There and back again: the dynamic evolution of panarthropod germ cell specification mechanisms

Germ cells enable the reproduction of an organism and the continuity of its lineage. These crucial cells are segregated from the soma at different times and places and via distinct mechanisms across the animal tree of life. Making sense of the evolution of germ cell specification across animals is complicated by the difficulty of making meaningful comparisons across the diversity of animal embryonic development. Here, we characterize germ cell specification within an ancient clade, that encompasses massive animal biodiversity but within which we can conduct meaningful comparative embryogenesis: the Panarthropoda. We amass data from centuries of studies describing the timing and mechanisms of germ cell formation, and apply ancestral state reconstruction to these data to propose novel hypotheses about the trajectory of evolution in this process. Furthermore, we speculate on the mechanisms underlying these evolutionary dynamics by considering the relationships among germ cell specification, concurrent developmental processes, and the germ line gene network. Collectively, this study derives new insights from a rich historical database of embryological observations, and has broad implications for understanding the evolution of metazoan germ cells.