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The ability to tolerate temperature stress is an important component of adult fitness. In holometabolous insects like Drosophila melanogaster, adult stress resistance can be affected by growth conditions experienced during the larval stages. Although evolution under crowded larval conditions is known to lead to the correlated evolution of many adult traits, its consequences on adult heat stress tolerance have not been investigated. Therefore, in the present study, we assessed the adult heat stress tolerance in populations of D. melanogaster adapted to a stressful larval crowding environment. We used replicate populations of D. melanogaster, selected for adaptation to larval crowding stress (MCUs), for more than 230 generations, and their respective controls (MBs). Larvae from selected and control populations were grown under crowded and uncrowded conditions, and their adult heat shock resistance at two different temperatures was measured. Further, we compared Hsp70 expression in crowded and uncrowded larvae of both populations and also measured the Hsp70 expression after a mild heat treatment in adults of selected and control populations. Our results showed that adaptation to larval crowding leads to the evolution of Hsp70 gene expression in larval stages and improves adult heat stress tolerance ability in males, but not in females.

Author contributions: MGL, EIS and AN designed the study. MGL and AN carried out the experiments. EIS and his former field assistants and PhD-students collected the observational long-term data. MGL conducted the statistical analyses and wrote the text with substantial input from EIS and AN.Acknowledgements: Observational data used in this study has been collected with the help of numerous PhD-students, field assistants and interns over the years. Sofie Nilén and Emily Scott helped collecting I. elegans individuals in the mating trial experiments. EIS was financially supported by research grants from Gyllenstiernska Krapperupsstiftelsen, Olle Engqvist Byggmästare Foundation (postdoctoral scholarship to MGL), Lunds Djurskyddsfond and the Swedish Research Council (VR; grant no. 2016-03356). Data Accessibility Statement: Data will be accessible in a public repository upon acceptance Word count (excluding figure legends and references): 6910 Text includes 4 figures, 2 table and 4 supplementary tables. ABSTRACTCondition-dependent sexual selection has been suggested to reduce mutation load, and sexual selection might also accelerate local adaptation and promote evolutionary rescue through several ecological and genetic mechanisms. Research on conditiondependent sexual selection has mainly been performed in laboratory settings, while data from natural populations are lacking. One ecological factor that can cause condition-dependent sexual selection is parasitism. Here, we quantified ectoparasite load (Arrenurus water mites) in a natural population of the common bluetail damselfly (Ischnura elegans) over 15 years. We estimated parasite-mediated sexual selection in both sexes and investigated how parasite resistance and tolerance changed over time and influenced population density. Parasites reduced mating success in both sexes, but the effects were stronger in males than in females. The male fitness advantage of carrying fewer parasites was higher under experimental low-density conditions than under high-density field conditions, suggesting that male-male competition could reduce parasite-mediated sexual selection. We further show that population density declined during the study period, while parasite resistance and male fitness tolerance (fecundity) increased, suggestive of increasing local adaptation against parasites and ongoing evolutionary rescue. We suggest that condition-dependent sexual selection can facilitate population persistence and promote evolutionary rescue by increasing local adaptation against parasites.

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