Abstract
Despite specific knowledge from selected model systems, a unified theory about the evolutionary origin of germ cell specification still requires detailed analyses in other species. In this thesis, I present evidence on the establishment of the germline and its correlation with pluripotency and embryonic patterning in the polychaete Platynereis dumerilii, an emerging model system that is well suitable for comparative developmental studies across Bilateria. In the first section, I describe the germline development of Platynereis with the help of several molecular germline markers, including vasa, nanos and par-1. This analysis revealed that the primordial germ cells originate from the mesodermal lineage and emerge during the final stages of larval development from a region of undifferentiated cells, namely the posterior growth zone. In juveniles, the primordial germ cells remain quiescent in the former larval segments, while the polychaete grows by addition of new segments from the posterior growth zone. At the onset of gametogenesis, these cells proliferate, migrate to the posterior segments and differentiate into gametes. I extended this analysis in the second section by investigating the expression of germline genes in two not terminally differentiated tissues: the growth zone and tail regenerates. From the expression pattern of vasa in these tissues and its comparison to several genes that are expressed in totipotent and stem cells, such as pl10 and piwi, I suggest that there is an ancestral link between these processes, which is conserved in Platynereis. The third part of my thesis addresses the relationship between germline determination and axis formation. Here, I compared the expression pattern of nanos – one of the posterior genes in Drosophila responsible for the maintenance of germ cell fate and establishment of the posterior segments – to the mRNA distribution of the gap gene hunchback in the early larva. My analysis is consistent with a conserved role of Nanos (and possibly its partner Pumilio) in the translational repression of hunchback, as it is the case in Drosophila, arguing that this system might be a conserved mechanism to establish embryonic polarity in the protostomes.
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