Abstract
In insects, extra-molting has been viewed as a compensatory mechanism for nymphal growth that contributes to optimize body weight for successful reproduction. However, little is known on the capacity of extra-molting to evolve in natural populations, which limits our understanding of how selection acts on nymphal growth. We used a multi-generational pedigree, individual monitoring and quantitative genetics models to investigate the evolution of extra-molting and its impact on nymphal growth in a solitarious population of the desert locust, Schistocerca gregaria. Growth compensation via extra-molting was observed for 46% of the females, whose adult weight exceeded by 4% that of other females, at a cost of a 22% longer development time. We found a null heritability for body weight threshold only, and the highest and a strongly female-biased heritability for extra molting. Our genetic estimates show that (1) directional selection can act on growth rate, development time and extra-molting to optimize body weight threshold, the target of stabilizing selection, (2) extra-molting can evolve in natural populations, and (3) a genetic conflict, due to sexually antagonistic selection on extra-molting, might prevent its fixation. Finally, we discuss how antagonistic selection between solitarious and gregarious environments and/or genetic correlations between growth and phase traits might also impact the evolution of extra-molting in locusts.
Highlights
Adult body size is a crucial quantitative life-history trait, closely related to individual fitness [1,2,3,4,5]
We investigate the genetic parameters of these traits by using a 2-generational pedigree to identify which trait involved in nymphal development might be able to respond to selection in S. gregaria
We included in the analysis of nymphal growth the 110 individuals that emerged as adults and for which maximal nymphal weight could be estimated
Summary
Adult body size is a crucial quantitative life-history trait, closely related to individual fitness [1,2,3,4,5]. The body size threshold is especially thought to be under strong selection as it determines the minimum size individuals need to attain for successful reproduction [5]. In species in which growth only occurs during juvenile development, the optimal size for reproductive success may be determined before sexual maturity, at the transition to the adult life history stage. Different values of adult body size can be attained by altering growth rate and/or development time [6], both strategies with their own costs and constraints and not mutually exclusive [5,7]. Selection should always optimize body size, life histories can be timeconstrained, for example in seasonal environments where organisms may have a limited time to reach their adult body size threshold [8].
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