Geographic clines in wing morphology relate to colonization history in New World but not Old World populations of yellow dung flies

Abstract

Geographic clines offer insights about putative targets and agents of natural selection as well as tempo and mode of adaptation. However, demographic processes can lead to clines that are indistinguishable from adaptive divergence. Using the widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), we examine quantitative genetic differentiation (Q ) of wing shape across North America, Europe, and Japan, and compare this differentiation with that of ten microsatellites (F ). Morphometric analyses of 28 populations reared at three temperatures revealed significant thermal plasticity, sexual dimorphism, and geographic differentiation in wing shape. In North America morphological differentiation followed the decline in microsatellite variability along the presumed route of recent colonization from the southeast to the northwest. Across Europe, where S. stercoraria presumably existed for much longer time and where no molecular pattern of isolation by distance was evident, clinal variation was less pronounced despite significant morphological differentiation (Q >F ). Shape vector comparisons further indicate that thermal plasticity (hot-to-cold) does not mirror patterns of latitudinal divergence (south-to-north), as might have been expected under a scenario with temperature as the major agent of selection. Our findings illustrate the importance of detailed phylogeographic information when interpreting geographic clines of dispersal traits in an adaptive evolutionary framework.

Life history traits, but not body size, vary systematically along latitudinal gradients on three continents in the widespread yellow dung fly

“…Neutral population differentiation of yellow dung fly populations, F ST , based on 10 microsatellite markers is very low but significantly greater than zero overall (p < 0.001). The worldwide F ST is 0.044 (95% CI: 0.034, 0.059), but differentiation was much lower within continents: 0.005 (95% CI: 0.002, 0.007) in Europe, 0.014 (95% CI: 0.001, 0.019) in North America, and 0.006 (95% CI: -0.003, 0.017) in Japan (elaborated in Schäfer et al 2018; cf. Supplementary material Appendix 1 Table A2).…”

“…Once there, flies were fed and allowed to copulate (again) with a random male from the same population within few days to generate a laboratory F1 generation (at 18°C, 12 h photoperiod, and 60% relative humidity) for our common garden rearing described below. Thereafter, the fieldcaught flies were preserved in pure ethanol and/or frozen at -80°C for later population genetic analysis of 10 polymorphic microsatellite loci to calculate F ST , as described in detail by Schäfer et al (2018). Supplementary material Appendix 1 Table A2 provides essential information on sample sizes, number of alleles, and mean observed (H O ) and expected (H E ) heterozygosity for each locus for North American, European and Japanese fly populations.…”

“…Confidence intervals of Q ST were calculated by a parametric bootstrap method adapted from O'Hara and Merilä (2005) and implemented in the R package 'QstFstComp' (< https://github.com/kjgilbert/ QstFstComp >). Neutral population variation F ST was calculated in a standard way as explained in Schäfer et al (2018), from which we took the estimates for our F ST /Q ST comparison here.…”

“…Their wide latitudinal range notwithstanding, the experienced climatic conditions at the beginning and the end of the season may actually not differ so much for flies at the various sites because the season begins later and ends earlier at higher latitudes (as winter length increases: Supplementary material Appendix 1 Table A1). Particularly in light of the species' great dispersal capacity mediating gene flow (Kaufmann et al 2013, Schäfer et al 2018), generally weaker selection may thus produce only slight differentiation in growth, development and final body size, presumably because these traits are optimized according to similar costs and benefits everywhere, ultimately resulting in the rather shallow clines observed. Body size (hind tibia length), in contrast, varied considerably with latitude, with specifics differing even qualitatively among continents (Fig.…”

Intraspecific mating system evolution and its effect on complex male secondary sexual traits: Does male–male competition increase selection on size or shape?

“…That is, we scaled the dot product of A and S by their norm (cf. Claude, ; Pitchers, Pool, & Dworkin, ; Schäfer et al, ). To test for selection on nonallometric shape effects (i.e.…”