Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, 4210 Silver Hill Road, Suitland, MD 20746, USA.
Mol Ecol. 2009 Dec;18(23):4888-903. doi: 10.1111/j.1365-294X.2009.04381.x. Epub 2009 Oct 27.
Hybrid zones are often characterized by narrow, coincident clines for diverse traits, suggesting that little introgression occurs across them. However, this pattern may result from a bias in focussing on traits that are diagnostic of parental populations. Such choice of highly differentiated traits may cause us to overlook differential introgression in nondiagnostic traits and to distort our perception of hybrid zones. We tested this hypothesis in an avian hybrid zone by comparing cline structure in two sets of molecular markers: isozyme and restriction fragment length polymorphism markers chosen for differentiation between parental forms, and microsatellite markers chosen for polymorphism. Two cline-fitting methods showed that cline centre positions of microsatellite alleles were more variable than those of isozyme and restriction fragment length polymorphism markers, and several were significantly shifted from those of the diagnostic markers. Cline widths of microsatellite alleles were also variable and two- to eightfold wider than those of the diagnostic markers. These patterns are consistent with the idea that markers chosen for differentiation are more likely to be under purifying selection, and studies focussed on these markers will underestimate overall introgression across hybrid zones. Our results suggest that neutral and positively selected alleles may introgress freely across many hybrid zones without altering perceived boundaries between hybridizing forms.
杂种区通常以多种特征的狭窄、一致的渐变带为特征，表明很少有基因渐渗发生在这些特征之间。然而，这种模式可能是由于我们专注于那些对亲本种群具有诊断意义的特征而产生的偏见。这种对高度分化特征的选择可能会导致我们忽略非诊断特征中的差异基因渐渗，并扭曲我们对杂种区的认识。我们通过比较两组分子标记的渐变结构来检验这个假设：同工酶和限制性片段长度多态性标记，这些标记是为了区分亲本形式而选择的；微卫星标记，这些标记是为了多态性而选择的。两种渐变拟合方法表明，微卫星等位基因的渐变中心位置比同工酶和限制性片段长度多态性标记的位置更具可变性，其中几个位置与诊断标记的位置显著不同。微卫星等位基因的渐变带宽度也具有可变性，比诊断标记的宽度宽两到八倍。这些模式与这样的观点一致，即选择用于分化的标记更可能受到纯化选择的影响，而专注于这些标记的研究将低估整个杂种区的基因渐渗。我们的研究结果表明，中性和正选择的等位基因可能会在许多杂种区自由地渐渗，而不会改变杂交形式之间的感知边界。
