Gatesy J, Hayashi C, Cronin M A, Arctander P
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson 85721.
Mol Biol Evol. 1996 Sep;13(7):954-63. doi: 10.1093/oxfordjournals.molbev.a025663.
The inferred transition from terrestrial hoofed mammal to fully aquatic cetacean has been intensively studied with fossil evidence. However, large sections of this remarkable evolutionary sequence are missing. Phylogenetic analysis of extant taxa may help to fill in some of these gaps. In this report, kappa-casein (exon 4) and beta-casein (exon 7) milk protein genes from cetaceans and other placental mammals were PCR-amplified, sequenced, and aligned to previously published sequences. Phylogenetic analyses of the casein data suggest that hippopotamid artiodactyls are more closely related to cetaceans than to other artiodactyls (even-toed hoofed mammals). An analysis of the nuclear casein sequences combined with published mitochondrial cytochrome b DNA sequences also supports the Cetacea/Hippopotamidae sister group. This affinity implies that some of the aquatic traits of cetaceans were derived in the common ancestor of Cetacea and Hippopotamidae. An extant "missing link" to Cetacea may have been overlooked by science since the description of the semiaquatic Hippopotamus in 1758. Paleontological information is grossly inconsistent with this hypothesis. If the casein phylogeny is accurate, large gaps in the fossil record as well as extensive morphological reversals and convergences must be acknowledged.
利用化石证据，人们对从陆生有蹄类哺乳动物到完全水生的鲸类动物的推断过渡进行了深入研究。然而，这一显著进化序列的大部分内容缺失。对现存分类群的系统发育分析可能有助于填补其中一些空白。在本报告中，对鲸类动物和其他胎盘哺乳动物的κ-酪蛋白（外显子4）和β-酪蛋白（外显子7）乳蛋白基因进行了PCR扩增、测序，并与先前发表的序列进行比对。酪蛋白数据的系统发育分析表明，河马科偶蹄目动物与鲸类动物的关系比与其他偶蹄目动物（偶蹄有蹄类哺乳动物）的关系更为密切。对核酪蛋白序列与已发表的线粒体细胞色素b DNA序列的分析也支持鲸目/河马科姐妹群。这种亲缘关系意味着鲸类动物的一些水生特征是在鲸目和河马科的共同祖先中衍生出来的。自1758年对半水生河马进行描述以来，鲸类动物现存的“缺失环节”可能一直被科学界忽视。古生物学信息与这一假设严重不符。如果酪蛋白系统发育是准确的，那么就必须承认化石记录中的巨大空白以及广泛的形态逆转和趋同现象。
