Origin of Germ Cells, Sex Determination, and Sex Inversion in Medusae of the Genus Clytia (Hydrozoa, Leptomedusae): The Influence of Temperature

In Cnidaria, a separation between soma and germline remains unclear. In this work, we studied the origin of germinal cells and determination of the sexual phenotype in Clytia hemisphaerica and Clytia sp. Colonies of C. Hemisphaerica were cultivated and the medusae liberated by each colony raised until maturity. Two hermaphrodite colonies were obtained, liberating male and female medusae. These two colonies and their medusae were raised at 15°C, 21°C, or 24°C. The medusae budded and cultured at 24°C were mainly female (80%). In contrast, if the medusae were released at 15°C, at whatever temperature they were raised later, they were mainly male (85%). The same occurred if, after release at 24°C but before the formation of the gametes, they were kept at 15°C for at least 24 hr. We suggest that there are two subpopulations of germ cells. The female line will be dominant at 24°C but temperature sensitive, with inhibition of this line by a temperature drop to 15°C, this inverting the population sex-ratio. The irreversible action of a temperature drop to 15°C supports the view that the germ cells are isolated very early. In C. hemisphaerica, hermaphrodite medusae were never observed. On the contrary, in Clytia sp., probably a new species, we have found male, female, but also hermaphrodite specimens. This is the second definite example of hermaphroditism described in any hydromedusan. The transformation of female into hermaphrodite then into male specimens occurs at 13°C. These results demonstrate the unstable character of genetic sex determination in cnidarians, at least in certain species. J. Exp. Zool. 287:233–242, 2000. © 2000 Wiley-Liss, Inc.