Geographic variation in phenotypic traits is commonly correlated with spatial variation in the environment, e.g. seasonality and mean temperature, providing evidence that natural selection generates such patterns. In particular, both body size and egg size of ectothermic animals are commonly larger in northern climates, and temperature induces plastic responses in both traits. Size-independent egg quality can also vary with latitude, though this is rarely investigated. For the widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae) we investigated whether there are latitudinal clines in reproductive traits (clutch size, egg size and egg composition), whether these clines are due to variation in body and/or egg size, and whether such clines replicate across independent experiments performed on different continents (North America and Europe). Egg size generally increased with latitude (especially in Europe), an effect largely explained by body size of the mother, while clutch size did not; overall reproductive effort thus increased with latitude. Both the absolute and relative (correcting for egg size) amount of egg protein increased with latitude, egg glycogen decreased with latitude, while latitudinal trends for egg lipids and total egg energy content were complex and non-linear. Altitude sometimes showed relationships analogous to those of latitude (egg proteins and glycogen) but occasionally opposite (egg size), possibly because latitude and altitude are negatively related among populations of this cold-adapted species. There was no evidence of a tradeoff between egg size and number across latitudinal populations; if anything, the relationship was positive. All traits, including body and egg size, varied with rearing temperature (12°C, 18°C, 24°C), generally following the temperature–size rule. Clines based on common garden rearing, thus reflecting genetic differentiation, were qualitatively but not always quantitatively consistent between continents, and were similar across rearing temperatures, suggesting they evolved due to natural selection, although the concrete selective mechanisms involved require further study.
|Number of pages
|Published - Nov 2018
Bibliographical noteFunding Information:
Acknowledgements – We thank U. Briegel for technical assistance, and Craig Stillwell, Bill Wallin and Goggy Davidowitz for help and advice at various stages of this long-term project. We are especially grateful to the numerous fly collectors listed in Table 1. Funding – This work was supported by the German research council (DFG grant BA3760/1-1) to SSB, by the University of Kentucky Agricultural Experiment Station to CWF, and by several grants over the years from the Swiss National Foundation (most notably grant no. 3100A0-111775), the Zoological Museum Zurich, and the University of Zurich.
© 2018 The Authors
- egg composition
- egg size
- latitudinal cline
- reproductive investment
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics