Phylogeography and population genetics of pine butterflies: sky islands increase genetic divergence

The sky islands of southeastern Arizona (AZ) mark a major transition zone between tropical and temperate biota and are considered a neglected biodiversity hotspot. Dispersal ability and host plant specificity are thought to impact the history and diversity of insect populations across the sky islands. We aimed to investigate the population structure and phylogeography of two pine-feeding pierid butterflies, the pine white (Neophasia menapia) and the Mexican pine white (N. terlooii), restricted to these "islands" at this transition zone. Given their dependence on pines as the larval hosts, we hypothesized that habitat connectivity affects population structure and is at least in part responsible for their allopatry. We sampled DNA from freshly collected butterflies from 17 sites in the sky islands and adjacent high-elevation habitats and sequenced these samples using ddRADSeq. Up to 15,399 SNPs were discovered and analyzed in population genetic and phylogenetic contexts with Stacks and pyRAD pipelines. Low genetic differentiation in N. menapia suggests that it is panmictic. Conversely, there is strong evidence for population structure within N. terlooii. Each sky island likely contains a population of N. terlooii, and clustering is hierarchical, with populations on proximal mountains being more related to each other. The N. menapia habitat, which is largely contiguous, facilitates panmixia, while the N. terlooii habitat, restricted to the higher elevations on each sky island, creates distinct population structure. Phylogenetic results corroborate those from population genetic analyses. The historical climate-driven fluxes in forest habitat connectivity have implications for understanding the biodiversity of fragmented habitats.

美国亚利桑那州东南部的天空群岛（sky islands）是热带与温带生物群落间的重要过渡带，被视作一处长期被忽视的生物多样性热点区域。学界普遍认为，扩散能力与宿主植物特异性会影响该区域内昆虫种群的演化历史与物种多样性。本研究旨在探究两种取食松树的粉蝶科（Pieridae）昆虫的种群结构与系统地理学特征，它们分别为松粉蝶（*Neophasia menapia*）与墨西哥松粉蝶（*N. terlooii*），二者均局限分布于该过渡带的天空群岛生境中。鉴于其幼虫阶段完全依赖松树作为宿主，我们提出假说：生境连通性会对种群结构产生影响，并至少部分解释二者的异域分布格局。研究团队从天空群岛及邻近高海拔生境的17个采样点采集的新鲜蝴蝶样本中提取基因组DNA，采用ddRADSeq技术完成测序。借助Stacks与pyRAD分析流程，共发掘并分析了至多15399个单核苷酸多态性（Single Nucleotide Polymorphism, SNPs）位点，并开展种群遗传与系统发育层面的分析。种群遗传分析结果显示，松粉蝶（*N. menapia*）的遗传分化程度极低，提示其种群为泛交种群（panmictic）。与之相反，墨西哥松粉蝶（*N. terlooii*）则存在显著的种群遗传结构。每个天空群岛大概率都存在其特有种群，且聚类结果呈层级结构，邻近山地的种群间亲缘关系更为紧密。松粉蝶的生境整体连续，为泛交种群的形成提供了有利条件；而墨西哥松粉蝶的生境局限于各天空群岛的高海拔区域，因此形成了独特的种群遗传结构。系统发育分析结果与种群遗传分析结果相互印证。历史上由气候驱动的森林生境连通性动态变化，对于理解破碎化生境的生物多样性格局具有重要的科学启示意义。