Hardening capacity in the Drosophila melanogaster species group is constrained by basal thermotolerance

1. To determine the relationship between basal thermotolerance and hardening capacity across related taxa, these traits were quantified for eight Drosophila species: D. melanogaster, D. simulans, D. sechellia and D. yakuba of the melanogaster subgroup and D. auraria, D. vulcana, D. serrata and D. birchii of the montium subgroup. The effects of pretreatment temperature and recovery time between pretreatment and heat shock on thermotolerance were also assessed. 2. For any given species, all elevated temperature pretreatments (31 degrees C, 33 degrees C and 35 degrees C) equally increased thermotolerance. 3. For any given species, increasing recovery time between pretreatment and heat shock increased thermotolerance, though this effect may be related to factors other than hardening. 4. Basal thermotolerance and absolute hardening capacity (pretreated thermotolerance minus basal thermotolerance) varied considerably among the species, and was not phylogenetically clustered. 5. Though unmodified and phylogenetically corrected correlations indicated a relationship between basal thermotolerance and absolute hardening capacity, this result was largely due to the presence of one species, the highly thermotolerant D. melanogaster. 6. Relative hardening capacity (absolute hardening capacity divided by basal thermotolerance) was fixed for all species at approximately 0.25.