Data from: Replicated latitudinal clines in reproductive traits of European and North American yellow dung flies

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 trade-off 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.