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Species of the mussel genus Mytilus possess maternally and paternally transmitted mitochondrial genomes. In the interbreeding taxa Mytilus edulis and M. galloprovincialis, several genomes of both types have been fully sequenced. The genome consists of the coding part (which, in addition to protein and RNA genes, contains several small noncoding sequences) and the main control region (CR), which in turn consists of three distinct parts: the first variable (VD1), the conserved (CD), and the second variable (VD2) domain. The maternal and paternal genomes are very similar in gene content and organization, even though they differ by >20% in primary sequence. They differ even more at VD1 and VD2, yet they are remarkably similar at CD. The complete sequence of a genome from the closely related species M. trossulus was previously reported and found to consist of a maternal-like coding part and a paternal-like and a maternal-like CR. From this and from the fact that it was extracted from a male individual, it was inferred that this is a genome that switched from maternal to paternal transmission. Here we provide clear evidence that this genome is the maternal genome of M. trossulus. We have found that in this genome the tRNAGln in the coding region is apparently defective and that an intact copy of this RNA occurs in the CR, that one of the two conserved domains is missing essential motifs, and that one of the two first variable domains has a high rate of divergence. These features may explain the large size and mosaic structure of the CR of the maternal genome of M. trossulus. We have also obtained CR sequences of the maternal and paternal genomes of M. californianus, a more distantly related species. We compare the control regions from all three species, focusing on the divergence among genomes of different species origin and among genomes of different transmission routes. [ABSTRACT FROM AUTHOR]

Both the maternal (F-type) and paternal (M-type) mitochondrial genomes of the Mytilus species complex M. edulis/galloprovincialis contain a noncoding sequence between the l-rRNA and the tRNATyr genes, here called the large unassigned region (LUR). The LUR, which is shorter in M genomes, is capable of forming secondary structures and contains motifs of significant sequence similarity with elements known to have specific functions in the sea urchin and the mammalian control region. Such features are not present in other noncoding regions of the F or M Mytilus mtDNA. The LUR can be divided on the basis of indels and nucleotide variation in three domains, which is reminiscent of the tripartite structure of the mammalian control region. These features suggest that the LUR is the main control region of the Mytilus mitochondrial genome. The middle domain has diverged by only 1.5% between F and M genomes, while the average divergence over the whole molecule is ∼20%. In contrast, the first domain is among the most divergent parts of the genome. This suggests that different parts of the LUR are under different selection constraints that are also different from those acting on the coding parts of the molecule. [ABSTRACT FROM AUTHOR]

In Mytilus, females carry predominantly maternal mitochondrial DNA (mtDNA) but males carry maternal mtDNA in their somatic tissues and paternal mtDNA in their gonads. This phenomenon, known as doubly uniparental inheritance (DUI) of mtDNA, presents a major departure from the uniparental transmission of organelle genomes. Eggs of Mytilus edulis from females that produce exclusively daughters and from females that produce mostly sons were fertilized with sperm stained with MitoTracker Green FM, allowing observation of sperm mitochondria in the embryo by epifluorescent and confocal microscopy. In embryos from females that produce only daughters, sperm mitochondria are randomly dispersed among blastomeres. In embryos from females that produce mostly sons, sperm mitochondria tend to aggregate and end up in one blastomere in the two- and four-cell stages. We postulate that the aggregate eventually ends up in the first germ cells, thus accounting for the presence of paternal mtDNA in the male gonad. This is the first evidence for different behaviors of sperm mitochondria in developing embryos that may explain the tight linkage between gender and inheritance of paternal mitochondrial DNA in species with DUI. [ABSTRACT FROM AUTHOR]

Explores the genetics of mother-dependent sex ratio in blue mussels. Implications for doubly uniparental inheritance of mitochondrial DNA; Model for mother-dependent sex determination; Fitting the model to sex ratios produced by wild-caught animals.