Morphology, phylogeny, and evolutionary development in the weevils (Insecta: Coleoptera: Curculionoidea)

Abstract

Weevils (superfamily Curculionoidea) rank among the most diverse groups of extant organisms, with ~60,000 described species. They are extremely important economically, being of great agricultural significance because they associate with all major groups of plants, often with negative consequences for crops. For these reasons, weevils are also of great systematic interest: few weevil systematists remain active, and (to date) few phylogenetic studies have focused on weevils and their relatives (Curculionoidea), all utilizing few taxa (~150 taxa), leaving knowledge of weevil relationships scanty at best. In this study, the densest taxon sampling to date was implemented, consisting of 282 morphological characters scored for 577 taxa. It also represents the first study to incorporate numerous fossils into a formal cladistic analysis. In order to develop a more robust morphological character system for cladistic analysis of the higher lineages and to gain a comprehensive understanding of rostrum structure prior to developmental studies examining its formation, a comparative study also was conducted of rostrum structure throughout Curculionoidea through examination of semi-thin sections. The weevil rostrum, for example, is a key evolutionary innovation that has enabled this group to feed on and oviposit in nearly all plant tissues, giving rise to diverse life histories and tremendous diversity in rostrum form. Insights into comparative development of the rostrum will provide insight into the evolution of this key innovation that may be responsible for the explosive radiation of the lineage. Although weevils are an enormous group and countless species are significant agricultural pests, no weevil species have been utilized in developmental studies. In order to better understand the formation and evolution of this structure, transcriptomes from the developing head tissue of 4 weevil species, representing disparate clades and divergent rostral forms, and 1 outgroup (non-weevil species) were generated. While there are difficulties in assessing differences among transcriptomes from divergent taxa, tests for differential expression patterns of transcripts were performed and a refined list of candidate genes has been produced. RNA interference experiments were performed on a subset of candidate genes to test function. This study provided insight into the developmental underpinnings that produced the profound phenotypic diversity observed in the rostrum and the genetic framework that permitted the diversification of such an immense lineage as the weevils.