Hedrick P W, Lee R N, Garrigan D
Department of Biology, Arizona State University, Tempe, AZ 85287, USA.
Mol Ecol. 2002 Oct;11(10):1905-13. doi: 10.1046/j.1365-294x.2002.01579.x.
We examined variation at a class II major histocompatibility complex (MHC) gene (DRB1) in the captive red wolf population and samples of coyotes from Texas and North Carolina. We found 4 alleles in the 48 red wolves, 8 alleles in the 10 coyotes from Texas and 15 alleles in the 29 coyotes from North Carolina. Two of the four alleles found in red wolves, Caru-2 and Caru-4, were found in both the Texas and North Carolina coyote samples. Allele Caru-1, previously found in gray wolves, was also found in the North Carolina sample. The most frequent red wolf allele, Caru-3, was not found in any of the coyote samples. However, an allele found in both the Texas and North Carolina coyote samples is only one nucleotide (one amino acid) different from this red wolf allele. Overall, it appears from examination of this MHC gene that red wolves are more closely related to coyotes than to gray wolves. There were a number of different types of evidence supporting the action of balancing selection in red wolves. Namely, there was: (i) an excess of heterozygotes compared with expectations; (ii) a higher rate of nonsynonymous than synonymous substitution for the functionally important antigen-binding site positions; (iii) an eight times higher average heterozygosity of individual amino acids at the positions identified as part of the antigen-binding site than those not associated with it; (iv) the amino acid divergence of four red wolf alleles was greater than that expected from a simulation of genetic drift; and (v) the distribution of alleles, and the distributions of amino acids at many positions were more even than expected from neutrality. Examination of the level and pattern of linkage disequilibria between pairs of sites suggest that the heterozygosity, substitution and frequencies at individual amino acids are not highly dependent upon each other.
我们检测了圈养红狼种群以及来自得克萨斯州和北卡罗来纳州的郊狼样本中II类主要组织相容性复合体（MHC）基因（DRB1）的变异情况。我们在48只红狼中发现了4个等位基因，在来自得克萨斯州的10只郊狼中发现了8个等位基因，在来自北卡罗来纳州的29只郊狼中发现了15个等位基因。在红狼中发现的4个等位基因中的两个，即Caru - 2和Caru - 4，在得克萨斯州和北卡罗来纳州的郊狼样本中均有发现。之前在灰狼中发现的等位基因Caru - 1，在北卡罗来纳州的样本中也被发现。红狼中最常见的等位基因Caru - 3，在任何郊狼样本中均未发现。然而，在得克萨斯州和北卡罗来纳州的郊狼样本中都发现的一个等位基因与这个红狼等位基因仅相差一个核苷酸（一个氨基酸）。总体而言，从对这个MHC基因的检测来看，红狼与郊狼的亲缘关系似乎比与灰狼的更近。有许多不同类型的证据支持红狼中平衡选择的作用。具体来说，有：（i）杂合子数量比预期的多；（ii）在功能重要的抗原结合位点位置，非同义替换率高于同义替换率；（iii）在被确定为抗原结合位点一部分的位置，单个氨基酸的平均杂合度比那些与之无关的位置高八倍；（iv）四个红狼等位基因的氨基酸差异大于遗传漂变模拟预期的差异；（v）等位基因的分布以及许多位置的氨基酸分布比中性预期更为均匀。对位点对之间连锁不平衡的水平和模式的检测表明，单个氨基酸的杂合度、替换和频率彼此之间并非高度依赖。
