Hallström Björn M, Kullberg Morgan, Nilsson Maria A, Janke Axel
Department of Cell and Organism Biology, Division of Evolutionary Molecular Systematics, University of Lund, Lund, Sweden.
Mol Biol Evol. 2007 Sep;24(9):2059-68. doi: 10.1093/molbev/msm136. Epub 2007 Jul 13.
The phylogenetic positions of the 4 clades, Euarchontoglires, Laurasiatheria, Afrotheria, and Xenarthra, have been major issues in the recent discussion of basal relationships among placental mammals. However, despite considerable efforts these relationships, crucial to the understanding of eutherian evolution and biogeography, have remained essentially unresolved. Euarchontoglires and Laurasiatheria are generally joined into a common clade (Boreoeutheria), whereas the position of Afrotheria and Xenarthra relative to Boreoeutheria has been equivocal in spite of the use of comprehensive amounts of nuclear encoded sequences or the application of genome-level characters such as retroposons. The probable reason for this uncertainty is that the divergences took place long time ago and within a narrow temporal window, leaving only short common branches. With the aim of further examining basal eutherian relationships, we have collected conserved protein-coding sequences from 11 placental mammals, a marsupial and a bird, whose nuclear genomes have been largely sequenced. The length of the alignment of homologous sequences representing each individual species is 2,168,859 nt. This number of sites, representing 2840 protein-coding genes, exceeds by a considerable margin that of any previous study. The phylogenetic analysis joined Xenarthra and Afrotheria on a common branch, Atlantogenata. This topology was found to fit the data significantly better than the alternative trees.
真灵长总目、劳亚兽总目、非洲兽总目和异关节总目这4个分支的系统发生位置，一直是近期胎盘类哺乳动物基部关系讨论中的主要问题。然而，尽管付出了巨大努力，这些对于理解真兽类进化和生物地理学至关重要的关系，基本上仍未得到解决。真灵长总目和劳亚兽总目通常被归入一个共同的分支（北方真兽高目），而非洲兽总目和异关节总目相对于北方真兽高目的位置一直不明确，尽管使用了大量的核编码序列，或者应用了诸如反转录转座子等基因组水平的特征。这种不确定性的可能原因是，这些分歧发生在很久以前且时间窗口很窄，只留下了很短的共同分支。为了进一步研究真兽类基部关系，我们从11种胎盘类哺乳动物、1种有袋类动物和1种鸟类中收集了保守的蛋白质编码序列，这些物种的核基因组已基本完成测序。代表每个物种的同源序列比对长度为2,168,859个核苷酸。这个位点数量代表了2840个蛋白质编码基因，大大超过了以往任何研究。系统发生分析将异关节总目和非洲兽总目置于一个共同的分支——大西洋兽类上。发现这个拓扑结构比其他替代树显著更符合数据。
