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Title Η αναστροφή του τρόπου μεταβίβασης του μιτοχονδριακού DNA στα δίθυρα και οι φυλογενετικές της προεκτάσεις
Alternative Title The reversal of the transmission route of mitochondrial DNA in bivalves and its phylogenetic implications
Author Θεολογίδης Ιωάννης
Thesis advisor Ζούρος, Ελευθέριος
Abstract Mitochondrial DNA (mtDNA) is maternally inherited in most metazoan species. In contrast, biparental transmission is the standard mode of mtDNA inheritance in several bivalve species. Female individuals inherit only the maternal genome (type F) and are homoplasmic, while males inherit, not only the maternal, but also the paternal genome (type M) and are heteroplasmic for both types. This mode of mtDNA transmission is termed Doubly Uniparental Inheritance (DUI) and it was, until recently, detected in five bivalve families (Mytilidae, Unionidae, Hyriidae, Margaritiferidae and Veneridae). In mussels of genus Mytilus, which are the best studied organisms with DUI, the two mtDNA types (F, M) differ by 20%. In natural populations of Mytilus there have been observed male individuals that do not contain the M molecule (atypical males). These males are homoplasmic for one F type, or heteroplasmic for two F types. This observation provided the basis for the masculinization or role reversal hypothesis, according to which an F genome may become paternally transmitted. The masculinized molecule functions as M. Masculinization has important implications for mechanism and evolution of DUI. However, it is not yet understood, and the observations that support its existence are fragmentary. We tested the hypothesis of masculinization in a systematic basis by studying the content of gonadal and somatic tissues extracted from atypical Mytilus galloprovincialis male mussels from populations of the Mediterranean and the Black Sea. We found that, in all atypical males, the gonad contained a genome that carried a compound control region. The 5 and 3 flanking domains of the control region of this genome were of type F, whereas its central domain consisted of tandemly repeated Mtype elements. This new genome was called C (Compound) and it was fully sequenced. Apart from its control region, the C genome was an F-like genome and it was considered to be masculinized because it was present in all atypical male gonads. The detection of the C genome provides direct evidence for the masculinization hypothesis in Mytilus galloprovincialis. It was also detected in the gonads of atypical Mytilus edulis males sampled from the eastern coast of Canada, a finding that suggests a common origin of masculinized genomes in the sibling species M. galloprovincialis and M. edulis. The population study for the C genome revealed that homoplasmic males do not exist in Mytilus galloprovincialis. This observation rejects the previous hypothesis that masculinization events may have origined from failure of the male mussel to inherit the paternal molecule and subsequent invasion of a typical F-type molecule into the male transmission route. According to the new evidence, masculinization seems to be always associated with the existence of the C genome. Masculinization can affect the phylogenetic relationships of maternal and paternal mtDNA molecules in the class of bivalvia, because it sets the divergence time between the maternal and paternal genomes to zero. Yet, evidence for the distribution of DUI in bivalvia is incomplete. For this reason, we tried to detect DUI in other genera and families. We tested some representatives of Veneridae, Arcidae and Donacidae. It was revealed that male individuals of species Donax trunculus (Bivalvia, Donacidae) were heteroplasmic for the two genetic loci tested (Cytb and lrRNA), whereas females were homoplasmic, containing only one of the two types that were found in males. This observation is a landmark of DUI and provides evidence for the existence of this system of mtDNA inheritance in a new bivalve family. After the addition of Donacidae in the list of families with DUI, we proceeded to produce a common phylogenetic analysis of F and M molecules. We analyzed three partial mtDNA genes (Cytb, COI and lrRNA) using sequence data from five families (Mytilidae, Unionidae, Hyriidae, Veneridae, Donacidae). According to the analysis, and by evaluating the potential role of masculinization in phylogeny, we discuss the two dominant hypotheses about the distribution of DUI. The first hypothesis claims that DUI has had multiple and independent origins during the evolution of bivalvia, as many, as the number of the distinct clusters that the maternal and the paternal molecules form together in phylogenetic trees. The alternative hypothesis assumes that DUI origined only once in primitive bivalves and it was subsequently retained or secondarily lost in several genera and families. Although the second hypothesis cannot by itself explain the phylogeny of the maternal and paternal molecules, we proposed that, if combined with the phenomenon of masculinization, it provides a more plausible scenario than the hypothesis of multiple origins.
Language Greek
Issue date 2007-05-11
Collection   School/Department--School of Sciences and Engineering--Department of Biology--Doctoral theses
  Type of Work--Doctoral theses
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