| Summary: | Metazoans normally possess a single lineage of mitochondria inherited from the mother (female-type mitochondria) while paternal mitochondria are absent or eliminated in fertilized eggs. In doubly uniparental inheritance (DUI), which is specific to the bivalve Glade including the ocean quahog, Arctica islandica, male-type mitochondria are retained in male gonads and, in a few species, small proportions of male-type mitochondria co-exist with female-type in somatic tissues. To the best of our knowledge, we report, for the first time in metazoan, the natural occurrence of male and female individuals with exclusively male-type mitochondria in somatic tissues of the bivalve A. islandica. Mitochondrial genomes differ by similar to 5.5% at DNA sequence level. Exclusive presence of male-type mitochondria affects mitochondrial complexes partially encoded by mitochondrial genes and leads to a sharp drop in respiratory capacity. Through a combination of whole mitochondrial genome sequencing and molecular assays (gene presence and expression), we demonstrate that 1) 11% of individuals of an Icelandic population appear homoplasmic for male-type mitochondria in somatic tissues, 2) male-type mitochondrial genes are transcribed and 3) individuals with d-type mitochondria in somatic cells lose 30% of their wild-type respiratory capacity. This mitochondrial pattern in A. islandica is a special case of DUI, highlighted in individuals from both sexes with functional consequences at cellular and conceivably whole animal level.
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