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RESEARCH PRODUCT

The Peculiarities of Gonad Transformation in Teleosts

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In teleosts the problem of sex determination and sex differentiation is more puzzling than in other vertebrates. It is not known whether the intriguing variety in the conditions of sex results from the position of these fishes within the vertebrate pedigree. They represent the final products of actinopterygian evolution toward the end of the age of reptiles, that is late in the Jurassic era. However, with that evolutionary fact in mind it is more plausible that teleosts deviate in many ways from the pattern that we are used to attribute to vertebrates in general. The following features of sex differentiation are striking examples of teleost peculiarity: (1) In both sexes there is no morphologic connection between the genital and the excretory system. (2) The somatic tissue of the gonad lacks a dual origin [16, 29]. (3) The natural diversity of chromosomal conditions in relation to the phenotypic expression of sex is unparalleled among other vertebrates [19]. (4) Teleosts are the only vertebrates among which various types of ambisexuality (the simultaneous or sequential occurrence of both functional sexes in the same individual) can be found often. Scattered in different orders there are numerous cases of simultaneous hermaphroditism or functional sex inversion (protogyny and protandry) at the adult stage [4, 51]. The many studies on the sex of teleosts carried out during the last three decades provide a challenge in current discussions concerned with fundamental aspects of sex determination and sex differentiation.