Data from: Lack of genetic variation prevents adaptation at the geographic range margin in a damselfly

What limits a species' distribution in the absence of physical barriers? Genetic load due to asymmetric gene flow and the absence of genetic variation due to lack of gene flow are hypothesized to constrain adaptation to novel environments in marginal populations, preventing range expansion. Here, we examined the genetic structure and geographic variation in morphological traits in two damselflies (Ischnura asiatica and I. senegalensis) along a latitudinal gradient in Japan, which is the distribution centre of I. asiatica and the northern limit of I. senegalensis. Genomewide genetic analyses found a loss of genetic diversity at the edge of distribution in I. senegalensis but consistently high diversity in I. asiatica. Gene flow was asymmetric in a south–north direction in both species. Although body size and wing loading showed decreasing latitudinal clines (smaller in north) in I. asiatica in Japan, increasing latitudinal clines (larger in north) in these phenotypic markers were observed in I. senegalensis, particularly near the northern boundary, which coincided well with the location where genetic diversity began a sharp decline. In ectothermic animals, increasing latitudinal cline in these traits was suggested to be established when they failed to adapt to thermal gradient. Therefore, our findings support the possibility that a lack of genetic variation rather than geneflow swamping is responsible for the constraint of adaptation at the margin of geographic distribution.

在缺乏物理屏障的前提下，是什么限制了一个物种的分布范围？有研究假说提出，由不对称基因流（asymmetric gene flow）引发的遗传负荷（genetic load），以及因基因流缺失导致的遗传变异（genetic variation）匮乏，会制约边缘种群（marginal populations）对新生境的适应性，进而阻碍物种的分布范围扩张（range expansion）。
本研究以日本境内沿纬度梯度分布的两种豆娘为研究对象，分别为蓝尾狭翅蟌（Ischnura asiatica）与塞内加尔狭翅蟌（Ischnura senegalensis）；其中日本是蓝尾狭翅蟌的分布中心，同时也是塞内加尔狭翅蟌的分布北界。全基因组遗传分析（genomewide genetic analyses）结果显示，塞内加尔狭翅蟌在分布边缘区域存在遗传多样性（genetic diversity）丧失的情况，而蓝尾狭翅蟌的遗传多样性始终维持在较高水平。两种豆娘的基因流均呈现出由南向北的不对称模式。
就体型（body size）与翼载荷（wing loading）而言，蓝尾狭翅蟌的这两项指标呈现出随纬度升高而降低的纬度渐变群（latitudinal clines，即北部个体体型更小）；而塞内加尔狭翅蟌的上述表型指标则呈现出随纬度升高而增大的纬度渐变群（即北部个体体型更大），这一现象在其分布北界附近尤为显著，且与遗传多样性开始急剧下降的区域高度吻合。
此前研究表明，对于变温动物（ectothermic animals）而言，当无法适应温度梯度（thermal gradient）时，其相关性状会形成随纬度升高而增大的渐变群。因此，本研究结果支持如下假说：在地理分布的边缘区域，制约物种适应性进化的主要因素是遗传变异的匮乏，而非基因流淹没（gene flow swamping）。