicken (Dodgson, Strommer & Engel, 1979),/3-like globin clusters. These repetitive sequences are characterized by the following properties: the repeats are transcribed by RNA polymerase III in vitro (Duncan et al., 1979); the repeats share sequence homology with an abundant small nuclear RNA molecule and with double-stranded heterogeneous nuclear RNA (Jelinek et al., 1980); a detailed study of the rabbit g-like globin gene cluster revealed a complex array of sequences that are repeated within the gene cluster as well as throughout the genome (Shen & Maniatis, 1980). All these facts suggest that such repetitive sequences are involved in globin gene expression in development. On the other hand, in addition to the unique sequences, the H n R N A also contains sequences complementary to the repetitive part of the genome. It has been estimated that the repetitive sequences comprise about 10% of the sequences in H n R N A (Smith et al., 1974). The competition hybridization experiments suggest that there are considerable differences in sequence homology between repetitive RNA from various tissues and developmental stages (Church & McCarthy, 1967). One particularly striking example is the lack of sequence homology between oocytes and blastulas in Xenopus. In D. melanogaster, about 12% of its genome is composed of moderately repetitive DNA sequences. Approximately 75 % of this moderately repetitive DNA is composed of at least 40 different sequence families, which in general are dispersed throughout the genome as determined by in situ hybridization to polytene chromosomes (Young, 1979). Among which, one clearly distinct structural class is the copia-like elements. Six such repeated sequence families with element sizes ranging from 5 to 8.5 kb have been studied in detail. They are the copia, 412, 297, B104, mdgl k and mdg 3. An interesting

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fact is the abundance of RNAs, which encoded by different copia-like elements appear to be modulated independently during development. According to the report of Flavell et al. (1980), the steady state level of total cell copia-specific R N A changed several-fold during development, being low in 6-8 hr embryos and abundant in larvae, and low in adult. The times of onset of accumulation of cytoplasmic poly(A)÷RNA homologous to copia, 412, B104, and 297 during the 22 hr o f embryonic development were determined by Scherer (Spradling & Rubin, 1981). According to his work, the copia-specific R N A begins to accumulate at 10--15 hr, 412 at 15 hr, B104 at 2-5 hr the 297-specific R N A is found in 0--90 min RNA, but disappears at later times. Therefore, we suppose that the programming action o f gene groups is advanced by the interaction of hormones and repetitive sequences. As mentioned above, the gross interspersion pattern of repetitive sequences in eukaryotic D N A suggests that it controls the expression of genes more in the group than in the individual. Probably, in the development process, a certain hormone is associated with them. It goes without saying that not all hormones and repetitive sequences are involved in the action of gene groups but only part of them. Probably, in this interaction of the hormones and repetitive sequences, the repetitive sequence as a good candidate must contain the recognizable site for hormone and must be located in the flank of the 5'-end o f the structure gene. The hormone as a good candidate which must be stage-specific and can induce a group o f genes but not an individual gene. We propose that there are two connexion genes in A, B, C gene group (Fig. 2), one o f which is a normal connexion gene which can activate the next epigenetic gene group, the other one is conservative connexion gene which had connected with the first gene group but is now in a low level of transcription, owing to an abnormal repetitive sequence which is contiguous to it, so the interaction between repetitive sequence and hormone is very weak.

R

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FIG. 2. A diagram of the genes and repetitive sequences in the A, B, C gene group. [:], gene; I~l,conservative connexion gene; II, normal connexion gene. R is a repetitive sequence and r an abnormal repetitive sequence.

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Therefore, the action of the two genes is as a result of dosage effect on the transcriptional level. In some particular condition, so long as the activity of the conservative connexion gene prevails over the activity o f the normal connexion gene, it will activate the first gene group as mentioned in the Axolotl or some coelenterate (e.g. Actiniaria, Stephanoscyphus, Nausitho~, Gastra etc.) In addition, there is only one connexion gene in the last gene group D. Obviously, this gene is a conservative gene, because it connects with the first gene group directly. In germ cells, the action of this gene is strong and it can activate the first gene group, but in somatic cells it is also in a very low level o f transcription as the others of A, B, C gene groups, so it can't activate the first gene group but only make the progressive inhibition of the last gene group. However the details of the interaction between the hormones and the repetitive sequences is entirely ignorant, it is still too early to advance a concrete scheme o f the interaction (a activationinhibition system) between the gene groups. We can understand completely, the products of the connexion gene may be as a positive control factor to the next gene group and also as a negative factor to its self-group at the same time. This is similar to the Or gene of the bacteriophage A to some extent. Use of this Model to Explain Some Vital Problems of Biology (A) DEVELOPMENT AND DIFFERENTIATION According to our model, development and differentiation are not the same thing. Development is defined as the programming action o f gene groups, and differentiation can be defined as the expression o f genes during the programming action o f gene groups. The programming action o f gene groups is an ordered transcription in a nucleus and differentiation is a gene expression which is accomplished in the cytoplasm. In other words, every gene o f one group may be transcribed in a nucleus but not necessarily find expression in the cytoplasm. There is an identical nRNAs group in the nucleus during a given development stage whatever the different types or functions o f cells, but the mRNAs in cytoplasm is only a part of nRNAs and is diverse. For example, in the salivary gland of Drosophila, it was found that the picture of chromosomal puffs remained stable in the periods between moultings, but changed dramatically in the larval moulting and metamorphosis. All these changes are only related to the development stages, but have nothing to do with the differentiation of salivary gland. Furthermore, in spite of functional differences in the proximal and distal parts o f

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the salivary gland in D. melanogaster, the puffing picture in these locations is identical (Belyaeva & Zhimulev, 1974). During sea urchin embryogenesis the complexity of nuclear RNA is unchanged from the blastula to the pluteus stage (Kleene & Humphreys, 1977), while the complexity of the polysomal mRNA drops by a factor of about two (Hough-Evans et al., 1977). As is known to all, mouse brain mRNA is not found in kidney polysomes but is found in kidney nuclear RNA (Davidson & Britten, 1979), and tobacco leaf mRNAs are not found in tobacco stem polysomes but are found in tobacco stem nuclear RNA (Kamalay & Goldberg, 1980). Our model regards the transcription of nRNAs as a developmental event in which not all genes act simultaneously, but a series gene group act sequentially. It is a programming controller of the development and the base of differentiation. (B) SENESCENCE Our model is to view from the angle of the programming action of gene groups. The whole programming action of gene groups is the gear of the hour hand of the biological clock and its rotation is dependent on the action of every gene group which is to activate the next one and give a feedback self-inhibition gradually. However, in the somatic cells of a adult body, this biological clock will be stopped at the last gene group and will not be able to activate the first gene group. Because the reactivation of the first gene group means the beginning of a new life cycle, it is only started in germ cells line (sexual and asexual). Therefore, in the soma cells of an adult body, there is only the progressive inhibition to the action of genes. Probably, this is the essential reason of senescence. Consequently, the senescence is an inevitable outcome of the programming action of chromosomal genes in the soma cells and is not the accumulation of the DNA mutation or the error catastrophe of transcription and translation (Orgel, 1963), otherwise we shall not be able to tell why the germ cells line is immortal and why the accumulation of mutation or the error catastrophe cannot befall germ cells? Therefore, the senescence is a development event but not as a result of terminal differentiation (Hayflick, 1975). In ciliates the micronuclear and the macronuclear coexist in one high differentiational cell; but the genetic materials of micronuclear are immortal, and the macronuclear must eventually collapse because the micronuclear can pass through the first programming action continuously by the conjugation.

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(C) C A N C E R

As shown in model, the somatic cells can only arrive at the programming action o f the last gene group. The senescence will be inevitable. If the first gene group is activated in the somatic cells, it will be cancerous. As shown in Fig. 1, under the abnormal condition, the epigenetic gene group cannot enter the action of the next epigenetic gene group, but will connect to the first gene group A. If the A group is only started in germ cells, it will be the poedogenesis. If the A group is started in somatic cells it will enter into the first programming action again and begin a new round of development. Owing to this it acts and expresses in a wrong time and space, and the cancer cells have not been the normal germ cells or embryonic cells, so cannot promote normal development and differentiation. Consequently, cancer occurrs. In view of above, we can anticipate that in some species, those chromosomes (or chromosomal regions) which control the embryonic stage development have lost in development, and some species which can be reproduced by budding (e.g. coelenterate, sponge, echinoderms etc) will not be cancerous. As mentioned in this paper, there are some animals which have a regular loss of some chromosomes of somatic cells during embryonic development. If we get quite clear on its mechanism, it will be advantageous to the control of cancer. According to our model, the so-called oncogene (or proto-oncogene) will not be any gene of groups (as shown in Fig. 1.) but is the conservative connexion gene of somatic cells. Therefore, there is an oncogene in every gene group. In normal development, the conservative connexion gene of soma cells either unactive or transcriptive in a low level, so the action o f gene groups proceed along the solid lines as shown in Fig. 1. As a result of some carcinogenic factor, the transcriptive level of the conservation connexion gene can be risen greatly and resume the connexion to group A in somatic cells, and the cancer will be occurred. Therefore, the occurrence of cancer is a competitive process between two kinds of the connection gene in soma cells. According to our model, the carcinogenesis is the somatic cells which have now entered the first programming action again and begun a new round of ontogenesis. However, there are many antigens, enzymes and hormones of early embryo that have reappeared in cancer cells. It indicates that many genes o f an early embryonic stage are expressed again. However, owing to the unsuitable environment the cancer cells are not the normal embryonic cells, so it is an abnormal expression and cannot carry out a

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normal differentiation. Thus we can understand, if a normal early embryonic cells implant into the mouse test of same species, it will be induced to teratocarcinoma. In addition, if we put the cancer ceils or their nuclei into a suitable development orbit, it is possible that they may develop normally and make cells lose their cancerous characteristics. Fortunately, there are some experiments which may support this idea such as the transplant o f nuclei from frog adenocarcinoma cells into enucleated eggs of normal frog (McKinnell Deggins & Labat, 1969). A number o f these transplant embryos developed into normal tadpoles. These tadpoles were developed from the cancerous nuclei, but had no cancerous signs. The mouse teratocarcinoma cells, after a transplant within the abdominal cavity o f a normal mouse (homologous in genome) through 2000 generation; and then transplanted the teratocarcinoma cells into the blastocyst of a normal mouse. These malignant cells can work in coordination with blastocyst cells to promote the normal development and produced a normal genetic mosaic mouse (Mintz & Illmensee, 1975). Occasionally, we can find the genome of teratocarcinoma cells in the germ cells line o f mosaic mouse. We consider it is not the reverse o f cancer cells, but is the normalization o f cancer cells. In a word, every viewpoint advanced in this paper, is the logical extension of the concept of our model. It is conducive to the exploration o f the problems of development, cancer and senescence, but is not the final solution. All these problems are so complicated, we have a long way to go. Lastly, we consider that there is no contradiction between our model and the Davidson & Britten model (1979); the latter is a differentiation model which rests on the basis o f post-transcriptional control and ours is a development model which rest on the basis of gene action. However, our model is different from the Morgan model (1934) and the Caplan model (Caplan & Ordhal, 1978). Our model regards the action of genes in development as a chain-reaction of inhibition-activation. In each and every stage there are both activation and the inhibition, they are a double-effect of the same gene (or genes) and occur at the same time. The Morgan model considered the action o f genes from entire inhibition to respective activation and the Caplan model is from entire activation to respective inhibition. REFERENCES ALEXANDER, M. (1976). In: The Genetics and Biology of Drosophila Vol. 1 (Ashburner, M. & Novitski, E., eds). Pp. 1365-1419. New York: Academic Press. ASHBURNER, M. (1967). Chromosoma 21, 398. BELYAEVA, E. ~. ZHIMULEV, I. (1974). Genetica (Russ) 10, 74. BELYAEVA, E., KOROCHKINA, L. ZHIMULEV, I. R, NAZAROVA, N. (1974). Cytologia (Russ) 16, 440. BERENDES, H. D., (1965). Chromosoma 17, 35.

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