An unusual case of Gender-Associated Mitochondrial DNA heteroplasmy: the Mytilid Musculista senhousia (Mollusca: Bivalvia).

A new case of Doubly Uniparental Inheritance (DUI) of mitochondrial DNA is described in the mussel Musculista senhousia (Mollusca: Bivalvia: Mytilidae). Its heteroplasmy pattern, showing male (M) and female (F) mitochondrial haplotypes, is in line with standard DUI. However, one trait appears absolutely unexpected: F haplotypes sequence variability is significantly higher than that of M haplotypes. This is the first time that such feature is observed in a DUI system, and it is challenging most of the rationales proposed to account for sex-linked mtDNA evolution. Moreover, the system, tested for F mtDNA variability in somatic tissues, shows that F mitochondrial haplotypes experience a higher mutation rate in males than in females, thus suggesting that there might be some mechanism to keep a low mitochondrial DNA mutation rate in females. This fits well with evolutionary predictions: antioxidant gene complexes, evolved to protect mitochondria from oxidative damages, might be under relaxed selection in males. Phylogenetic analysis clusters together M and F types, suggesting that M. senhousia may have experienced ‘masculinization’ events, occurring in the lineage leading to the taxon; this also suggests that during the evolutionary history ‘masculinization’ might have been present in mytilids, other than Mytilus.