Population dynamics of notothenioid fishes. - Examination of adaptive evolution and neutral evolution -

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In this project, the origin of antifreeze glycopeptides of notothenioid fishes was estimated to be about 21.8-24.3 million years ago, which corresponded to the time when fishes of notothens, plunderfishes, dragonfishes, and icefishes, were about to be diverged.Near et al. (2006) have recently identified that one species of icefish has retained alpha globin gene but not other 15 species of icefishes and have suspected that at least the two haplotypes had coexisted for the period of successive icefish speciations. A large effective population size is expected for two pseudogene haplotypes to be maintained neutrally for millions of years but various estimates of effective population size inferred for several existing species from mtDNA polymorphism (several hundred thousands to millions) are barely the minimum effective size of population to account for the situation. Considering that the most species studied have likely been subject to recent population expansion and that apparent positive selection is detected for the icefish alpha hemoglobin gene, some type of selection is speculated to maintain allelic pseudogenes for prolonged period in the past.Despite of limited nucleotide diversity of mitochondrial gene observed for Antarctic toothfish Dissostichus mawsoni, nucleotide sequence variation among individuals is detected at major histocompatibility complex gene class II beta genes, indicating larger population size of the species in the past. For the immunoglobulin gene, nucleotide duplication has caused rapid change of protein structure as in the case of antifreeze glycopeptides and it raised possibility that gene and nucleotide duplications have played significant roles in the evolution of notothenioid genome.