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.

在无物理屏障的条件下，哪些因素会限制物种的分布范围？研究假说提出，不对称基因流（asymmetric gene flow）引发的遗传负荷（genetic load），以及基因流缺失导致的遗传变异匮乏，会制约边缘种群对新生境的适应能力，进而阻碍物种分布范围的扩张。本研究针对日本境内沿纬度梯度分布的两种异痣蟌属昆虫——亚洲蓝尾蟌（Ischnura asiatica）与塞内加尔异痣蟌（Ischnura senegalensis）——开展了遗传结构与形态性状地理变异的分析：其中日本为亚洲蓝尾蟌的分布中心，同时也是塞内加尔异痣蟌的分布北界。全基因组遗传分析结果显示，塞内加尔异痣蟌的分布边缘区域存在遗传多样性（genetic diversity）丢失现象，而亚洲蓝尾蟌的遗传多样性始终维持在较高水平。两个物种的基因流均呈现出南-北向的不对称模式。在日本境内的亚洲蓝尾蟌种群中，体型大小与翼载荷（wing loading）表现出随纬度升高而降低的纬度渐变群（latitudinal cline），即北部个体体型更小；而塞内加尔异痣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，这一特征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。已有研究表明，对于变温动物（ectothermic animals）而言，当其无法适应温度梯度（thermal gradient）时，相关功能性状便会形成正向纬度渐变群。因此，本研究结果支持如下假说：地理分布边缘的适应限制，主要由遗传变异匮乏而非基因流淹没（gene flow swamping）所导致。