Population genetic structure of Picea engelmannii, P. glauca and their previously unrecognized hybrids in the central Rocky Mountains

Areas of geographic overlap between potentially hybridizing species provide the opportunity to study interspecific gene flow and reproductive barriers. Here we identified hybrids between Picea engelmannii and P. glauca by their genetic composition at 17 microsatellite markers, and determined the broad-scale geographic distribution of hybrids in the central Rocky Mountains of North America, a geographic region where hybrids and isolation between species had not previously been studied. Parameter estimates from admixture models revealed considerable variation in ancestry within and among collection sites, suggesting that within this area of geographic overlap, the interaction of the two species varies extensively. The results document a previously unrecognized patchy distribution of hybrids between P. engelmannii and P. glauca, including locations where hybrids were not known or expected to exist. Further, the ancestry of many hybrids was consistent with multiple generations of hybridization, with probable directional backcrossing to P. engelmannii, suggesting a relatively porous species boundary. The identification and characterization of hybridization between these spruce in this region raises the question of what factors maintain barriers to gene flow in these long-lived forest trees. The current research lays the groundwork for future study of the ecological and evolutionary contexts of their hybridization, as well as of differential introgression and permeability of species boundaries.

存在潜在杂交可能的物种间的地理重叠区域，为研究种间基因流与生殖隔离屏障提供了研究契机。本研究借助17个微卫星标记（microsatellite markers）的遗传组分，鉴定了恩氏云杉（Picea engelmannii）与白云杉（P. glauca）之间的杂交个体，并明确了北美落基山脉中部区域杂交个体的大范围地理分布格局——此前该区域尚未开展过物种间杂交与生殖隔离的相关研究。基于遗传混合模型的参数估计结果显示，采样位点内部及位点之间的祖先遗传组分存在显著差异，表明在该地理重叠区域内，两个物种的相互作用存在广泛变异。研究结果揭示了恩氏云杉与白云杉之间此前未被认知的杂交个体斑块状分布格局，涵盖了一些此前未被发现或推测存在杂交的区域。此外，多数杂交个体的祖先遗传组分符合多代杂交的特征，且可能存在向恩氏云杉的定向回交，这暗示二者之间存在较为松散的物种边界。对该区域内这两种云杉属植物间杂交事件的鉴定与表征，引出了一个新的科学问题：究竟是哪些因素维持了这些长寿林木的基因流屏障？本研究为后续探究二者杂交的生态与进化背景、以及差异化渐渗与物种边界通透性的相关研究奠定了坚实基础。