Heteroplasmy in a deep-sea protobranch bivalve suggests an ancient origin of doubly uniparental inheritance of mitochondria in Bivalvia

Most metazoan species have strict maternal inheritance of the mitochondrial genome. In bivalves, a unique inheritance pattern called doubly uniparental inheritance (DUI) occurs in at least seven bivalve families. In this system of mitochondrial inheritance, males inherit and carry mtDNA from both parents, while females only carry mtDNA from the mother. Here, we present evidence of mitochondrial heteroplasmy in deep-sea protobranch bivalves. Divergent 16S rRNA and cytochrome b sequences were obtained within individuals of Ledella ultima. Ledella sublevis also exhibited divergent 16S sequences. Levels of divergence between 16S sequences within individuals were 27 and 15 % for each species, respectively. Ratios of homoplasmic to heteroplasmic individuals were not significantly different from 1:1, in agreement with sex ratios in protobranchs. The results provide the first evidence for mitochondrial heteroplasmy in the protobranchs and suggest DUI might have evolved much earlier in the evolution of the Bivalvia than previously thought.