Mitochondrial phylogeny of notothenioids: a molecular approach to Antarctic fish evolution and biogeography

Antarctic waters represent a unique marine environment delimited by an oceanographic barrier, the Polar Front Zone, and characterized by constant subzero temperatures and presence of sea ice. A group of teleost fish, the Notothenioidei, have adapted to these challenging environmental conditions, undergoing a remarkable diversification. In the present study a total of 798 base pairs, generated from partial sequencing of 16S and 12S mitochondrial ribosomal RNA genes, were examined in 33 notothenioid species representative of all families included in the suborder Notothenioidei. Phylogenetic trees, reconstructed on the basis of sequence data using different methods, indicate that traditional hypotheses on notothenioid systematics and biogeography might be in need of reexamination. Molecular evidence suggests that vicariant speciation could be invoked to explain the early divergence of Eleginops maclovinus, a species previously included in the family Nototheniidae, which is now proposed as the closest sister group to all the rest of notothenioids apart from bovichtids. On the other hand, repeated, independent dispersal through the Polar Front is proposed for the divergence of other subantarctic notothenioid species. Likewise, multiple, independent transitions from benthic to pelagic habit are inferred from molecular data, at variance with the more conservative hypothesis based on cladograms reconstructed from morphological data.

南极水域是受海洋学屏障——极锋带（Polar Front Zone）所界定的独特海洋环境，其特征为持续的亚零低温与海冰广泛分布。一类隶属于南极鱼亚目（Notothenioidei）的真骨鱼类，已适应了这一极具挑战性的环境条件，并经历了显著的物种辐射演化。本研究针对南极鱼亚目所有科的代表类群，共选取33种南极鱼类，对其16S与12S线粒体核糖体RNA基因的部分测序片段（总长798碱基对）开展了分析。基于序列数据通过多种方法构建的系统发育树显示，此前关于南极鱼类系统分类学与生物地理学的传统假说或需重新审视。分子证据表明，可通过异域物种形成解释马氏南冰鱼（Eleginops maclovinus）的早期分化——该物种此前被归入南极鱼科（Nototheniidae），现被认为是除牛帆鱼科（Bovichtidae）外所有其余南极鱼类的最近姊妹群。另一方面，其余亚南极区域的南极鱼类的分化则被认为是通过多次独立穿越极锋带的扩散事件所促成。同样，分子数据推断存在多次独立的底栖-远洋生活习性转变，这与基于形态学数据构建的分支图所提出的更为保守的假说相悖。