Largely flat latitudinal life history clines in the dung fly Sepsis fulgens across Europe (Diptera: Sepsidae)
Clinal variation in body size and related life history traits is common and has stimulated the postulation of several eco-geographical rules. Whereas some clinal patterns are clearly adaptive, the causes of others remain obscure. We investigated intra-specific body size, development time and female fecundity (egg size and number) clines across 13 European populations of the dung fly Sepsis fulgens spanning 20° latitude from southern Italy to Estonia in a genetic common garden approach. Despite very short generation times (ca. 2 weeks at 24 °C), we found a converse Bergmann cline (smaller size at higher latitudes). As development time did not change with latitude (flat cline), integral growth rate thus likely declines towards the pole. At the same time, early fecundity, but not egg size, increased with latitude. Rather than being mediated by seasonal time constraints, the body size reduction in the northernmost flies from Estonia could suggest that these are marginal, edge populations, as when omitting them the body size cline became flat as well. Most of the other sepsid species investigated to date also show flat body size clines, a pattern that strikingly differs from Drosophila. We conclude that S. fulgens life history traits appear to be shaped by similar environmental pressures and selective mechanisms across Europe, be they adaptive or not. This reiterates the suggestion that body size clines can result as a secondary consequence of selection pressures shaping an entire life history syndrome, rendering them inconsistent and unpredictable in general.
KeywordsBody size Development time Egg size Fecundity Geographic differentiation Genetic differentiation Latitudinal cline Life history
We thank Martin Schäfer, Juan Pablo Busso, Anders Kjaersgaard, Nalini Puniamoorthy, Toomas Esperk, and Toomas Tammaru for collecting flies, and the entire Wolfpack for various support. This study was supported by Grant No. SNF 31003A-143787 from the Swiss National Science Foundation.
Author contribution statement
JR, WUB and PTR conceived and designed the study, and contributed all to the statistical analysis and the writing of the manuscript. JR and PTR performed the experiments.
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