Are recession populations of the desert locust (Schistocerca gregaria) remnants of past swarms?

The desert locust (Schistocerca gregaria) undergoes crowding-induced phase transformation from solitary to gregarious, which involves changes in behav iour, colour, development, morphometry, fecundity and endocrine physiology. During recession, solitary locusts persist in the central, drier part of t he species' range in small pocket populations that are prone to extinction. During the intermittent upsurges and the subsequent plagues, gregarious sw arms attain huge population size and invade a vast area causing major damag e to agriculture. A highly variable nuclear DNA marker, a noncoding 3' end fragment of an antennapedia-class homeobox gene, was screened in locust sam ples from Eritrea. Despite the homogenizing potential of plague swarms, the last of which was in 1986-89 and originated in this region, the population genetic structure of solitary phase locusts along the Red Sea coast of Eri trea revealed significant divergence. The pattern of divergence indicated t hat the invasion of the western and northern plains in the summer of 1995 m ay not, as reported then, have originated in eastern Chad or western Sudan. A number of interrelated hypotheses have been presented to explain the obs erved genetic heterogeneity between the sampled populations. We conclude, w ith caution due to the limited sample sizes, that: (i) geographical isolati on between breeding sites during plagues and recession; (ii) the marked dif ferences in the flight behaviour of plague swarms and recession populations ; (iii) possible failure of gregarious locusts to solitarize and re-establi sh in recession areas; and (iv) the effect of repeated extinction and recol onization in the meta-population contribute to the maintenance of the genet ic structure of recession populations. Potentially productive future resear ch has been identified.