Abstract
Widespread species often show geographic variation in thermally-sensitive traits, providing insight into how species respond to shifts in temperature through time. Such patterns may arise from phenotypic plasticity, genetic adaptation, or their interaction. In some cases, the effects of genotype and temperature may act together to reduce, or to exacerbate, phenotypic variation in fitness-related traits across varying thermal environments. We find evidence for such interactions in life-history traits of Heteronympha merope, a butterfly distributed across a broad latitudinal gradient in south-eastern Australia. We show that body size in this butterfly is negatively related to developmental temperature in the laboratory, in accordance with the temperature-size rule, but not in the field, despite very strong temperature gradients. A common garden experiment on larval thermal responses, spanning the environmental extremes of H. merope's distribution, revealed that butterflies from low latitude (warmer climate) populations have relatively fast intrinsic growth and development rates compared to those from cooler climates. These synergistic effects of genotype and temperature across the landscape (co-gradient variation) are likely to accentuate phenotypic variation in these traits, and this interaction must be accounted for when predicting how H. merope will respond to temperature change through time. These results highlight the importance of understanding how variation in life-history traits may arise in response to environmental change. Without this knowledge, we may fail to detect whether organisms are tracking environmental change, and if they are, whether it is by plasticity, adaptation or both.
Highlights
Studies on broadly distributed insects have repeatedly shown geographic patterns in larval development time, growth rate and adult size in response to climatic gradients across the landscape [1,2]
The thorax length of male butterflies were on average smaller (61.160.25 mm) than their female counterparts (67.760.24 mm)
In the 8uC treatment, no female adults successfully emerged, while only 8 male butterflies emerged but with abnormally large thoraces, mouth parts and legs, and with disproportionately small wings; these individuals survived for up to only three days. These responses to the 8uC treatment were evenly spread throughout the populations, suggesting that chronic exposure to 8uC during larval development is close to the lower thermal limit for H. merope
Summary
Studies on broadly distributed insects have repeatedly shown geographic patterns in larval development time, growth rate and adult size in response to climatic gradients across the landscape [1,2]. Geographic variation in these traits may arise in response to multiple selection pressures, involving phenotypic plasticity, genetic adaption, or both. These life-history traits are often linked; changes in larval growth and development can have a direct impact on adult size [3,4]. As body temperature rises, metabolic processes increase but the reaction norms are such that maturation increases proportionately more than growth, leading to a decrease in body size at maturity in response to increasing temperature
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