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

Abstract

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 (a) increased male body size with latitude, but no latitudinal effect on female body size, resulting in reduced sexual dimorphism towards the range limit, (b) a steeper decline in male genetic similarity with increasing geographic distance than in females, (c) male‐biased genetic migration propensity and (d) 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.

Body and wing size, but not wing shape, vary along a large-scale latitudinal gradient in a damselfly

“…The observed differences in body size between the sexes along the latitudinal gradient, might be explained by latitudinal variation of sex-specific selection on body size. A recent study on the damselfly Ischnura elegans (Vander Linden, 1820) has found a decrease in sexual size dimorphism at north edge populations, as a result of an increase in male body size 16 . Our results show, however, a likely increase in female body size at north edge populations, resulting in an increase in sexual size dimorphism.…”

“…Our results show, however, a likely increase in female body size at north edge populations, resulting in an increase in sexual size dimorphism. One reason for these differences might be related to sex biases in dispersal: Ischnura damselflies can show male-biased dispersal 16 , while Lestes damselflies have been suggested to show female-biased dispersal 50 . Moreover, since females invest more in fecundity than males, females are probably under stronger fecundity selection at edge populations.…”

“…However, dispersal and reproductive traits can be subject to conflicting selection pressures at edge populations 15 , possibly limiting the range expansion process 7 . Moreover, intralocus sexual conflict can vary along a species distribution range, potentially leading to smaller sex differences in traits under relaxed selection at edge populations 16 , 17 . Exploring variation of morphological traits across large latitudinal ranges and the signatures of selection and spatial dynamics at edge populations is important to understand the evolutionary and ecological fate of species facing environmental change 18 , 19 .…”

Temporal and spatial variation in sex-specific abundance of the avian vampire fly (Philornis downsi)

“…In general, sexual conflict and sexual dimorphism have been shown to drive sex-specific distributions in arthropods (Foster and Soluk 2006 ; Romey and Wallace 2007 ; Stanley et al 2018 ). Patterns of male and female abundance can differ due to sex-biased dispersal, with extremes where one sex disperses while the other is sedentary, which is especially relevant during range expansions or invasions (Beirinckx et al 2006 ; Dudaniec et al 2021 ; Miller and Inouye 2013 ). In other cases, females may aggregate together, away from areas of high male density, to avoid harassment from males, which is seen in systems with high costs to females from multiple mating (Roswell et al 2019 ; Stanley et al 2018 ; Stone 1995 ; Warburg and Yuval 1997 ).…”

Understanding climate change response in the age of genomics

“…The study by Moerman et al highlights both the incredible power of using experimental evolution to isolate and study mechanisms and contingencies of evolution during range shifts and, as emphasised by the authors, some important limitations of such approaches, with regard to inherent biases related to the precision by which, and type of, adaptive genetic change that can be detected. Also in this Special Feature, Dudaniec et al (2021) study genomic changes associated with a rapidly range shifting damselfly species, Ischnura elegans, in Northern Europe. Specifically, they find that temperature poses a strong barrier to dispersal in both male and female I. elegans, which would predict a slowing of the range shift at higher latitudes where cooler temperatures are encountered.…”