Genetic drift vs. natural selection in a long-term small isolated population:: major histocompatibility complex class II variation in the Gulf of California endemic porpoise (Phocoena sinus)
Genetic drift vs. natural selection in a long-term small isolated population:: major histocompatibility complex class II variation in the Gulf of California endemic porpoise (Phocoena sinus)
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DOI：
10.1111/j.1365-294x.2007.03319.x
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发表时间：
2007-10-01
影响因子：
4.9
通讯作者：
Flores-Ramirez, Sergio
中科院分区：
文献类型：
--
作者：
Munguia-Vega, Adrian;Esquer-Garrigos, Yareli;Flores-Ramirez, Sergio
尽管许多研究证实，长期的小型孤立种群（例如岛屿特有种群）通常由于遗传漂变而维持低中性遗传变异，但目前尚不清楚适应性或有害基因的选择如何与随机力相互作用。我们研究了墨西哥加利福尼亚湾上部特有的鼠海豚（Phocoena sinus，或小头鼠海豚）的两个主要组织相容性复合体 (Mhc) II 类基因座的序列变异。其独特的种群数量下降估计约为 500 只。单链构象多态性分析揭示了一个假定的功能等位基因固定在 DQB 位点 (n = 25)。在 DRB 基因座，我们发现了两个假定的功能等位基因 (n = 29)，其差异在于单个非同义核苷酸取代，该取代可以增加杂合个体上 αβ-异二聚体二聚体界面的稳定性。相同的反式特异性 DQB1 和 DRB1 等位基因在 P. sinus 和它的近亲——伯迈斯特江豚 (Phocoena spinipinnis) 之间被鉴定出来。与对四种岛屿特有哺乳动物的研究进行比较表明，由于遗传漂变，一个等位基因的固定通常发生在 DQA 或 DQB 位点（有效中性）。同样，影响较小的有害等位基因实际上也是中性的，并且可以被固定。小头鼠海豚高频率的解剖畸形为这一预测提供了实证支持。相反，DRB 基因座上低但功能性多态性的保留与较高的选择强度一致。这些观察结果表明，即使面对强烈且长期的遗传漂变和近交，自然选择也可以维持（也可能清除）一些关键的等位基因，这表明长期的小种群应该表现出较低的近交抑制。低水平的 Mhc 变异预示着小头鼠海豚对新型病原体和疾病的高度易感性。
Although many studies confirm long-term small isolated populations (e.g. island endemics) commonly sustain low neutral genetic variation as a result of genetic drift, it is less clear how selection on adaptive or detrimental genes interplay with random forces. We investigated sequence variation at two major histocompatibility complex (Mhc) class II loci on a porpoise endemic to the upper Gulf of California, Mexico (Phocoena sinus, or vaquita). Its unique declining population is estimated around 500 individuals. Single-strand conformation polymorphism analysis revealed one putative functional allele fixed at the locus DQB (n = 25). At the DRB locus, we found two presumed functional alleles (n = 29), differing by a single nonsynonymous nucleotide substitution that could increase the stability at the dimer interface of alpha beta-heterodimers on heterozygous individuals. Identical trans-specific DQB1 and DRB1 alleles were identified between P. sinus and its closest relative, the Burmeister's porpoise (Phocoena spinipinnis). Comparison with studies on four island endemic mammals suggests fixation of one allele, due to genetic drift, commonly occurs at the DQA or DQB loci (effectively neutral). Similarly, deleterious alleles of small effect are also effectively neutral and can become fixed; a high frequency of anatomical malformations on vaquita gave empirical support to this prediction. In contrast, retention of low but functional polymorphism at the DRB locus was consistent with higher selection intensity. These observations indicated natural selection could maintain (and likely also purge) some crucial alleles even in the face of strong and prolonged genetic drift and inbreeding, suggesting long-term small populations should display low inbreeding depression. Low levels of Mhc variation warn about a high susceptibility to novel pathogens and diseases in vaquita.