The Timing of Insect/Plant Diversification: Might Tetraopes (Coleoptera: Cerambycidae) and Asclepias (Asclepiadaceae) Have Co-Evolved?

Abstract

Ehrlich and Raven's essay on coevolution has stimulated voluminous work on the mechanisms of insect/plant interaction, but few explicit tests of their model's prediction that the evolutionary success of entire insect and plant clades is governed by their putative reciprocal adaptations. This paper begins an inquiry into possible coevolutionary diversification for North American milkweeds of the genus Asclepias and one of their few major herbivores, the longhorn beetle genus Tetraopes, focusing first on the historical duration and continuity of the interaction. A phylogeny for Tetraopes and relatives, estimated from morphology and allozymes, shows evident similarity to a morphology based hostplant cladogram synthesized from the literature, though the significance of the correspondence under heuristic statistical tests depends on the treatment of one beetle species reported (without certainty) from multiple host species. Fossils and biogeography support the interpretation that cladogram correspondence reflects synchronous diversification of these two clades, hence opportunity for coevolution, rather than beetle 'host-tracking' of previously-diversified plants. Cladogram correspondence is more evident at higher than at lower levels, as expected under Ehrlich and Raven's model. An apparent phylogenetic progression in the potency and location of milkweed cardenolides: seemingly related to species diversity of both Asclepias and Tetraopes subclades, provides further suggestive evidence for that model. The phylogeography of the Tetraopes/Asclepias assemblage suggests that extant species evolved largely in their current, often quite localized biomes, facilitating potential experimental tests for hypotheses of adaptation and counteradaptation and their importance to diversification.