Gene to Genus: Systematics and Population Dynamics in Lamiini Beetles (Coleoptera: Cerambycidae) With Focus on Monochamus Dejean

Abstract

Herbivorous insects make up more than a third of the multicellular species on Earth. The discovery, description, and organization of this diversity is necessary to enable study of the mechanisms involved in the dynamic speciation process of phytophages. In this dissertation, each of these levels is addressed using molecular systematics of the tribe Lamiini and genus Monochamus, and population genomics with a focus on differentiation in the sky island inhabiting pine sawyer, Monochamus clamator. With this data, we have evaluated several variables that could promote herbivore speciation including geography, host-plant diversity, and climate. The genus Monochamus Dejean has long been considered to be a combination of species that do not belong in the same genus, but morphological characters have failed to delineate natural groups. We are the first to use multi-gene molecular data and coalescent modeling to estimate the phylogeny of this economically-important genus. Monochamus rev. nov. is a monophyletic group of Holarctic conifer-feeding species based on this data. The angiosperm feeding species currently placed in Monochamus are revealed to belong to other genera. We find that Monochamus is a derived conifer feeding genus that likely originated in southern Asia in the late Miocene and dispersed over the second Bering Bridge to North America and subsequently diversified. Modern multispecies coalescent species delimitation techniques exposed varied evolutionary histories for current species, that some subspecies are unsupported and others should be elevated to species status, and helped to discover a new species of Monochamus. The approach of phasing nuclear sequences to alleles for phylogeny and delimitation revealed that phylogeny node heights are impacted at multiple taxonomic levels by ambiguity codes in sequences. This can lead to incorrect divergence times and delimit incorrect numbers of species. The tribe that Monochamus belongs to, the Lamiini, is diverse and shares morphological characteristics with other tribes in the Lamiinae subfamily. We use molecular data to build a “backbone” phylogeny of the Lamiini and related tribes to test for evolutionary independence and determine if any morphological characters support the result. The data supports the tribes Batocerini, Gnomini, Monochamini, and Acridocephalini being synonyms of the Lamiini sensu novo. There are no synapomorphic morphological characters found to support the new tribe, but a group of characters will place most specimens and therefore allow more efficient identifications in this economically impactful and widespread group. At the population level, we address adaptive processes in Monochamus clamator in the climate-change produced Great Basin sky islands of western North America. With the use of RNA-Seq to build a genomic and gene expression dataset for multiple habitat islands with differing host plant composition, the relationship between genetic differentiation and habitat factors was explored in a geographically explicit framework. Genomic data revealed low genetic differentiation at the island level and high importance of immigration between islands across the Basin. Multi-matrix regression on factors of geography, host-plant diversity, and environment showed that only the environment has a significant relationship with genetic distance in the beetles. Gene expression measures reveal hundreds of differentially expressed genes between island beetle groups that may be the first sign of adaptation to their habitat. As a whole, this work contributes novel findings concerning the description, organization, and origins of diversity in herbivorous insects, a hyperdiverse group of organisms.