Comparative reproductive dormancy differentiation in European black scavenger flies (Diptera: Sepsidae)
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Seasonality is a key environmental factor that regularly promotes life history adaptation. Insects invading cold–temperate climates need to overwinter in a dormant state. We compared the role of temperature and photoperiod in dormancy induction in the laboratory, as well as winter survival and reproduction in the field and the laboratory, of 5 widespread European dung fly species (Diptera: Sepsidae) to investigate their extent of ecological differentiation and thermal adaptation. Unexpectedly, cold temperature is the primary environmental factor inducing winter dormancy, with short photoperiod playing an additional role mainly in species common at high altitudes and latitudes (Sepsis cynipsea, neocynipsea, fulgens), but not in those species also thriving in southern Europe (thoracica, punctum). All species hibernate as adults rather than juveniles. S. thoracica had very low adult winter survivorship under both (benign) laboratory and (harsh) field conditions, suggesting flexible quiescence rather than genetically fixed winter diapause, restricting their distribution towards the pole. All other species appear well suited for surviving cold, Nordic winters. Females born early in the season reproduce before winter while late-born females reproduce after winter, fulgens transitioning earliest before winter and thoracica and punctum latest; a bet-hedging strategy of reproduction during both seasons occurs rarely but is possible physiologically. Fertility patterns indicate that females can store sperm over winter. Winter dormancy induction mechanisms of European sepsids are congruent with their geographic distribution, co-defining their thermal niches. Flexible adult winter quiescence appears the easiest route for insects spreading towards the poles to evolve the necessary overwinter survival.
KeywordsDiapause Dormancy Diptera Genetic differentiation Overwinter survival Phylogenetic signal Plasticity Quiescence Species comparison Thermal adaptation
This work is in part based on an UZH Master’s Thesis (2015) by Valérian Zeender, and was supported over the years by the University of Zurich and several grants from the Swiss National Science Foundation, most recently Grant No. 31003A_143787.
Author contribution statement
WUB, MAS and PTR conceived and designed the entire study and analysed the final data; VZ conducted the laboratory diapause induction experiment (his MSc project); JR, AW, PTR and NG performed the field experiment over 2 years; WUB, PTR and MAS wrote the manuscript with input from all others.
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