Primordial germ cells (PGCs) are part of a unique and highly conserved cell lineage that gives rise to gametes. In mammals, PGCs arise outside of the embryo and must undergo migration in order to colonise the developing gonad. Once in the gonad, the surrounding somatic environment dictates their differentiation into sperm or eggs. The primary route for PGC migration in eutherian mammals is through the hindgut and dorsal mesentery. However, it has been noted that in marsupials, most PGCs instead migrate through the mesoderm surrounding the hindgut. This finding was interesting and unusual, considering the conserved nature of the germ cell lineage and consistency of their migratory path in eutherian mammals.
We looked at the process of sex determination in the fat-tailed dunnart (Sminthopsis crassicaudata), one of the most altricial marsupials at birth. To our surprise, we found that testis differentiation begins at 2 days post-partum (d.p.p.), and clear ovarian differentiation was evident by day 8 d.p.p. This is around the same time relative to birth as seen in less altricial marsupials. Thus, gonad formation in the dunnart occurred at a much earlier developmental stage than seen in other marsupials. We also found that PGCs enter the gonads of both sexes around 2 d.p.p. We noticed that in this species, PGCs appear to be migrating through the hindgut, as they do in eutherian mammals, suggesting that perhaps germ cell migration follows a conserved route after all.