Doubly Uniparental Inheritance (DUI), a peculiar way of inheritance of mitochondrial DNA in animals, has been detected in seven families of marine and freshwater bivalves, including Unionidae. DUI involves two independently inherited mitochondrial genomes: maternal (F genome) and paternal (M genome), which show different tissue localisation and wide genetic variation. F genomes occur in somatic tissues of both sexes and are inherited maternally (Strict Maternal Inheritance, SMI). M genomes are

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... ocated in male germ cells and transmitted to next generations along the male lineage, i.e. from fathers to male offspring. The objective of this study was detection of M genomes and characteristics of DUI in unionid bivalves from Poland, based on sequential analyses of seven mitochondrial genes. This is the study to analyse F and M haplotypes at intra-and interspecific level in seven species of freshwater mussels. DUI was first observed in species of the genus Unio (U. crassus, U. pictorum and U. tumidus), and the best M haplotype marker was gene cox1. In the studied bivalves F and M sequences showed a similar intraspecific variation, with differences among the genes. Three tRNA genes showed the smallest (ca. 20%) nucleotide variation, followed by the gene coding for RNA for the small ribosomal subunit, srRNA (24%); a significantly greater variation (exceeding 30%) was recorded for protein-coding genes (cox1, cytb) and the gene coding for RNA for the large ribosomal subunit, lrRNA. Interspecific variation of F sequences of the studied unionids ranged from 5% for tRNAs to 18% for cytb. Higher values were observed for M sequences: from 7% for tRNAs to 19% for cox1. The Chinese mussel occurring in Poland, despite the morphology-based identification as Anodonta / Sinanodonta woodiana, proved to be genetically more similar to A. arcaeformis than to Asian specimens of A. woodiana. Phylogenetic analyses showed that in the genus Unio the youngest species were U. pictorum and U. mancus, and the earliest species was U. tumidus showing the greatest genetic distinctness.