MALE-DEPENDENT DOUBLY UNIPARENTAL INHERITANCE OF MITOCHONDRIAL-DNA AND FEMALE-DEPENDENT SEX-RATIO IN THE MUSSEL MYTILUS-GALLOPROVINCIALIS

We have investigated sex ratio and mitochondrial DNA inheritance in pa ir-matings involving fire female and five male individuals of the Medi terranean mussel Mytilus galloprovincialis. The percentage of male pro geny varied widely among families and was found to be a characteristic of the female parent and independent of the male to which it was mate d. Thus sex-ratio in Mytilus appears to be independent of the nuclear genotype of the sperm. With a few exceptions, doubly uniparental inher itance (DUI) of mtDNA was observed in all families fathered by four of the five males: female and male progeny contained the mother's mtDNA (the F genome), but males contained also the father's paternal mtDNA ( the M genome). Two hermaphrodite individuals found among the progeny o f these crosses contained the F mitochondrial genome in the female gon ad and both the F and M genomes in the male gonad. All four families f athered by the fifth male showed the standard maternal inheritance (SM I) of animal mtDNA: both female and male progeny contained only the ma ternal mtDNA. These observations illustrate the intimate linkage betwe en sex and mtDNA inheritance in species with DUI and suggest different major roles for each gender. We propose a model according to which de velopment of a male gonad requires the presence in the early germ cell s of an agent associated with sperm-derived mitochondria, these mitoch ondria are endowed with a paternally encoded replicative advantage thr ough which they overcome their original minority in the fertilized egg and this advantage (and, therefore, the chance of an early entrance i nto the germ line) is countered by a maternally encoded egg factor.