Latitudinal clines in sexual selection, sexual size dimorphism, and sex-specific genetic dispersal during a poleward range expansion

Range expansions can be shaped by sex differences in behaviours and other phenotypic traits affecting dispersal and reproduction. Here, we investigate sex differences in morphology, behaviour and genomic population differentiation along a climate-mediated range expansion in the common bluetail damselfly Ischnura elegans in northern Europe. We sampled 65 sites along a 583 km gradient spanning the I. elegans range in Sweden and quantified latitudinal gradients in site relative abundance, sex ratio and sex-specific shifts in body size and mating status (a measure of sexual selection). Using single nucleotide polymorphism (SNP) data for 426 individuals from 25 sites, we further investigated sex-specific landscape and climatic effects on neutral genetic connectivity and migration patterns. We found evidence for sex differences associated with the I. elegans range expansion, namely (1) increased male body size with latitude, but no latitudinal effect on female body size, resulting in reduced sexual dimorphism towards the range limit, (2) a steeper decline in male genetic similarity with increasing geographic distance than in females, (3) male-biased genetic migration propensity, and (4) a latitudinal cline in migration distance (increasing migratory distances towards the range margin), which was stronger in males. Cooler mean annual temperatures towards the range limit were associated with increased resistance to gene flow in both sexes. Sex ratios became increasingly male-biased towards the range limit, and there was evidence for a changed sexual selection regime shifting from favouring larger males in the south, to favouring smaller males in the north. Our findings suggest sex-specific spatial phenotype sorting at the range limit, where larger males disperse more under higher landscape resistance associated with cooler climates. The combination of latitudinal gradients in sex-biased dispersal, increasing male body size, and (reduced) sexual size dimorphism should have emergent consequences for sexual selection dynamics and the mating system at the expanding range front. Our study illustrates the importance of considering sex differences in the study of range expansions driven by ongoing climate change.

Methods
Data are RADseq data as in Dudaniec et al (2018) Mol Ecol, filtered as described in this paper

种群扩张的过程可受到影响扩散与繁殖的行为及其他表型性状中的性别差异塑造。本研究以北欧分布的常见蓝尾蜻蛉（Ischnura elegans）为研究对象，针对其经气候介导的种群扩张梯度，探究其形态、行为及基因组种群分化的性别差异。我们在瑞典境内覆盖该物种完整分布范围的583千米纬度梯度上设置了65个采样位点，量化了各站点的相对丰度、性比，以及体型大小与交配状态（性选择的衡量指标）的性别特异性纬度梯度变化。我们利用25个采样位点共426个个体的单核苷酸多态性（Single Nucleotide Polymorphism, SNP）数据，进一步探究了性别特异性的景观与气候因子对中性遗传连通性及迁移模式的影响。本研究发现了与该蜻蛉种群扩张相关的性别差异证据，具体如下：（1）雄性体型随纬度升高而增大，但雌性体型未表现出显著的纬度梯度效应，最终导致种群分布边界处的性二态性降低；（2）相较于雌性，雄性的遗传相似性随地理距离增加的下降幅度更为陡峭；（3）存在雄性偏向的遗传迁移倾向；（4）迁移距离呈现纬度渐变群特征（向分布边缘的迁移距离逐渐增加），且该效应在雄性中更为显著。分布边缘区域的年均温更低，与两性的基因流阻力升高均存在显著关联。种群边界处的性比逐渐偏向雄性，且有证据表明性选择体系发生转变：南部种群偏向选择体型更大的雄性，而北部种群则偏向选择体型更小的雄性。本研究结果表明，种群分布边界处存在性别特异性的空间表型分选——在气候更寒冷、景观阻力更高的环境中，体型更大的雄性具有更强的扩散能力。性偏扩散的纬度梯度、雄性体型增大，以及（降低的）性尺寸二态性的共同作用，将对扩张种群前沿的性选择动态与交配系统产生涌现效应。本研究阐明了在气候变化驱动的种群扩张研究中，考虑性别差异的重要性。

方法
本研究使用的为限制性位点相关DNA测序（Restriction-site Associated DNA sequencing, RADseq）数据，数据处理流程参考Dudaniec等人（2018）发表于《Molecular Ecology》的研究，并按照本文所述方法完成过滤。