15. Sexuality and Sex-genes

By T.Yamamoto

Sexuality

Members belonging to the Class Pisces exemplify an almost complete range of the various types of sexuality from synchronous hermaphroditism, protandrous and protogynous hermaphroditism, to gonochorism.

At the outset, it is necessary to clarify the concept of sex. The male and female can best be defined as sperm and egg producers, respectively. This definition, although self-evident, is needed to correct statements that appear frequently in literature, with respect to spontaneous sex reversal in gonochorists. The term "bisexuality" used to denote hermaphroditism is likely to lead to a misunderstanding. This term should be used to describe gonochorist.

The term "intersex" (Goldschmidt, 1915, 1927) is used here to denote either sporadically appearing or experimentally produced hermaphroditic individuals of a species in which all or nearly all individuals are gonochoristic.

Whether hermaphroditism is the more primitive condition from which bisexuality or gonochorism may have arisen or a specialization derived from the more usual vertebrate gonochorism is a matter for debate. The solution of this interesting problem will require a great deal more information in the future. Nevertheless, fishes provide excellent material to approach the problems of sex differentiation and of evolution of sex among animals.

As Witschi (1914a, b, 1930) has pointed out for amphibians so also in gonochoristic fishes there are "undifferentiated" and "differentiated" species. In the former, the indifferent gonad first develops into an ovary-like gonad and then about one-half of individuals become males and the other half females. In the latter, the indifferent gonad directly differentiates into either a testis or an ovary. In both types, sex differentiation seems to be brought about by male- and female-inducing substances. It is natural that undifferentiated species are more unstable than differentiated ones in sexuality. Unfortunately, however, the two types have been studied embryologically in only a few fishes. In the absence of embryological evidence, it is as yet hazardous to correlate the two modes with the occurrence of sporadic intersexes which represent a remnant of the embryological condition manifested late in some adults.

However, a few species in which undifferentiated and differentiated conditions are known provide evidence that spontaneous intersexes among gonochorists occur mostly in the "undifferentiated" species while in the "differentiated" ones the occurrence of intersex is rarely or never seen.

Only a few fishes have been shown to be differentiated gonochorists. Sexuality of differentiated species is fairly stable among fishes. Wolf (1931) demonstrated that the "domesticated" platyfish, Xiphophorus (formerly Platypoecilus) maculatus, is a differentiated species. Bellamy and Queal (1951) stated that not a single sporadic intersex has ever been found among 50,000 specimens.

The medaka, Oryzias latipes, is a differentiated gonochorist (Yamamoto, 1953). The writer has not come across even a single spontaneous, true intersex.

Sex genes

The classic sex factor studies use notations such as FF = female, FM = male, where M>F in male heterogametic species and FM=female, MM=male, where F>M in female heterogametic forms. These are oversimplified and are not generally found in nature, if the F and M symbolize single sex determiners (genes). Hartmann (1951, 1956) has proposed the formulae (notAG)FF=female and (AnotG)MF=male, where the M and F represent the sex determiners (realizers), the AG stands for ambisexual potencies and the symbols notA and notG represent the inhibiting effects exerted by the combinations of sex realizers. The supposed existence of the double system is also a formalization and has been a matter for debate since experiments on the localization of the AG complex have not been fruitful. Kosswig (1964) is also an opponent to these formulae.

To grasp the real situation of sex determination, we must adopt the broader view originally advanced by Bridges (1922, 1925) for the fruit fly and elaborated by Winge (1934) for fish. "A given property, the sex included, depends upon all the chromosomes, some of which pull in one direction and others in the other direction, some strongly and others faintly or not demonstrably at all" (Bridges, 1939).

In fishes in which the homogamety/heterogamety is established, exceptional XX males or XY females and WZ(Y) males appear occasionally, although no exceptional ZZ(YY) females are reported. These exceptions have been misleadingly called genetic "sex reversals" even by distinguished geneticists. This is based on the concept that only sex chromosomes are carriers of the sex determiners.

Only a few geneticists - of whom Winge (1934) is the most celebrated -have grasped the intrinsic nature of sex differentiation. He has never referred to such exceptions as sex reversals because his theory is based on multiple sex factors with superior sex genes in the allosomes.

The genetic evidence indicates that the guppy, Poecilia (Lebistes) reticulata, has sex chromosomes, XX for female and XY for male (Winge, 1922b). However, minor or polygenic F and M genes are distributed throughout the autosomes. The Y has superior (epistatic) M gene(s) and the X is supposed to possess epistatic F gene(s) (Winge, 1934; Winge and Ditlevsen, 1947, 1948). In the majority of individuals, autosomal sex genes are more or less in balance. Consequently, sex in most individuals is determined by the allosomal combination. However, by fortuitous combinations of autosomes or recombinations a few exceptions may appear in which ·M> ·F in spite of an AAXX constitution. Mutatis mutandis, exceptional XY females are considered to be individuals in which the totality of sex genes becomes ·F> ·M. These exceptions cannot be regarded as genetic "sex reversals" since they have a genetic basis to develop either into males or females from the beginning.

Aida's breeding results (1930) in the medaka, Oryzias (Aplocheilus) latipes, can be interpreted by this postulation (Winge, 1930). Since polygenic sex genes are numerous, by selective breedings of XX-males the sex ratio of offspring can be changed. Aida (1936) established an XX-XX strain of the medaka and adopted a theory of polygenic sex differentiation which is somewhat different from that of Winge. He suggested that XX males may be the result of a lowering of the female determining potency of the X chromosome. It is unfortunate that he referred to XX males as "sex reversals".

In our d-rR strain of the medaka, where normally XrXr are females and XrYR males, about 0.5% of the progeny are exceptions (0.2% crossovers and 0.3% of XrXr males plus XrXR females, Yamamoto, 1959a, 1964a, b). These rare XrXr males and XrYR females are regarded as exceptions in which autosomal M genes and F genes, respectively, over-accumulate in such a way that the sum of autosomal sex genes outbalances the superior allosomal sex factors. As pointed out earlier (Yamamoto, 1963), the number of possible autosomal combinations is 2 exp 46 an astronomical number.

Even if only some of autosomes are assumed to be sufficiently different in respect to M and F genes (or modifiers), the rest being more or less in even balance, the number of combinations would be enormous.

In an AAXY constitution the AA curve is shifted in the male direction (right) because of epistatic M gene(s) in the Y, and in the AAXX constitution the curve is pulled to the female direction (left), perhaps because of the presence of epistatic F gene(s) in the X or absence of the Y. Exceptional XX males and XY females encountered in normal breeding may be regarded as individuals which fall in the two extreme regions of the AA curve (stippled and oblique hatched), i.e., the ·M greatly overweighs the ·F of AA or vice versa.

By inbreeding and systematic selections, the AA curve may be shifted to either direction. In fact, the concept of polygenic sex factors has been substantiated by selective breeding. Thus, Aida (1936) obtained a strain of the medaka, in which both female and male have the XX constitution. In Aida's stock, exceptional XX males were rare at first; by continued selection for XX males a breed was established in which XX males outnumbered XX females.

To sum up, the need to clarify our thinking about sexuality in fishes is emphasized. First, it is unsound to consider that sexuality in all members of the Class Pisces is labile. There are in fact many sex types: synchronous, protandrous, and protogynous hermaphrodites, as well as gonochorists. Among gonochorists, there are "undifferentiated" and "differentiated" species in the sense of Witschi. The latter are more stable than the former in sexuality. Evidently, there are graded levels in sexuality among fishes.

Second, sex is a phenotypic expression. In gonochorists, it is important to clarify the concept of sex to eliminate confusion. A male is a sperm producer and a female an egg producer. This definition, although self evident, is particularly important in avoiding controversy in the problem of sex reversal. By definition, sex genotypes of zygotes before sex differentiation such as AAXX and AAXY are usually presumptive or prospective females and males, respectively.

Although combinations of sex genes are determined at the time of fertilization, sex is actually determined by sex-gene-controlled sex inducers at a certain critical period of development, that is, sex genes act only indirectly upon sex differentiation. Their action is directly mediated by sex inducers.

Sex genes are present not only in the sex chromosomes but also distributed over a great number of autosomes. In view of multiple (polygenic) sex factors, exceptional AAXX males or AAXY females and WZ(Y) males are not "genetic sex reversals." These exceptions appear when the totality of sex genes become ·M>·F by fortuitous combinations of autosomes in spite of AAXX constitution and ·F>·M despite AAXY constitution. This concept may also be applied to exceptional WZ(Y) males in female heterogametic species. They have the genetic basis for either male or female. In general, sex differentiation is induced by sex inducers controlled by sex genes with or without (in species without sex chromosomes) sex chromosomes. Sex chromosomes have epistatic (superior) sex genes.

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