Data from: Coexistence and origin of trophic ecotypes of pygmy whitefish, Prosopium coulterii, in southwestern Alaskan lake

Ecologically, morphologically, and genetically distinct populations within single taxa often co-exist in postglacial lakes and have provided important model systems with which to investigate ecological and evolutionary processes such as niche partitioning and ecological speciation. Within the Salmonidae, these species complexes have been well studied, particularly within the Coregonus clupeaformis-C. laveratus (lake and European whitefish, respectively) group, but the phenomenon has been less well documented in the other whitefish genera, Prosopium and Stenodus. Here, we examined the morphology, feeding biology, and genetic structure of three putative forms of the pygmy whitefish, Prosopium coulterii, that were first reported from Chignik Lake, southwestern Alaska, over 40 years ago. Field collections and morphological analyses resolved a shallow water (< 5 m depth) low gill raker count form (< 15 first arch gill rakers), a deepwater (> 30 m), low gill raker form, and a deepwater, high gill raker count (> 15 gill rakers) form. The two low gill raker count forms fed almost exclusively on benthic invertebrates (mostly chironomids), while the deepwater, high gill raker count form fed almost exclusively on zooplankton, differences in diet that were also reflected in differences both in δ13C and δ15N stable isotopes. All three forms were characterized by the same major mitochondrial DNA clade that has been associated with persistence in, and postglacial dispersal from, a Beringian glacial refugium. Analysis of variation at nine microsatellite DNA loci indicated low, but significant differentiation amongst forms, especially between the two low gill raker count forms and the high gill raker count form. The extent of differentiation along phenotypic (considerable) and genetic (subtle) axes amongst the Chignik Lake forms is similar to that found amongst distinct taxa of Prosopium found in pre-glacial Bear Lake (Utah-Idaho, USA) which is probably at least ten times older than Chignik Lake. Our analyses illustrate the potential for the postglacial differentiation in traits subject to divergent natural selection across variable environments.

从生态学、形态学与遗传学层面来看，单个分类群（taxa）内部的不同种群常共存于冰川后湖泊中，它们已成为研究生态位分化、生态物种形成等生态与进化过程的关键模型体系。在鲑科（Salmonidae）鱼类中，这类物种复合体已得到充分研究，尤以湖白鲑（Coregonus clupeaformis）与欧洲白鲑（C. laveratus）类群为代表，但该现象在另外两个白鲑类群——北鲑属（Prosopium）与扁吻白鲑属（Stenodus）中的记录则相对较少。四十余年前，人们首次在阿拉斯加西南部的奇格尼克湖（Chignik Lake）中报道了矮白鲑（Prosopium coulterii）的三种推定类群，本研究对这三类矮白鲑的形态学特征、摄食生物学与遗传结构展开了分析。野外采集与形态学分析结果显示，存在三类形态类群：一类栖息于浅水区（水深<5米），第一鳃弓鳃耙数较低（<15）；一类栖息于深水区（水深>30米），鳃耙数同样较低；还有一类栖息于深水区，鳃耙数较高（>15）。两类低鳃耙数类群几乎完全以底栖无脊椎动物（主要为摇蚊类）为食，而高鳃耙数的深水区类群则几乎完全取食浮游动物；这种食性差异也在δ¹³C与δ¹⁵N稳定同位素的特征差异中得到了印证。三类类群均隶属于同一主要线粒体DNA（mitochondrial DNA, mtDNA）支系，该支系与白令冰期避难所的存续以及从该避难所出发的冰川后扩散事件密切相关。对9个微卫星DNA（microsatellite DNA）位点的变异分析显示，各类群间存在微弱但显著的遗传分化，其中尤以两类低鳃耙数类群与高鳃耙数类群间的分化最为突出。奇格尼克湖类群在表型层面（分化显著）与遗传层面（分化微弱）的分化程度，与美国犹他州-爱达荷州冰期前熊湖（Bear Lake）中北鲑属（Prosopium）的不同类群间的分化程度相近，而熊湖的形成时间或许至少是奇格尼克湖的10倍。本研究结果表明，在多变的环境中，受趋异自然选择作用的性状存在冰川后分化的潜力。