从12SrRNA部分基因序列探讨鳄的系统发生关系

本文用常规方法测定了雅卡凯门鳄（caiman yacane）、新几内亚鳄（crocodylus novaeguineae）等2种鳄类的约400bp的mtDNA12SrRNA基因片段的部分序列,所测序列与GenBank中的其他18种鳄类的12SrRNA基因相应片段同源序列一起经Clustal X1．8软件进行比对,并以乌龟（chinemys reevesii）为外群,用ClustalX（1．8）,Mega2软件以及paup软件将这20种鳄的12SrRNA基因部分序列进行比对后构建了20种鳄的进化关系树,并分别用NJ树,MP树探讨它们之间的系统发生关系。其结果显示：食鱼鳄属Gavialis与马来假食鱼鳄属Tomistoma的亲缘关系较近,构成姐妹群并位于鳄科分支的基部。非洲窄吻鳄（Crocodylus cataphractus）与骨喉鳄（Osteolaemus retraspis）形成姐妹群,它们的亲缘关系比与其它真鳄类的关系更近,赞同非洲窄吻鳄应从鳄属中独土出来归属于长吻鳄属Mecistops的观点,因此鳄科包括鳄属、骨喉鳄属和长吻鳄属,它们的关系为（鳄属（骨喉鳄属,长吻鳄属））。鼍科4属中凯门鳄属与黑鳄属关系最近,接下来的是古鳄属、鼍属,而且鼍属是鼍科中起源最早的类群。扬予鳄（Alligator sinensis）与密河鳄（Alligator mississippiensis）较近的亲缘关系得到支持,但它们并没有形成姐妹群关系。

Molecular Phylogeney of Crocodilifonnes Crocodilifomes Inferred from Mitochondrial 12S rRNA DNA Sequences

The crocodilifonnes crocodilifomes comprise three familiesy （Alligatoridae, Crocodylidae and Gavialidae） and about 23 pecies in the world. But three large group and the relationship between each group including the species, which have not been consistent. The intergenus and interspecies relationships of extant Crocodylia have became the focus of systematic biological studies. To ascertain the phylogenetic relationships and some long --standing taxonomic problems among 23 crocodile species, we detected part of the 12S rRNA gene sequences Based on 12S rRNAindividual gene sequence and concatenate datasets, Maximun parsimony method, Maximum likelihood method and Bayesian inference Neighbor--Joining method were applied to reconstructed phylogenetic tree of crocodiles. The results supported controversial genus Gavialis and Tomistoma had closer relationship, they formed a sister group and placed at the base ofal to Crocodylidae. The relationship between African slender--snouted crocodile （Crocodylus cataphractus） and dwarf crocodile （Osteolaemus tetraspis） was closer than other true crocodiles, which implied African slender--snouted crocodile should separate from genus Crocodylus form another single genus Mecistops. Thus, Crocodylidae should be composed of Crocodylus, Osteolaemus, Mecistops, and their relationships were （Crocodylus （Osteolaemus, Mecistops））. In conclusion, further research about the relationship between Gavialis andTomistoma, Alligator sinensis and Alligator mississippiensis, Crocodylus and Crocodylua cataphractu, need be taken in the furture. The proposed future research should focus on looking for a molecular genetic marker of faster evolution rate, and this can be explained the relationship of the evolutionary differences relatively late species in Crocodiliformes, our molecular data conflict with orthodox morphological systematic studies, suggesting further research should be taken in the furture.