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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CHAPTER 12 SEX AND EVOLUTION The evolution of sex determination in digametic species has been discus¬ sed in comprehensive works by Darhngton (1932, 1939), by White (1954) and by other authors who have primarily been concerned with the cytological aspects of the sex chromosome mechanisms. Very little information is available on the other hand about the evolution¬ ary relationships existing among the vast number of organisms which show monogeny, labile gonochorism, relative sexuality, irregular partheno- genetic cycles or hermaphroditism of different kinds. The discussion about evolution from monoecysm or hermaphroditism to dioecysm or gonochorism has scarcely yet begun. The reason for this situation can be traced to the limited number of obser¬ vations and experiments which have been made on sex determining mechan¬ isms other than those which are shown by sex digametic species. It has been a general opinion that organisms which do not show the sex chromosome mechanism represent exceptional cases or at least they belong to a quite distinct category of sex determination (the so-called phenotypic sex determination). For this reason not much interest has been paid to the various aspects of what can broadly be named the polyfactorial determination of sex. One school of thought discouraged attempts to approach the evolution of sex determination in accordance with the criteria that are used to analyse population genetics. Sex determiners were regarded as quite dissimilar from all other types of genetic factors, or it was assumed that the basic sex determin¬ ing system is beyond the reach of our present methods of genetic investigation. It is fortunate, therefore, that the presence of detectable sex chromosomes has allowed cytologists to carry out brilliant research within the field of sex digametic species. Recent comparative research indicated, however, that sex digamety—• although it had developed independently in several phyla—is present in a minority of organisms and that mechanisms of sex determination, which were formerly regarded as exceptional, are on the contrary much widespread. In order to understand why sex inheritance has been regarded by some authors as a different form of heredity, it must be recalled that research has concentrated on sex digamety which is obtained through a variety of highly specialized mechanisms. It is not surprising therefore that previous 238