Evolution of Spermophagus seed beetles (Coleoptera, Bruchinae, Amblycerini) indicates both synchronous and delayed colonizations of host plants

Abstract

Seed beetles are a group of specialized chrysomelid beetles, which are mostly associated with plants of the legume family (Fabaceae). In the legume-feeding species, a marked trend of phylogenetic conservatism of host use has been highlighted by several molecular phylogenetics studies. Yet, little is known about the evolutionary patterns of association of species feeding outside the legume family. Here, we investigate the evolution of host use in Spermophagus, a species-rich seed beetle genus that is specialized on two non-legume host-plant groups: morning glories (Convolvulaceae) and mallows (Malvaceae: Malvoideae). Spermophagus species are widespread in the Old World, especially in the Afrotropical, Indomalaya and Palearctic regions. In this study we rely on eight gene regions to provide the first phylogenetic framework for the genus, along with reconstructions of host use evolution, estimates of divergence times and historical biogeography analyses. Like the legume-feeding species, a marked trend toward conservatism of host use is revealed, with one clade specializing on Convolvulaceae and the other on Malvoideae. Comparisons of plants' and insects' estimates of divergence times yield a contrasted pattern: on one hand a quite congruent temporal framework was recovered for morning-glories and their seed-predators; on the other hand the diversification of Spermophagus species associated with mallows apparently lagged far behind the diversification of their hosts. We hypothesize that this delayed colonization of Malvoideae can be accounted for by the respective biogeographic histories of the two groups.

Keywords: Bruchinae; Convolvulaceae; Historical biogeography; Host-plant associations; Malvoideae; Spermophagus.