Variability of mitochondrial ORFans hints at possible differences in the system of doubly uniparental inheritance of mitochondria among families of freshwater mussels (Bivalvia: Unionida).

The ORFan proteins show a comparable organization of secondary structures among different families of freshwater mussels, which supports a conserved physiological role, but also have distinctive family-specific features. Given this latter observation and the fact that the ORFans can be either highly mutated or completely absent in species that secondarily lost DUI depending on their respective family, we hypothesize that some aspects of the connection among ORFans, sexual systems, and DUI may differ in the various lineages of unionids.