Decoupling of genetic and phenotypic divergence in a headwater landscape

In stream organisms, the landscape affecting intraspecific genetic and phenotypic divergence is comprised of two fundamental components: the stream network and terrestrial matrix. These components are known to differentially influence genetic structure in stream species, but, to our knowledge, no study has compared their effects on genetic and phenotypic divergence. We examined how the stream network and terrestrial matrix affect genetic and phenotypic divergence in two stream salamanders, Gyrinophilus porphyriticus and Eurycea bislineata, in the Hubbard Brook Watershed, New Hampshire, USA. Based on previous findings and differences in adult terrestriality, we predicted that genetic divergence and phenotypic divergence in body morphology would be correlated in both species, but structured primarily by distance along the stream network in G. porphyriticus, and by overland distance in E. bislineata. Surprisingly, spatial patterns of genetic and phenotypic divergence were not strongly correlated. Genetic divergence, based on AFLPs, increased with absolute geographic distance between sites. Phenotypic divergence was unrelated to absolute geographic distance, but related to relative stream vs. overland distances. In G. porphyriticus, phenotypic divergence was low when sites were close by stream distance alone and high when sites were close by overland distance alone. The opposite was true for E. bislineata. These results show that small differences in life history can produce large differences in patterns of intraspecific divergence, and the limitations of landscape genetic data for inferring phenotypic divergence. Our results also underscore the importance of explicitly comparing how terrestrial and aquatic conditions affect spatial patterns of divergence in species with biphasic life cycles.

对于溪流生物而言，影响其种内遗传与表型分化（intraspecific genetic and phenotypic divergence）的景观系统由两个核心组分构成：溪流网络（stream network）与陆地基质（terrestrial matrix）。已知这两类组分可对溪流物种的遗传结构（genetic structure）产生差异化影响，但据我们所知，目前尚无研究对比二者对遗传与表型分化的作用效果。我们以美国新罕布什尔州哈伯德布鲁克流域（Hubbard Brook Watershed）内的两种溪流蝾螈Gyrinophilus porphyriticus和Eurycea bislineata为研究对象，探究了溪流网络与陆地基质对其种内遗传与表型分化的调控作用。基于已有研究成果及成体陆生性（adult terrestriality）的差异，我们提出如下预测：两种蝾螈的躯体形态（body morphology）相关遗传分化与表型分化均会呈现相关性，但G. porphyriticus的分化格局主要由沿溪流网络的距离所决定，而E. bislineata的分化格局则主要受陆地距离（overland distance）调控。出乎意料的是，遗传与表型分化的空间格局并未呈现显著相关性。基于扩增片段长度多态性（AFLPs）的分析结果显示，遗传分化随采样点间的绝对地理距离（absolute geographic distance）增大而升高。表型分化与绝对地理距离无关，但与溪流距离与陆地距离的相对比值相关。对于G. porphyriticus而言，当采样点仅在溪流距离上较近时，表型分化程度较低；而当采样点仅在陆地距离上较近时，表型分化程度较高。E. bislineata则呈现出完全相反的格局。本研究结果表明，生活史（life history）特征的细微差异可导致种内分化格局产生显著差异，同时也凸显了利用景观遗传学（landscape genetics）数据推断表型分化时存在的局限性。此外，本研究结果也强调了需明确对比陆地与水生环境如何影响具有双相生活史（biphasic life cycles）物种的分化空间格局。