Bert Theresa M, Arnold William S
Florida Marine Research Institute, 100 Eighth Avenue Southeast, St. Petersburg, Florida, 33701.
Evolution. 1995 Apr;49(2):276-289. doi: 10.1111/j.1558-5646.1995.tb02240.x.
Two models developed to discern the mode of selection in hybrid zones differ in some predictions. The tension-zone model predicts that selection acts against hybrids and independently of the environment (endogenous selection) and that selection is invariant throughout the hybrid zone. The ecological selection-gradient, or ecotone, model maintains that fitness of different genotypes varies in response to environmental variation (exogenous selection) and thus, that in a region of the zone, fitness of hybrids is at least equal to that of the parental species. Therefore, to assess the predominant mode of selection operating in a hybrid zone, it is fundamental to evaluate whether selection is acting specifically against hybrid individuals, that is, whether hybridity alone is the basis for deficiencies of hybrids, and to evaluate whether the relative fitness of hybrids versus that of pure species varies across the zone. In a hardclam (genus Mercenaria) hybrid zone located in a polyhaline lagoon in east-central Florida, we used age-specific and location-specific analyses to determine that a hybrid deficit occurrs, that the deficit seems to be due to selection against hybrids, and that selection varies across the zone. Various measures of deviation from Hardy-Weinberg equilibrium, linkage disequilibrium analyses, and shifts in allele frequencies at semidiagnostic loci support the idea that selection is strongest in the northern region of the lagoon, the zone of sympatry and hybridization. Southward, into the range of M. mercenaria (the numerically predominant species), the percentage of hybrids remains relatively high and selection against hybrids decreases. For some genetic linkage groups, selection for M. mercenaria alleles seems to be occurring, but selection seems to be acting principally against alleles characteristic of M. mercenaria and, to a lesser degree, for alleles characteristic of M. campechiensis (the rarer species). These findings and others from previous analyses we have done on this hybrid zone demonstrate that selection in the zone is complex, and that characteristics of both the tension-zone and ecotone models are present. Supporting the tension-zone model, selection against hybrids per se clearly occurs, but specific genotypes seem to be at a selective disadvantage, whereas others have a selective advantage, and selection operates differentially on the two parental species within the zone. Supporting the ecotone model, the strength of overall selection varies throughout the zone, and environmentally mediated selection in which each species and hybrids have an advantage in specific habitats occurs, but some selection against hybrids is invariant throughout the zone. Thus, the structure and genetic architecture of this hybrid zone appear to be products of a complicated interaction between both types of selective forces cited in the two competing models.
为识别杂交带中选择模式而开发的两种模型在一些预测方面存在差异。张力带模型预测，选择作用于杂种且独立于环境（内源性选择），并且选择在整个杂交带中是不变的。生态选择梯度或生态交错带模型则认为，不同基因型的适合度会随着环境变化而变化（外源性选择），因此，在杂交带的某个区域，杂种的适合度至少与亲本物种相当。所以，要评估在杂交带中起主导作用的选择模式，关键在于评估选择是否专门针对杂种个体起作用，即杂种性本身是否是杂种缺陷的基础，以及评估杂种与纯种的相对适合度在整个杂交带中是否有所不同。在佛罗里达州中东部一个多盐度泻湖中的硬壳蛤（Mercenaria属）杂交带，我们通过年龄特异性和位置特异性分析确定存在杂种缺失，这种缺失似乎是由于对杂种的选择导致的，并且选择在整个杂交带中有所变化。各种偏离哈迪 - 温伯格平衡的度量、连锁不平衡分析以及半诊断位点上等位基因频率的变化都支持这样一种观点，即选择在泻湖的北部区域最强，该区域是同域分布和杂交区域。向南进入M. mercenaria（数量上占优势的物种）的分布范围，杂种的百分比仍然相对较高，对杂种的选择减少。对于一些遗传连锁群，似乎存在对M. mercenaria等位基因的选择，但选择似乎主要针对M. mercenaria特有的等位基因起作用，在较小程度上针对M. campechiensis（较稀有物种）特有的等位基因起作用。这些发现以及我们之前对这个杂交带所做的其他分析结果表明，该杂交带中的选择是复杂的，并且同时存在张力带模型和生态交错带模型的特征。支持张力带模型的是，对杂种本身的选择显然存在，但特定基因型似乎处于选择劣势，而其他基因型则具有选择优势，并且选择在杂交带内对两个亲本物种的作用不同。支持生态交错带模型的是，整体选择的强度在整个杂交带中有所变化，并且存在环境介导的选择，即每个物种和杂种在特定栖息地具有优势，但对杂种的一些选择在整个杂交带中是不变的。因此，这个杂交带的结构和遗传结构似乎是这两种相互竞争模型中所提及的两种选择力之间复杂相互作用的产物。
