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.