Phenotypic variability and thermal adaptation in social spider mites: insights into speciation and local adaptation.

Thermal adaptation plays a crucial role in shaping the development, reproduction and population dynamics of ectothermic organisms. In this study, we compared thermal life history traits among three closely related social spider mites: Stigmaeopsis sabelisi, S. miscanthi high-aggression (HG) form, and their common ancestral group, S. miscanthi mild-aggression (ML) form. We investigated the minimum temperature thresholds for development by measuring the days required for egg hatching under five constant temperature conditions (15 °C, 20 °C, 25 °C, 30 °C, 32 °C) and estimating the thresholds using linear and nonlinear regression models. Additionally, we assessed their reproductive diapause attributes. Our results revealed that the minimum development thresholds were slightly lower in S. sabelisi from colder regions compared to S. miscanthi HG form and S. miscanthi ML form distributed in warmer and subtropical regions. Notably, high-temperature stress negatively affected development only in S. sabelisi, suggesting local adaptation. Reproductive diapause attributes also varied: reproductive diapause was induced under short-day conditions in S. sabelisi, whereas the other two species lacked such diapause. Moreover, phenotypic variation in the number of days required for egg hatching was highest in S. miscanthi ML form, suggesting retained ancestral variability that may have facilitated subsequent divergence. These findings support the hypothesis that populations from colder environments exhibit lower thermal thresholds and more intense diapause than those from warmer environments, and also provide insights into the mechanisms driving local adaptation and speciation in the social spider mites.