Differences in population structure estimated within maternally- and paternally-inherited forms of mitochondria in Lampsilis siliquoidea (Bivalvia: Unionidae)

Mussels in several orders possess two separate mitochondrial lineages: a standard female-inherited form and one inherited only through males. This system of doubly uniparental inheritance (DUI) for mitochondrial genes provides an opportunity to compare the population structure of gene-lineages passed either mother-to-daughter or father-to-son. In the present study, we contrast variation in the male and female haplotype lineages of the American freshwater mussel species, Lampsilis siliquoidea (sometimes called Lampsilis radiata luteola), throughout the Lake Erie, Ohio River, and upper Mississippi River watersheds, and contrast variation with the sequences obtained for the related species/subspecies Lampsilis radiata radiata from Maine. The genetic markers were fragments of the cytochrome c oxidase subunit I gene (COI), which occurs in both mitochondrial types, F (female) and M (male). High haplotype diversity was found in the two independent lineages, although purifying selection against amino acid change appeared to be stronger in the female than the male lineage. Phylogeographical patterns also varied between mitochondria passing through females and males. The female lineage exhibited more population structure, with the occurrence of private or nearly-private haplotypes within two streams, and three others showed restricted haplotype distributions. By contrast to the F-haplotypes, complex phylogenetic structure occurred for M-haplotypes, yet this phylogenetic variation coincided with almost no geographical pattern within haplotypes. Basically, F-haplotypes showed isolation, especially above physical barriers, whereas M-haplotypes did not. A few individuals in the eastern Lake Erie watershed even possessed M-haplotypes of an Atlantic Slope (L. radiata radiata) origin, although their F-haplotypes were typical of Midwestern L. siliquoidea. The finding that mussels package sperm as spermatozuegmata, which float downstream, may underlie greater gene mobility in male-inherited mitochondria.