Data from: Genetic and phenotypic variation across a hybrid zone between ecologically divergent tree squirrels (Tamiasciurus)

A hybrid zone along an environmental gradient should contain a clinal pattern of genetic and phenotypic variation. This occurs because divergent selection in the two parental habitats is typically strong enough to overcome the homogenizing effects of gene flow across the environmental transition. We studied hybridization between two parapatric tree squirrels (Tamiasciurus spp.) across a forest gradient over which the two species vary in coloration, cranial morphology, and body size. We sampled 397 individuals at 29 locations across a 600-km transect to seek genetic evidence for hybridization; upon confirming hybridization, we examined levels of genetic admixture in relation to maintenance of phenotypic divergence despite potentially homogenizing gene flow. Applying population assignment analyses to microsatellite data, we found that T. douglasii and T. hudsonicus form two distinct genetic clusters but also hybridize, mostly within transitional forest habitat. Overall, based on this nuclear analysis, 48% of the specimens were characterized as T. douglasii, 9% as hybrids, and 43% as T. hudsonicus. Hybrids appeared to be reproductively viable, as evidenced by the presence of later-generation hybrid genotypes. Observed clines in ecologically important phenotypic traits—fur coloration and cranial morphology—were sharper than the cline of putatively neutral mtDNA, which suggests that divergent selection may maintain phenotypic distinctiveness. The relatively recent divergence of these two species (probably late Pleistocene), apparent lack of pre-zygotic isolating mechanisms, and geographic coincidence of cline centers for both genetic and phenotypic variation suggest that environmental factors play a large role in maintaining the distinctiveness of these two species across the hybrid zone.

沿环境梯度分布的杂交带应呈现遗传与表型变异的渐变群格局。该现象的成因在于，两个亲本生境中的歧化选择强度通常足以克服环境过渡区中基因流所带来的同质化效应。
我们针对沿森林梯度分布的两种邻域树松鼠（*Tamiasciurus* spp.，美洲红松鼠属）开展杂交研究，该梯度下两物种在毛色、颅骨形态与体型上均存在差异。我们在600公里长的样带内的29个采样点采集了397号个体样本，以寻找杂交的遗传证据；在确认存在杂交现象后，我们进一步分析了遗传渐渗水平与表型分化维持之间的关联，尽管存在可能引发同质化的基因流。
我们对微卫星（microsatellite）数据开展群体分配分析，结果显示道格拉斯红松鼠（*T. douglasii*）与哈德逊红松鼠（*T. hudsonicus*）可分为两个独立的遗传聚类群，同时二者存在杂交现象，杂交个体主要分布于过渡林生境中。整体而言，基于本次核基因分析结果，48%的标本被鉴定为道格拉斯红松鼠，9%为杂交个体，43%为哈德逊红松鼠。
杂交个体似乎具备繁殖可育性，这一点可从后代杂种基因型的存在得到佐证。观测到的生态重要表型性状（毛色与颅骨形态）渐变群相较于推定中性的线粒体DNA（mtDNA）渐变群更为陡峭，这表明歧化选择或许维持了两物种的表型独特性。
这两个物种的分化时间相对较近（大概率为更新世晚期），且未发现明显的合子前生殖隔离机制，同时遗传与表型变异的渐变群中心在地理上重合，这些证据均表明环境因子在维持该杂交带内两物种的独特性方面发挥了重要作用。