Génétique des populations de sous-espèces pullulantes et non-pullulantes du criquet migrateur Locusta migratoria

The migratory locust, Locusta migratoria, is an agricultural pest of major importance in large areas of the Ancient world. Nine geographical taxa of L. migratoria with different propensities to display outbreaks were described, and considered as subspecies. Whether the population dynamics associated with outbreak events influence the genetic diversity within and between populations is still unknown. This question was addressed by analysing the genetic diversity at microsatellite loci in two sets of populations characterized by contrasted patterns of outbreak events. In the north-west Mediterranean area, only uncommon and low intensity outbreaks have been reported. This area, which includes the subspecies L. m. cinerascens and L. m. migratoria, is hence considered as a non-outbreaking area. In Madagascar, frequent and intense outbreak events have been recorded over the last century (36 years of pullulation vs. 64 years of remission). Madagascar (L. m. capito subspecies) is thus considered as a frequently outbreaking area. Nine and twelve populations were sampled over a similar geographical scale in the north-west Mediterranean area and in Madagascar respectively, and analysed at 14 polymorphic microsatellite markers. A similar level of within population diversity was observed in both areas (mean number of alleles: 13.039 vs. 13.232 and mean expected heterozygosity: 0.843 vs. 0.833 in north-west Mediterranean and in Madagascar respectively). This suggests that effective sizes in non-outbreaking subspecies are similar to the harmonic means of effective population sizes in outbreaking subspecies. On the other hand, genetic differentiation between populations was significantly lower in the Malagasy outbreaking subspecies (global Fst = 0.002) than in the north-west Mediterranean subspecies (global Fst = 0.058). Altogether, these results suggest a higher effective migration rate in the outbreaking subspecies than in non-outbreaking subspecies.