Department of Anatomy, School of Dental Medicine, Tsurumi University, Yokohama, Japan.
Mol Biol Evol. 2011 Jun;28(6):1943-56. doi: 10.1093/molbev/msr015. Epub 2011 Jan 27.
The study describes >400 major histocompatibility complex (MHC) class II B exon 2 and 114 intron 2 sequences of 36 passerine bird species, 13 of which belong to the group of Darwin's finches (DFs) and the remaining 23 to close or more distant relatives of DFs in Central and South America. The data set is analyzed by a combination of judiciously selected statistical methods. The analysis reveals that reliable information concerning MHC organization, including the assignment of sequences to loci, and evolution, as well as the process of species divergence, can be obtained in the absence of genomic sequence data, if the analysis is taken several steps beyond the standard phylogenetic tree construction approach. The main findings of the present study are these: The MHC class II B region of the passerine birds is as elaborate in its organization, divergence, and genetic diversity as the MHC of the eutherian mammals, specifically the primates. Hence, the reported simplicity of the fowl MHC is an oddity. With the help of appropriate markers, the divergence of the MHC genes can be traced deep in the phylogeny of the bird taxa. Transspecies polymorphism is rampant at many of the bird MHC loci. In this respect, the DFs behave as if they were a single, genetically undifferentiated population. There is thus far no indication of alleles that could be considered species, genus, or even DF group specific. The implication of these findings is that DFs are in the midst of adaptive radiations, in which morphological differentiation into species is running ahead of genetic differentiation in genetic systems such as the MHC or the mitochondrial DNA. The radiations are so young that there has not been enough time to sort out polymorphisms at most of the loci among the morphologically differentiating species. These findings parallel those on Lake Victoria haplochromine fishes. Several of the DF MHC allelic lineages can be traced back to the MHC genes of the species Tiaris obscura, which we identified previously as the closest extant relative of DFs in continental America.
该研究描述了 36 种雀形目鸟类的>400 个主要组织相容性复合体（MHC）II B 外显子 2 和 114 个内含子 2 序列，其中 13 种属于达尔文雀（DFs），其余 23 种属于中美洲和南美洲 DFs 的近亲或更远的亲缘关系。该数据集通过明智选择的统计方法组合进行分析。分析表明，如果分析超出标准系统发育树构建方法的几个步骤，可以在没有基因组序列数据的情况下获得有关 MHC 组织、包括序列分配给基因座以及进化和物种分化过程的可靠信息。本研究的主要发现如下：雀形目鸟类的 MHC II B 区域在组织、分化和遗传多样性方面与真兽类哺乳动物，特别是灵长类动物的 MHC 一样复杂。因此，报道的禽类 MHC 的简单性是一种反常现象。借助适当的标记，可以在鸟类分类群的系统发育中追踪 MHC 基因的分化。在许多鸟类 MHC 基因座中，种间多态性猖獗。在这方面，DFs 表现得好像是一个单一的、遗传上未分化的群体。到目前为止，还没有任何迹象表明存在可以被认为是物种、属甚至 DF 组特有的等位基因。这些发现意味着 DFs 正处于适应性辐射之中，在这种辐射中，形态分化成物种的速度快于 MHC 或线粒体 DNA 等遗传系统中的遗传分化。辐射如此年轻，以至于在形态分化的物种中，大多数基因座的多态性还没有足够的时间进行分类。这些发现与维多利亚湖haplochromine 鱼类的发现相似。一些 DF MHC 等位基因谱系可以追溯到我们之前确定的美洲大陆上与 DFs 关系最密切的物种 Tiaris obscura 的 MHC 基因。
