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 thi s work, we studied the origin of germinal cells and determination of the se xual phenotype in Clytia hemisphaerica and Clytia sp. Colonies of C. hemisp haerica were cultivated and the medusae liberated by each colony raised unt il maturity. Two hermaphrodite colonies were obtained, liberating male and female medusae. These two colonies and their medusae were raised at 15 degr ees C, 21 degrees C, or 24 degrees C. The medusae budded and cultured at 24 degrees C were mainly female (80%). In contrast, if the medusae were relea sed at 15 degrees C, at whatever temperature they were raised later, they w ere mainly male (85%). The same occurred if, after release at 24 degrees C but before the formation of the gametes, they were kept at 15 degrees C for at least 24 hr. We suggest that there are two subpopulations of germ cells . The female line will be dominant at, 24 degrees C but temperature sensiti ve, with inhibition of this line by a temperature drop to 15 degrees C, thi s inverting the population sex-ratio. The irreversible action of a temperat ure drop to 15 degrees 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 mal e, female, but also hermaphrodite specimens. This is the second definite ex ample of hermaphroditism described in any hydromedusan. The transformation of female into hermaphrodite then into male specimens occurs at 13 degrees C. These results demonstrate the unstable character of genetic sex determin ation in cnidarians, at least in certain species. J. Exp. Zool. 287:233-242 , 2000. (C) 2000 Wiley-Liss, Inc.