Genetic determinism of gregarization in the migratory locust Locust migratoria

The density-dependent transition between the solitarious and gregarious phases, called gregarization, is a defining feature of the migratory locust, Locusta migratoria, being central to the biology of the species and to its significance as a serious pest. Nine geographical taxa of L. migratoria with different propensities to display outbreaks were described and considered as subspecies. Among these subspecies, swarming and the expression of gregarization are highly correlated. Differences in the propensity to gregarization between subspecies may reflect differences in their environments, some being rarely and others cyclically favourable to increases in population density (e.g. paleartic zone vs. intertropical zone, respectively). On the other hand, it is still unknown whether the gregarization associated with outbreaks has a genetic determinism. This question was addressed by analysing the magnitude of behavioural and morphometrical gregarization under environmental control in two populations characterized by contrasted patterns of outbreak events. In France, only uncommon and low intensity outbreaks have been reported. This area (L. m. cinerascens) is hence considered as a non-outbreaking area. In Madagascar, frequent and intense outbreak events have been recorded over the last century. Madagascar (L. m. capito) is thus considered as a frequently outbreaking area. Our results show that Malagasy population expressed significantly more pronounced gregarization relative to French population. Because both populations were raised in a common environment during our experiment, it is highly likely that a genetically based adaptive process is responsible for the gregarization differences between subspecies. More specifically the high propensity for gregarization of the Malagasy population may have evolved due to selection for maintaining phase plasticity. The lower propensity for gregarization of the French population may rather be an experimentally induced re-expression of the ancestral gregarious character, which evolved by genetic drift since selection for this trait was relaxed (atavism). (Texte intégral