The exceptional mitochondrial DNA system of the mussel family Mytilidae

Species of the families Mytilidae (sea mussels) and Unionidae (fresh water mussels) contain two types of mitochondrial DNA (mtDNA), the F that behaves as the standard animal mtDNA and the M that is transmitted through the spe rm and establishes itself only in the male gonad. The two molecules have, t herefore, separate transmission routes, one through the female and the othe r through the male lineage. The system has been named doubly uniparental in heritance (DUI). Another important feature of sea mussels is that the sex r atio among offspring of a pair mating is determined by the female parent on ly. The mechanism of DUI remains unknown. One hypothesis that is consistent with all observations is that the standard maternal inheritance was modifi ed in mussels via the evolution of a suppressor gene that is expressed duri ng oogenesis and has two alleles, the inactive and the active allele. In th e presence of the active allele in the mother's genotype the egg is supplie d with a substance that interferes and the normal mechanism of elimination of sperm mitochondria. This will explain why half of mussels have the fathe r's mtDNA and half do not, but would not explain why presence/absence of pa ternal mtDNA is linked with the male and female gender, respectively. To pr ovide an explanation for this linkage, one would have to assume that there is a causal relationship between retention of paternal mtDNA and sex determ ination.