Ross Charles L, Harrison Richard G
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USA.
Evolution. 2002 Nov;56(11):2296-312. doi: 10.1111/j.0014-3820.2002.tb00153.x.
The pattern of character variation within a hybrid zone, the hybrid zone structure, has been used to infer the processes that maintain hybrid zones. Unfortunately it is difficult to infer process from structure alone because many different processes can produce the same pattern of character variation. Mosaic hybrid zones may be maintained by exogenous selection in a heterogeneous environment and/or endogenous selection against hybrid individuals; habitat preference, premating isolating barriers and/or fertility selection can also contribute. The spatial scale at which a hybrid zone is sampled affects its apparent structure; a hybrid zone may appear clinal at one scale and mosaic at another. Here, we sample the mosaic hybrid zone between two field crickets, Gryllus firmus and G. pennsylvanicus, at a scale that spans the boundaries between individual soil-habitat patches. From our analysis, we find that at fine scales, the mosaic hybrid zone resolves into a set of steep clines across patch boundaries. Both morphological and molecular traits exhibit sharp and generally concordant clines. However, clines for mitochondrial DNA and one anonymous nuclear marker are clearly displaced as a result of current hybridization or past introgression (the "ghost of hybridization past"). Thus, scale is important for the structure of this and probably other hybrid zones. The extremely sharp, concordant clines across patch boundaries indicate that the cricket hybrid zone is undoubtedly structured by selection. However, the detailed mechanisms responsible for the maintenance of the hybrid zone--whether endogenous selection against hybrids, exogenous selection by the environment, and/or behavioral preferences for mates or habitats--remain to be elucidated. Determining these mechanisms will depend on closer inspection of the organisms themselves and their interactions, as is the case for all hybrid zones.
杂交区内性状变异的模式，即杂交区结构，已被用于推断维持杂交区的过程。不幸的是，仅从结构很难推断过程，因为许多不同的过程可能产生相同的性状变异模式。镶嵌杂交区可能由异质环境中的外源选择和/或针对杂交个体的内源选择维持；栖息地偏好、交配前隔离屏障和/或育性选择也可能起作用。对杂交区进行采样的空间尺度会影响其表观结构；杂交区在一个尺度上可能呈现渐变，而在另一个尺度上则呈现镶嵌状。在这里，我们在跨越单个土壤栖息地斑块边界的尺度上，对两种田蟋（坚实田蟋和宾夕法尼亚田蟋）之间的镶嵌杂交区进行采样。通过分析，我们发现在精细尺度上，镶嵌杂交区在斑块边界处分解为一组陡峭的渐变群。形态和分子性状都表现出明显且通常一致的渐变群。然而，由于当前的杂交或过去的基因渗入（“过去杂交的幽灵”），线粒体DNA和一个匿名核标记的渐变群明显偏移。因此，尺度对这个杂交区以及可能其他杂交区的结构很重要。跨越斑块边界的极其明显、一致的渐变群表明，田蟋杂交区无疑是由选择构建的。然而，维持杂交区的详细机制——无论是针对杂交种的内源选择、环境的外源选择，和/或对配偶或栖息地的行为偏好——仍有待阐明。确定这些机制将取决于更仔细地观察生物体本身及其相互作用，所有杂交区都是如此。
