Elements of divergence in germline determination in closely related species

Evolutionary transitions enable the wide diversity in life histories of plants and animals. This is particularly germane in the development of the germ line in which fitness is a direct readout of evolutionary change. Here, we focused on the gem line of two distinct sea urchin species who shared a common ancestor 50 million years ago. Even though they both rely on inherited mechanisms to specify their germ line, the integration of stage-matched single cell RNA-seq (scRNA-seq) datasets from these two sea urchins revealed a variety of differences in gene expression, including a broader expression of the germ line factor Nanos2 in Lytechinus variegatus ( Lv ) compared to Strongylocentrotus purpuratus ( Sp ). In Sp, Nanos2 mRNA expression is highly restricted to the primordial germ cells (PGCs) by a lability element in its 3’UTR. This element is lacking in the mRNA of Lv Nanos2 , explaining how this mRNA more broadly accumulates in the Lv embryos. We discovered that the Lv Nanos2 3’UTR instead leads to a germline specific translation of the protein. The results emphasize that regulatory mechanisms resulting in germline diversity rely less on transcriptional regulation and more on post-transcriptional and post-translational restrictions of key gene products, such as Nanos2. Highlights - The first integration of scRNA-seq datasets comparing two echinoderm species. - We find Nanos2 positive cells in the embryonic soma of Lytechinus variegatus , an unusual occurrence, but not in Strongylocentrous purpuratus . - We discovered that this somatic Nanos2 mRNA is lacking an important regulatory element (GNARLE) in its 3’UTR - Instead, in Lv , the 3’UTR of Nanos2 leads to its specific translation in the germ cells.