Animal mitochondrial genomes are typically circular, 14-20 kb in length, maternally inherited, contain 13 coding genes, two ribosomal genes and are homoplasmic. In contrast, plant mitogenomes display frequent gene rearrangements, often contain greatly expanded repetitive regions, encode various open reading frames of unknown function and may be heteroplasmic due to differential repeat expansions between molecules. Error correction by recombination is common in plant mitochondria and has been

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... posed as the driver behind the rearrangements and repeat expansions that are often observed. In contrast, most animal mitochondria never or only very seldomly recombine and their utilisation of other repair mechanisms for mitochondrial genome error correction is a potential driver of their non-coding DNA reduction. Bivalve molluscs are one of the few animal groups in which direct observation of mitochondrial recombination has been repeatedly demonstrated and this coincides with numerous reports of extensive variation in gene order, genome length and repeat content. Here we present the expanded 46 kb mitochondrial genome of the quagga mussel, Dreissena rostriformis and demonstrate the use of PacBio long reads to identify heteroplasmy in the form of variable repeat lengths within two repetitive blocks of the sequenced individual. We investigate the transcriptional dynamics of the mitochondria over the course of development and provide evidence that bivalve mollusc mitochondrial evolutionary dynamics are influenced by similar processes to those that occur in plants.