Elements of divergence in germline determination in closely related species

Evolutionary transitions are particularly important in development of the germ line, cells which directly impact sexual reproduction. Differences in the primordial germ cells (PGCs) of two sea urchin species were examined here by stage-matched, integrated, single cell RNA-seq (scRNA-seq) datasets. Even though both species rely on inherited mechanisms to specify their germ line, this analysis revealed a variety of differences in germline gene expression, including a broader expression of the germline factor Nanos2 (Nan2) in Lytechinus variegatus (Lv) compared to Strongylocentrotus purpuratus (Sp). In Sp, Nan2 mRNA expression is highly restricted to the PGCs by a lability element in its 3'UTR, which is lacking in the mRNA of Lv-Nan2, thus explaining the difference. We discovered that the Lv-Nan2 3'UTR instead leads to its specific translation in the PGCs. The results emphasize that regulatory mechanisms resulting in germline specification rely greatly on post-transcriptional restrictions of key gene products.