Constrained body shape among highly genetically divergent allopatric lineages of the supralittoral isopod Ligia occidentalis (Oniscidea)

Multiple highly divergent lineages have been identified within Ligia occidentalis sensu lato, a rocky supralittoral isopod distributed along a ~3000 km latitudinal gradient that encompasses several proposed marine biogeographic provinces and ecoregions in the eastern Pacific. Highly divergent lineages have nonoverlapping geographic distributions, with distributional limits that generally correspond with sharp environmental changes. Crossbreeding experiments suggest postmating reproductive barriers exist among some of them, and surveys of mitochondrial and nuclear gene markers do not show evidence of hybridization. Populations are highly isolated, some of which appear to be very small; thus, the effects of drift are expected to reduce the efficiency of selection. Large genetic divergences among lineages, marked environmental differences in their ranges, reproductive isolation, and/or high isolation of populations may have resulted in morphological differences in L. occidentalis, not detected yet by traditional taxonomy. We used landmark-based geometric morphometric analyses to test for differences in body shape among highly divergent lineages of L. occidentalis, and among populations within these lineages. We analyzed a total of 492 individuals from 53 coastal localities from the southern California Bight to Central Mexico, including the Gulf of California. We conducted discriminant function analyses (DFAs) on body shape morphometrics to assess morphological variation among genetically differentiated lineages and their populations. We also tested for associations between phylogeny and morphological variation, and whether genetic divergence is correlated to multivariate morphological divergence. We detected significant differences in body shape among highly divergent lineages, and among populations within these lineages. Nonetheless, neither lineages nor populations can be discriminated on the basis of body shape, because correct classification rates of cross-validated DFAs were low. Genetic distance and phylogeny had weak to no effect on body shape variation. The supralittoral environment appears to exert strong stabilizing selection and/or strong functional constraints on body shape in L. occidentalis, thereby leading to morphological stasis in this isopod.

研究在广义西方潮虫（Ligia occidentalis sensu lato）中鉴定出多个高度分化的支系。该类群为岩栖潮间带等足类动物（isopod），沿约3000公里的纬度梯度分布，覆盖东太平洋中多个已被提出的海洋生物地理省与生态区。这些高度分化的支系具有互不重叠的地理分布，其分布界限通常与剧烈的环境变化相吻合。杂交实验表明，部分支系间存在交配后生殖隔离；对线粒体与核基因标记的调查未发现杂交的证据。该物种种群间高度隔离，部分种群规模极小，因此遗传漂变（genetic drift）的作用预计会降低自然选择的效率。支系间巨大的遗传分化、分布区显著的环境差异、生殖隔离及/或种群间的高度隔离，可能导致广义西方潮虫出现传统分类学尚未检出的形态差异。本研究采用基于地标点的几何形态测量学分析（landmark-based geometric morphometric analyses），检验广义西方潮虫各高度分化支系间以及支系内种群间的体型差异。我们共分析了从加利福尼亚南部海湾至墨西哥中部（含加利福尼亚湾）的53个沿海采样点的492个个体。通过对体型形态测量数据开展判别函数分析（DFA, discriminant function analyses），评估遗传分化支系及其种群间的形态变异。此外，我们还检验了系统发育（phylogeny）与形态变异间的关联，以及遗传分化是否与多变量形态分化相关。研究检测到高度分化支系间以及支系内种群间的体型存在显著差异。然而，无论是支系还是种群，均无法通过体型进行区分，因为交叉验证判别函数分析的正确分类率极低。遗传距离与系统发育对体型变异的影响微弱甚至无影响。潮间带环境似乎对广义西方潮虫的体型施加了强烈的稳定选择及/或严苛的功能约束，从而导致该等足类动物出现形态停滞（morphological stasis）。