RPHM21 is a recently discovered protein encoded by a male-specific mitochondrial genome, and with a putative role in the paternal inheritance of sperm mitochondria in the Manila clam Ruditapes philippinarum, a species characterized by the doubly uniparental inheritance of mitochondria (DUI). The available evidence suggests a viral origin of RPHM21 and supports its activity during spermatogenesis: RPHM21 is progressively accumulated in mitochondria and nuclei of spermatogenic cells, and we hypothesize it can influence mitochondrial inheritance and sexual differentiation.
We propose a testable model that describes how the acquisition of selfish features by a mitochondrial lineage might have been responsible for the emergence of DUI, and for the evolution of separate sexes (gonochorism) from hermaphroditism.
The appearance of DUI in a species most likely entailed the invasion of at least one selfish element, and the extant DUI systems can be seen as resolved conflicts. It was proposed that hermaphroditism was the ancestral condition of bivalves, and a correlation between DUI and gonochorism was documented. We hypothesize that DUI might have driven the shift from hermaphroditism to gonochorism, with androdioecy as a transition state. The invasion of sex-ratio distorters and the evolution of suppressors can prompt rapid changes among sex-determination mechanisms, and DUI might have been responsible for one of such changes in some bivalve species. If true, DUI would represent the first animal sex-determination system involving mtDNA-encoded proteins.