Butterfly egg plasticity in physiologically and reproductive relevant traits: adaptation to climate?

Embryos of terrestrial animals developing inside immobile eggs face variable environmental conditions. Insect egg pores, critical for gas exchange (aeropyles) and fertilisation (micropyles), are ecologically important yet understudied. While aeropyles likely adapt to climatic respiratory demands, and micropyles may respond to environmental pressures, their inter- and intra-specific variation remains poorly studied. Here, we compared egg pore morphology of three closely related Pieris butterflies sampled in the Netherlands. In addition, we compared egg traits of different P. napi populations sampled along a 4,000-km latitudinal gradient from northern to southern Europe linked these traits with available climatic data. After several generations under identical greenhouse conditions, egg pore traits remained highly variable within and between species and groups. Remarkably, P. napi populations from divergent climates differed more in pore number and size than did co-occurring Pieris species. Aeropyle number and width were strongly associated with climatic variables such as maximum temperature, independent of egg size. Micropyle width varied with seasonality. Additionally, we observed the micropylar pit of older eggs covered by a substance more often than fresh eggs. Our findings indicate that insect egg pores are highly dynamic traits, shaped by ecological, behavioural and climatic factors and an under-recognised layer of egg adaptation.