Complex response in size-related traits of bulb mites (Rhizoglyphus robini) under elevated thermal conditions - an experimental evolution approach.

Temperature is a key environmental factor affecting almost all aspects of life history in ectotherms. Theory predicts that they grow faster, reach smaller sizes and produce smaller offspring when temperature increases. In addition, temperature changes, through their effects on metabolism, may also influence the expression of alternative reproductive phenotypes (ARPs) in ectotherms. Although many studies have investigated the phenotypic plasticity of life history traits in relation to temperature change, little is known about how those traits and phenotypic plasticity may evolve together. In our study we subjected bulb mites (non-model, soil organisms that normally experience rather stable thermal conditions) to experimental evolution in two temperature treatments: control (24°C) and elevated (28°C). After 18 generations, we measured adult body size, egg size and development time of both treatments at control as well as at elevated temperatures (test temperatures). Thus, we were able to detect genetic changes (the effects of selection temperature) and environmental effects (the effects of test temperature). We also observed the ARP expression throughout the experimental evolution. Our results revealed quite complex patterns of life history in traits response to temperature. Mites developed faster and reached smaller sizes at increased temperature, but evolutionary responses to increased temperature were not always parallel to the observed phenotypic plasticity. Additionally, despite smaller body sizes, females laid larger eggs at higher temperature. This effect was more pronounced in animals evolving at elevated temperature. Evolution at increased temperature also affected ARP expression, with the proportion of armored fighters decreasing from generation to generation. We propose that this could be the consequence of temperature sensitivity of the cost-to-benefits ratio of expressing ARPs.