| dc.description | In the blue mussel 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 has been termed doubly uniparental inheritance (DUI) of mtDNA. In this study, the behavior of sperm mitochondria in M. edulis embryos was monitored. In female embryos, sperm mitochondria are randomly dispersed among blastomeres. In male embryos, sperm mitochondria tend to aggregate and land in one blastomere at the 2- and 4-cell stages. It is postulated that this aggregate eventually ends up in the first germ cells, thus accounting for the presence of paternal mtDNA in the male gonad. To investigate the mechanism of sex-ratio determination in Mytilus, pedigreed crosses were made for several generations. The data confirm the previous observations that in mussel crosses there is a strong sex-ratio bias and that the bias is a characteristic of the female. Furthermore, the data suggest that this bias is controlled by the mother's nuclear rather than mitochondrial genotype. A female-dependent sex determination model is proposed and tested by the data from the crosses made with wild mothers. To understand mtDNA features of DUI species, sequences from Mytilus F(emale) and M(ale) lineages involving various regions (the large unassigned region (LUR), 16S, 12S, tRNAs, COI and Cytb) were compared. Secondary structure modeling of 16S rRNA, 12S rRNA and tRNAs was performed. The LUR is identified as the control region of mussel mtDNA and appears to contain gender-specific motifs. M genomes evolve faster than F genomes for all regions examined, but they resemble each other in terms of RNA secondary structures, base compositions and codon usage. Different start codons are found to be used for the Cytb gene by M. trossulus F and M molecules. The location of the light strand replication origin is speculated. The base composition in the third codon positions (P3) and four-fold degenerate sites (P4FD) was analyzed for each of 12 M. edulis F mtDNA genes. In both P3 and P4FD the nucleotide frequencies for each gene are correlated with the gene's duration at the single-stranded state during replication and transcription, suggesting that an asymmetrical composition bias exists and that the bias is the main source shaping the codon usage. The hydrolytic deamination and oxidation of mtDNA during replication and transcription are likely responsible for the composition asymmetry in mussels. | en_US |
