Patterns of host tree use within a lineage of saproxlic snout-less weevils (Coleoptera: Curculionidae: Scolytinae: Scolytini)

Abstract

The influence of plants in the diversification of herbivorous insects, specifically those that utilize moribund and dead hosts, is little explored. Host shifts are expected because the effectiveness of toxic secondary chemicals is lessened by decay of dead plants. Feeding on dead plants also releases herbivorous insect lineages from diversifying within a particular plant lineage. Thus, phylogenetic constraints on the herbivorous insect lineage imposed by the host plants are diminished and repeated patterns of species diversification in an association with unrelated host trees is hypothesized (i.e., taxon cycle). Scolytini, a diverse weevil tribe, specialize on many different dead and moribund plant taxa as a source of food. These species and their hosts offer an opportunity to examine the association between dead host plants and the extent of phylogenetic constraints. A phylogeny of the Scolytini was reconstructed with likelihood and Bayesian analyses of DNA sequence data from nuclear (28S, CAD, ArgK) and mitochondrial (COI) genes. Ancestral host usage and geography was reconstructed using likelihood criteria and conservation of host use was tested. Results supported a monophyletic Scolytini, Ceratolepis, Loganius, and a paraphyletic Scolytus, Camptocerus and Cnemonyx. Diversification of the Scolytini generally occurred well after their host taxa diversified and suggests a sequential evolution of host use. In this scenario the beetle imposes little selection pressure on the tree but the tree provides a platform for beetle evolution. Major changes in host tree use occurred during periods of global cooling associated with changes in beetle biogeography. Diversification of beetles occurred on common and widespread hosts and there was likely a single origination of conifer-feeding from angiosperm-feeding species during the early Pliocene and a radiation of beetle species from the Palearctic to the Nearctic. Overall, the observed patterns of Scolytini host use are conserved and are similar to those expected in a taxon pulse diversification. That is, after a host switch to an unrelated tree, the beetles diversify within the host plant lineage. The need to locate an ephemeral food resource, i.e., a dying tree, likely maintains host specificity once a host shift occurs. These findings suggest that characteristics of dead and moribund host plants (e.g. secondary chemicals) influence the diversification of these saproxlic weevils despite the reduction of selection pressures.