Sex determination.

Bacci, Guido.

Date:: [1965]

Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Sex determination. Source: Wellcome Collection.

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SEX AND EVOLUTION 239 writers have been led to emphasize differences rather than basic similarities between sex inheritance and inheritance of other characters. The recent trend of research points on the other hand to the role of genes, that can hardly be regarded as sex genes, in the simple systems of polyfactorial determination and indicates that it is impossible to draw a straight line of separation across a graduated series of sex determiners which begins with sex modifiers and ends with blocks of sex genes, or supergenes. Polyfactorial sex determination often imphes the possibility of rapid changes in the sexual structure of populations. Such changes are clearly related with a variability of both the sex phenotype and of the sex genotype which, although it is particularly marked in polyfactorial sex determination, is by no means absent in sex digametic populations. A discussion on the evolution of sex both within and between major systems of sex determination can now be initiated and a few concrete examples be given. Data are rather scarce indeed but a fresh outlook will provide useful indications for future work. It appears expedient, however, first to discuss a system of classification which takes into account the results on polyfactorial sex determination and on sex in bacteria and can possibly replace the misleading separation into two categories of genetic and phenotypic sex determination. A Tentative Classification A classification, even a good one, may prove a useful as well as a dangerous tool because, besides expressing new positive results, it may conceal a lack of knowledge in some important fields. It is important therefore to distin¬ guish between the categories that are mainly based on positive information and the categories that have mainly been established for mechanisms which cannot be made to fit within the boundaries of the other well-investigated categories. Such is the case of the protosexual as opposed to eusexual and parasexual processes (see Chapter 3). Recombination is brought about in bacteria and viruses through mechanisms that are different from those which have been well investigated in eusexual organisms. It must not be forgotten on the other hand that very little is known about such basic problems as those concerning prophage recombination or replacement of recipient by donor genes after transduction. Until such and other major mechanisms have been clarified, protosexuality will stand as a provisory category whose utility lays only in stressing that cycles of bacteria and viruses are not based on the regular alternation of caryogamy and meiosis and are possibly more primitive than the eusexual cycle. Similar considerations can be made about parasexuality in Fungi as problems posed by somatic crossing-over are far from being solved.