When insects help to resolve plant phylogeny:: evidence for a paraphyletic genus Acacia from the systematics and host-plant range of their seed-predators

In this study we use an indirect method to address the issue of the systematics of the large and economically important genus Acacia (Leguminosae, Mimosoideae, Acacieae). We propose the use of host-preference data in closely related insect species as a potentially useful tool to investigate host systematic issues, especially when other approaches yield inconsistent results. We have examined the evolution of host-plant use of a highly specialized group of seed-feeders who predate Acacia - the seed-beetles (Coleoptera, Chrysomelidae, Bruchinae). First, the evolution of host-plant preferences in a large clade of Bruchidius species was investigated using molecular phylogenetics and character optimization methods. Second, the scope of our study was enlarged by critically reviewing the host-plant records of all bruchine genera associated with Acacia. Both morphological and molecular data were used to define relevant insect clades, for which comparisons of host-plant range were performed. Interestingly, the analyses of host-plant preferences from 163 seed-beetle species recovered similar patterns of host-plant associations in the distinct clades which develop within Acacia seeds. Our results clearly support the hypothesis of Acacia being a paraphyletic genus and provide useful insights with reference to the systematics of the whole subfamily as well. This study should also be of interest to those involved in the numerous biological control programs which either already use or aim to use seed-beetles as auxiliary species to limit the propagation of several invasive legume tree species.