Primordial germ cell expression of SSEA1 and DDX4 (VASA) in female Trichosurus vulpecula (Marsupialia) reveals conserved and unique molecular patterns during marsupial germ cell development.

Abstract

Development of primordial germ cells (PGCs: precursors to adult gametes) is a key process in vertebrate sexual differentiation. Marsupials are ideal to investigate this phenomenon because much of PGC migration and development unusually occurs postnatally in pouch young. However, investigation of the molecular dynamics underpinning PGC development is restricted to one marsupial model species: the tammar wallaby (Macropus eugenii). Given the reproductive diversity among clades, marsupial PGCs likely exhibit diversity in molecular patterns that could help uncover their developmental dynamics. Here we characterise PGC marker expression (SSEA1 and DDX4) in developing ovaries of the brushtail possum, Trichosurus vulpecula. Female germ cells expressed DDX4 from 6 days postpartum (dpp) and almost all germ cells expressed DDX4 by meiosis (40 dpp), consistent with M. eugenii and eutherian mammals. In contrast, PGCs and oogonia expressed SSEA1 from 12 dpp, throughout proliferation and until entry into meiosis (40-63 dpp). SSEA1 expression was temporally distinct from that of M. eugenii, in which SSEA1 expression persists only until 14 dpp, indicating differential expression between marsupial species at equivalent stages of germ cell development. Hence, the molecular characteristics of M. eugenii germ cells cannot be assumed for all marsupials, as at least one key molecule exhibits species-specific expression.