Sexual differentiation and germ cell transfer in the colonial ascidian Botryllus schlosseri

The sexual differentiation in the colonial ascidian Botryllus schlosseri was considered with respect to colony sexualization and sexualization of the single zooids of the same blastogenic generation. The colony sexualization was investigated histologically in a series of young colonies reared in the laboratory and fixed periodically throughout a sequence of blastogenic generations. The newly set colonies lack gonad blastemata; next, germ cells appear and several generations are involved in various phases of hyposexuality prior to the appearance of generations in which mature both eggs and sperm. The sexual contribution of the zooids of the same generation was found to vary according to an intrinsic bilateral asymmetry (the gonadogenic potential differs both contralaterally in each zooid and between zooids of a different, dextral or sinistral, parental origin) and in proportion to the quantity of germ cells that have been transferred from preceding generations. The transfer of germ cells from one to the following generations was investigated by fusing colonies of opposite pigment genotypes, then crossing the separated parabionts to a common genotype, and scoring the offspring pigment phenotypes. This experiment has shown that not only immature oocytes but also undifferentiated cells were exchanged between the parabionts and then gradually matured as eggs or sperm in a prolonged series of generations. The primordial germ cells differ in size from hemoblasts, the source of hemocytes. However, further experiments are needed in order to decide between the two contrasting hypotheses of germ cell origin: either segregation from the atrial epithelium of buds or development from hemoblasts.