Evolutionary aspects of host plant specialisation - a study on bruchids (Coleoptera: Bruchidae)

We have studied the relationship of bruchids and their legume hosts by extensive field sampling throughout Hungary over 17 years and by rearing the beetles from the samples in the laboratory. The value of the system lies in the accuracy of host affiliations. A total of 138 species and subspecies of plants were sampled, representing approximately 87% of the Leguminosae in Hungary. Only 51 legume species (37.0%) harboured bruchids (12 Bruchus spp. and 15 Bruchidius spp). Bruchids occurred in 38.3% of the host plant samples. Thirty-seven plant species congeneric with bruchid host species were unoccupied by bruchids. The degree of host specificity ranged from monophagy (at least ecological monophagy) to oligophagy. On the basis of presence/absence data we tested the null hypothesis assuming that plant taxa and seed consuming bruchid species form congruent phyletic relations at the species level. We argue that the case for coevolution between the beetles and their hosts is weak. A comparison of the available taxonomic relationships (and presumed phylogenies) best fits a case of sequential evolution, with stronger phylogenetic conservatism in Bruchus species than in Bruchidius species. Bruchid and host plant taxa showed rough congruence only at the tribe level. We suggest that host shifts are best explained by the changes in the bruchids' chemosensory system that determines the females' host selection behaviour.