Meyer-Lucht Yvonne, Otten Celine, Püttker Thomas, Sommer Simone
Evolutionary Genetics, Leibniz Institute for Zoo and Wildlife Research, Alfred-Kowalke-Str, 17, D-10315 Berlin, Germany.
BMC Genet. 2008 Jun 5;9:39. doi: 10.1186/1471-2156-9-39.
Research on the genetic architecture and diversity of the MHC has focused mainly on eutherian mammals, birds and fish. So far, studies on model marsupials used in laboratory investigations indicated very little or even no variation in MHC class II genes. However, natural levels of diversity and selection are unknown in marsupials as studies on wild populations are virtually absent. We used two endemic South American mouse opossums, Gracilinanus microtarsus and Marmosops incanus, to investigate characteristic features of MHC selection. This study is the first investigation of MHC selection in free-ranging Neotropical marsupials. In addition, the evolutionary history of MHC lineages within the group of marsupials was examined.
G. microtarsus showed extensive levels of MHC diversity within and among individuals as 47 MHC-DAB alleles and high levels of sequence divergence were detected at a minimum of four loci. Positively selected codon sites were identified, of which most were congruent with human antigen binding sites. The diversity in M. incanus was rather low with only eight observed alleles at presumably two loci. However, these alleles also revealed high sequence divergence. Again, positive selection was identified on specific codon sites, all congruent with human ABS and with positively selected sites observed in G. microtarsus. In a phylogenetic comparison alleles of M. incanus interspersed widely within alleles of G. microtarsus with four alleles being present in both species.
Our investigations revealed extensive MHC class II polymorphism in a natural marsupial population, contrary to previous assumptions. Furthermore, our study confirms for the first time in marsupials the presence of three characteristic features common at MHC loci of eutherian mammals, birds and fish: large allelic sequence divergence, positive selection on specific sites and trans-specific polymorphism.
主要针对真兽类哺乳动物、鸟类和鱼类开展了关于主要组织相容性复合体（MHC）的遗传结构和多样性的研究。到目前为止，针对实验室研究中使用的有袋类动物模型的研究表明，MHC II类基因几乎没有变异，甚至没有变异。然而，由于几乎没有对野生种群的研究，有袋类动物的自然多样性水平和选择情况尚不清楚。我们利用两种南美洲特有小鼠负鼠，即小跗 Gracilinanus microtarsus 和灰腹小鼠负鼠 Marmosops incanus，来研究MHC选择的特征。本研究是对自由放养的新热带有袋类动物MHC选择的首次调查。此外，还研究了有袋类动物群体内MHC谱系的进化历史。
小跗负鼠在个体内部和个体之间均表现出广泛的MHC多样性，因为在至少四个位点检测到47个MHC - DAB等位基因和高水平的序列差异。鉴定出了正选择密码子位点，其中大多数与人类抗原结合位点一致。灰腹小鼠负鼠的多样性较低，在大概两个位点仅观察到8个等位基因。然而，这些等位基因也显示出高序列差异。同样，在特定密码子位点鉴定出了正选择，所有这些位点都与人类抗原结合位点以及在小跗负鼠中观察到的正选择位点一致。在系统发育比较中，灰腹小鼠负鼠的等位基因广泛散布在小跗负鼠的等位基因中，两个物种共有4个等位基因。
我们的研究揭示了自然有袋类动物种群中广泛存在的MHC II类多态性，这与之前的假设相反。此外，我们的研究首次在有袋类动物中证实了在真兽类哺乳动物、鸟类和鱼类的MHC位点常见的三个特征：大的等位基因序列差异、特定位点的正选择和跨物种多态性。
