Plague dynamics and population genetics of the desert locust: can turnoverduring recession maintain population genetic structure?

The desert locust (Schistocerca gregaria) undergoes crowding-induced phase transformation from solitary form to gregarious form. The transformations i nvolves changes in behaviour, colour, development, morphometry, fecundity a nd endocrine physiology. Recession populations of the desert locust exist p rimarily in the solitary phase as small populations in patchy environments and are prone to extinction because of climatic events. Significant genetic differentiation among recession populations along the Red Sea coast of Eri trea was previously reported. It was hypothesized that despite the mixing e ffect of recurrent swarms, metapopulation dynamics could have produced gene tic divergence among these highly scattered recession populations. A Monte Carlo simulation of the population dynamics of the desert locust in a metap opulation setting, with a realistic range of parameter values clearly demon strated that this is possible. Population growth was represented by a discr ete-time logistic equation. The duration of recessions and swarms was sampl ed from normal distributions whose means and standard deviations were varie d based on reported estimates. An average recession duration of 10 +/- 3 ge nerations and swarm periods half as long but almost twice as variable produ ced a partitioning of the total genetic variance most similar to that in th e empirical study. In conventional metapopulation analysis, whether turnove r leads to increased or reduced divergence is dependent on the number of co lonists relative to the number of recurrent migrants, and on whether the co lonists arise from a single patch or many patches. In the case of locusts, the stochastic boom and bust cycle is the overriding factor. Divergence bet ween patches during recession due to founder effect and recurrent drift is balanced by the high rate of mixing during plagues.