Data from: Climatic factors shape plastic trade-offs in the polyphenic black scavenger fly Sepsis thoracica (Diptera: Sepsidae)

Aim: Trade-offs allow individuals to optimize their fitness by tailoring
the investment into different traits to variable environmental conditions,
such as along geographic gradients. Trade-offs thus can help in adjusting
to changing thermal and insolation profiles, especially in small
ectotherms, whose body temperature typically follows environmental
temperatures closely. Two traits usually involved in latitudinal
adaptation are body size and melanism. Since both traits are costly,
individuals need to optimize investment into each trait. Here we studied
how environmental factors influence this trade-off in the short and long
term. Location: Europe Methods: We raised flies from 15 latitudinal
populations at three constant temperatures in a laboratory common garden
to differentiate plastic and evolutionary responses to temperature. We
further analysed how the different insolation components of the
populations’ habitats influenced the evolution of the trade-off. Results:
Male Sepsis thoracica (Diptera: Sepsidae) feature a sigmoid relationship
between melanism and body size, defining two strikingly different male
morphs: obsidian (small and black) and amber (large and orange). This
relationship was altered by the developmental temperature, documenting its
plasticity. The relationship further evolved across populations in
response to the environmental characteristics of their habitat, notably
temperature, insolation and UV radiation, suggesting that plasticity also
has an underlying genetic basis. Nevertheless, melanism, but not body
size, merely slightly increased with latitude. Main Conclusions: As the
plastic and evolutionary responses of the relationship to temperature
differed, plasticity does not necessarily follow the direction of
evolution of this trade-off, but rather adds to it. Our study evinces the
role of several environmental factors in shaping the evolution of a
plastic melanism – body size relationship defining a rare male
polymorphism in temperate sepsid flies.