Extra Molting and Selection on Nymphal Growth in the Desert Locust

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.

Exploring the relationship between tychoparthenogenesis and inbreeding depression in the Desert Locust, Schistocerca gregaria

“…Finally, as expected, sex and extramolting had a significant effect on larval development time and on adult femur length, such that extramolting females had the longest development time and biggest size (Table 2; Pélissié et al., 2016). …”

“…These populations originated from the sampling of egg pods in locust laboratories in England, Belgium, France, and Mauritania. All these laboratory lines were first initiated from wild populations of North Africa (Berthier et al., 2010; Pelissié et al., 2016). They correspond to three types of laboratory rearing history: long‐term (England and Belgium), recent (France) and none (Mauritania; Table 1).…”

“…The English and Belgian populations have been reared in the laboratory for >25 years, that is, over 100 generations. The French population was collected from nine egg pods in Mauritania in 2010 and has been bred in the laboratory for seven generations (Pelissié et al., 2016). Finally, Mauritanian population is directly derived from six egg pods collected in the field in February and March 2013 by K. Maeno in Mauritania (JIRCAS, Kyoto University) and was immediately used for the experiment.…”

“…Rearing conditions in the laboratory were 32°C and 50% humidity with a photoperiod of 13/11 hours (Pelissié et al., 2016). The first experimental generation is composed by full‐sib families that were made of the egg pods collected from each population ( n  = 8; 8; 7 and 6 for England, Belgium, France, and Mauritania, respectively).…”

“…Between the third and fourth instars, desert locusts can undergo an extramolt, which leads to six rather than five instars. This extramolt is a catch‐up growth strategy allowing small‐size offspring to increase their body size at adult molt, at the cost of a longer development time (Pelissié et al., 2016). Therefore, for each individual we recorded the number of larval molts and used it as an explanatory variable.…”