Dowling Thomas E, Broughton Richard E, DeMarais Bruce D
Department of Biology, Arizona State University, Tempe, Arizona, 85287-1501.
Department of Wildlife and Fisheries Sciences, Texas A & M University, College Station, Texas, 77843.
Evolution. 1997 Oct;51(5):1574-1583. doi: 10.1111/j.1558-5646.1997.tb01480.x.
Samples of Luxilus cornutus, Luxilus chrysocephalus, and their hybrids were collected along hypothesized routes of dispersal from Pleistocene refugia to examine the significance of geographic variation in patterns of introgression between these species. Patterns of allozyme and mitochondrial DNA (mtDNA) variation were generally consistent with those from previous studies. Tests of Hardy-Weinberg equilibrium revealed significant deficiencies of heterozygotes in all samples, indicating some form of reproductive isolation. Mitochondrial DNAs of each species were not equally represented in F hybrids; however, this bias was eliminated when the two largest samples were excluded from the analysis. Backcross hybrids exhibited biased mtDNA introgression, as samples from Lake Erie (eastern) and Lake Michigan (western) drainages showed significant excesses of mtDNAs from L. chrysocephalus and L. cornutus, respectively, relative to frequencies of diagnostic allozyme markers. The extent and direction of allozyme and mtDNA introgression was quantified by calculating isolation index values from morphologically "pure" individuals of each species from each locality. Analysis of variance of these measures identified limited introgression of allozyme variants with no geographic pattern, but significant differences in direction of mtDNA introgression between drainages (i.e., postglacial dispersal route). Association between patterns of mtDNA introgression and dispersal route across the latitudinal width of the contact zone is best explained by genetic divergence during past isolation of ancestral populations from these drainages. These results identify a significant role for historical effects in the evolution of reproductive isolation and the process of speciation.
沿着从更新世避难所到推测的扩散路线，采集了角闪光唇鱼（Luxilus cornutus）、金黄头光唇鱼（Luxilus chrysocephalus）及其杂交种的样本，以研究这些物种之间基因渗入模式中地理变异的重要性。等位酶和线粒体DNA（mtDNA）变异模式总体上与先前研究的结果一致。哈迪-温伯格平衡检验表明，所有样本中的杂合子均存在显著不足，这表明存在某种形式的生殖隔离。在F代杂种中，每个物种的线粒体DNA的代表性并不相同；然而，当从分析中排除两个最大的样本时，这种偏差就消除了。回交杂种表现出偏向性的mtDNA基因渗入，因为来自伊利湖（东部）和密歇根湖（西部）流域的样本分别显示，相对于诊断性等位酶标记的频率，来自金黄头光唇鱼和角闪光唇鱼的mtDNA显著过量。通过计算来自每个地点的每个物种形态学上“纯合”个体的隔离指数值，对等位酶和mtDNA基因渗入的程度和方向进行了量化。对这些测量值的方差分析表明，等位酶变体的基因渗入有限，且无地理模式，但不同流域间mtDNA基因渗入方向存在显著差异（即冰期后扩散路线）。mtDNA基因渗入模式与接触区纬度宽度上的扩散路线之间的关联，最好用过去这些流域祖先种群隔离期间的遗传分化来解释。这些结果表明，历史效应在生殖隔离的进化和物种形成过程中发挥了重要作用。
