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

本项目估计南极鱼的抗冻糖肽的起源大约在2180万~ 2430万年前，与南极鱼、掠鱼、龙鱼、冰鱼等鱼类即将分化的时期相对应。Near等人（2006）最近发现，一种冰鱼保留了α -珠蛋白基因，而其他15种冰鱼没有，并怀疑至少这两种单倍型在连续的冰鱼物种时期共存。预计两个假基因单倍型在数百万年的时间里保持中立，但从mtDNA多态性推断出的几种现有物种的有效种群大小的各种估计（几十万到几百万）几乎不能解释这种情况的最小有效种群大小。考虑到所研究的大多数物种可能受到最近种群扩张的影响，并且在冰鱼α血红蛋白基因中检测到明显的正选择，推测在过去的一段时间内，某种类型的选择使等位伪基因维持了很长时间。尽管南极齿鱼Dissostichus mawsoni线粒体基因的核苷酸多样性有限，但在主要组织相容性复合体基因II类β基因上发现了个体间的核苷酸序列差异，表明该物种过去的种群规模较大。对于免疫球蛋白基因来说，核苷酸重复引起了蛋白质结构的快速变化，就像抗冻糖肽一样，这就提出了基因和核苷酸重复在南极鱼基因组进化中发挥重要作用的可能性。

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.