Ancient trans-Atlantic flight explains locust biogeography: molecular phylogenetics of Schistocerca

Abstract

The desert locust (Schistocerca gregaria) has been an important agricultural pest at least since biblical times. Although the ecology, physiology and behaviour of this insect species have been well characterized, its biogeographical origins and evolutionary history are more obscure. Schistocerca gregaria occurs throughout Africa, the Middle East and Western Asia, but all other species in the genus Schistocerca are found in the New World. Because S. gregaria has the capacity for extreme long-distance movement associated with swarming behaviour, dispersal may have played an important role in determining current distribution patterns. Some authors have argued that S. gregaria is the product of an eastward trans-Atlantic dispersal from North America to Africa; others consider it more likely that the New World taxa are the product of westward dispersal from Africa. Here, we present a mitochondrial DNA phylogeny of Schistocerca species that supports the monophyly of New World species (including the Galapagos endemic Halmenus) relative to S. gregaria. In concert with observed patterns of molecular divergence, and in contrast to previous morphological studies, our analysis indicates a single trans-Atlantic flight from Africa to South America, followed by extensive speciation and ecological divergence in the New World.

Figure 1

Nymphs of Schistocerca gregaria, solitarious phase on the left and gregarious phase on the right. Photograph by G. Sword.

Figure 2

Two biogeographical hypotheses to explain the distribution of species within the genus Schistocerca. Many species of Schistocerca are found in the Western Hemisphere, but only the desert locust (S. gregaria) is found in the Eastern Hemisphere. On the world map, the distribution of the desert locust is shown in light red and the distribution of Western Hemisphere species of Schistocerca is shown in blue. The dark red areas are occupied by species in other genera within the Cyrtacanthacridinae (which include the outgroups for our analysis). The trees show expected phylogenetic relationships based on the alternative hypotheses of (a) west to east or (b) east to west crossings of the Atlantic Ocean.

Figure 3

Maximum-likelihood tree showing relationships among Western Hemisphere Schistocerca species and the basal position of S. gregaria, a pattern observed using all phylogenetic methods. The tree is rooted using outgroup taxa from the subfamily Cyrtacanthacridinae. Bayesian posterior probabilities and parsimony bootstrap values (in parentheses) are included for nodes of major biogeographical interest. Colours reflect the distributions of species as depicted in figure 2 (dark red, outgroups; light red, S. gregaria; blue, Western Hemisphere species of Schistocerca). Swarming taxa are indicated in bold. Image of S. gregaria after Dirsh (1965).