Endogenized mitochondrial gene expressed in the germ line

Germ line is the cell lineage that transmits genes from generation to generation in sexually reproducing animals. Mitochondria, among many other factors, are responsible of the specification of primordial germ cells (PGCs), and their inheritance dynamics are relevant to understand this process. We studied these issues in the Manila clam Ruditapes philippinarum, a species with Doubly Uniparental Inheritance (DUI) of mitochondria, in which a novel protein (RPHM21) is encoded by the male-transmitted mtDNA and expressed in spermatozoa. RPHM21 was predicted in silico to be of viral origin – namely an endogenized residual of an ancestral infection. We analyzed mitochondrial transcription and expression patterns using qPCR and antibodies targeting the germ line marker VASPH (VASA homolog) and RPHM21. qPCR allowed to discriminate the phases in which vasph begins to be transcribed, and VASPH immunolabelling allowed to identify PGCs. Both targets were localized in the PGCs of males, but while VASPH was detected in all PGCs, RPHM21 appeared to be expressed only in a subpopulation. In R. philippinarum we identified a source of germ cell precursors that migrate from the gut and reach the presumptive gonad localization, where they rebuild the gonad at every reproductive season. Since RPHM21 might have a role in activation, proliferation, and migration of male PGCs, we propose that the germ cells expressing RPHM21 could gain advantage over others during spermatogenesis. The investigation of how co-option of foreign elements modifies existing biological pathways is fundamental to assess the impact of such events on the evolution of new developmental features.