Bert Theresa M, Harrison Richard G
Department of Biology, Osborn Memorial Laboratory, Yale University, New Haven, CT, 06511.
Evolution. 1988 May;42(3):528-544. doi: 10.1111/j.1558-5646.1988.tb04158.x.
Two distinct taxa of marine crabs (Menippe mercenaria and M. adina, sensu Williams and Felder [1986]) interbreed extensively in two disjunct areas of the coastal southeastern United States. We used variation in coloration and allele frequencies at three diagnostic loci to examine in detail the structure of the two zones of variation. A narrow hybrid zone in northwest Florida is situated at the junction of the present ranges of the two parental forms, in an area of strong ecological change and hydrological convergence. Despite extensive hybridization in this area, there is a significant deficiency of heterozygotes for the Alp-2 locus, nonrandom associations of alleles for two diagnostic loci, and an absence of certain combinations of phenotypes and genotypes. Along the Atlantic coast (east central Florida into South Carolina), a broad zone of increased variability exists within the range of M. mercenaria. Allele frequencies throughout this zone are similar to those of M. mercenaria but reflect apparent introgression from M. adina. In contrast, color patterns are quite variable, but only in the center of this zone. There is little evidence of a heterozygote deficiency, and the preferred habitat of M. mercenaria is not present. The Atlantic zone of variability is apparently expanding, with alleles at enzyme loci introgressing more rapidly than color characteristics. Despite these differences, certain features are common to both zones. These include 1) asymmetry in terms of the direction of introgression, 2) differential introgression of alleles, and 3) an almost complete absence of M. adina phenotypes that carry high proportions of M. mercenaria alleles. Differences between the two zones illustrate the influence that environmental setting, time of contact in relation to time of divergence, and location of the zone relative to the parental species ranges can have on hybridization events. However, observed similarities between the zones suggest that certain patterns of introgression and recombination may be independent of environmental setting. Thus, we suggest that factors inherent to the organism (intrinsic factors) and factors inherent to the environment (extrinsic factors) both act to structure and maintain the two hybrid zones.
两种不同的海洋蟹类分类群（根据威廉姆斯和费尔德[1986]的分类，即梅氏硬壳蟹和阿迪纳硬壳蟹）在美国东南部沿海的两个不连续区域广泛杂交。我们利用三个诊断位点的颜色变化和等位基因频率，详细研究了这两个变异区域的结构。佛罗里达州西北部的一个狭窄杂交带位于两个亲本形态当前分布范围的交界处，处于生态变化强烈和水文汇聚的区域。尽管该区域存在广泛杂交，但Alp - 2位点的杂合子显著不足，两个诊断位点的等位基因存在非随机关联，并且某些表型和基因型组合缺失。沿着大西洋海岸（从佛罗里达州中东部到南卡罗来纳州），在梅氏硬壳蟹的分布范围内存在一个变异性增加的广阔区域。该区域内的等位基因频率与梅氏硬壳蟹相似，但反映出明显来自阿迪纳硬壳蟹的基因渗入。相比之下，颜色模式变化很大，但仅在该区域的中部。几乎没有杂合子不足的证据，且不存在梅氏硬壳蟹偏好的栖息地。大西洋变异区域显然在扩大，酶位点的等位基因渗入速度比颜色特征更快。尽管存在这些差异，但两个区域有一些共同特征: 1) 基因渗入方向的不对称性；2) 等位基因的差异渗入；3) 几乎完全没有携带高比例梅氏硬壳蟹等位基因的阿迪纳硬壳蟹表型。两个区域的差异说明了环境背景、与分化时间相关的接触时间以及区域相对于亲本物种分布范围的位置对杂交事件可能产生的影响。然而，两个区域观察到的相似性表明，某些基因渗入和重组模式可能与环境背景无关。因此，我们认为生物体固有的因素（内在因素）和环境固有的因素（外在因素）都对两个杂交区域的结构和维持起到作用。
