Germ Cell Specification Requires Zygotic Mechanisms Rather Than Germ Plasm in a Basally Branching Insect

BACKGROUND Primordial germ cell (PGC) specification is a universal process across animals, but the molecular mechanisms specifying PGCs are remarkably diverse. In Drosophila, PGCs are specified by maternally provided, asymmetrically localized cytoplasmic factors (germ plasm). In contrast, historical literature on most other arthropods reports that PGCs arise from mesoderm during midembryogenesis, suggesting that an arthropod last common ancestor may have specified PGCs via zygotic mechanisms. However, there has been no direct experimental evidence to date for germ plasm-independent arthropod PGC specification. RESULTS Here we show that in a basally branching insect, the cricket Gryllus bimaculatus, conserved germ plasm molecules are ubiquitously, rather than asymmetrically, localized during oogenesis and early embryogenesis. Molecular and cytological analyses suggest that Gryllus PGCs arise from abdominal mesoderm during segmentation, and twist RNAi embryos that lack mesoderm fail to form PGCs. Using RNA interference we show that vasa and piwi are not required maternally or zygotically for PGC formation but rather are required for primary spermatogonial divisions in adult males. CONCLUSIONS These observations suggest that Gryllus lacks a maternally inherited germ plasm, in contrast with many holometabolous insects, including Drosophila. The mesodermal origin of Gryllus PGCs and absence of instructive roles for vasa and piwi in PGC formation are reminiscent of mouse PGC specification and suggest that zygotic cell signaling may direct PGC specification in Gryllus and other Hemimetabola.