Evaluating species boundaries using coalescent delimitation in pine-killing Monochamus (Coleoptera: Cerambycidae) sawyer beetles

Plant-feeding beetle species are diverse and often individually highly variable. Accurate classifications can be difficult to establish yet are essential for study of evolutionary patterns and processes. Molecular data are key to further characterizing morphologically difficult groups and defining genus and species boundaries.Monochamus Dejean species are ecologically and economically significant, and in coniferous forests they vector the nematode that causes Pine Wilt Disease. This study uses nuclear and mitochondrial genes to test the monophyly and relationships of Monochamus and applies coalescent methods to further delimit the conifer-feeding species.Monochamus has also included approximately 120 Old World species associated with diverse angiosperm tree species. We sample from these additional morphologically diverse species to determine their placement in the Lamiini. Through supermatrix and coalescent methods, the higher-level relationships of Monochamus show that conifer-feeders are a monophyletic group that includes the type species and has split into Nearctic and Palearctic clades. Molecular dating indicates a single dispersal of conifer-feeders to North America over the second Bering Land Bridge circa 5.3 Ma. All other Monochamus sampled fall in different parts of the Lamiini tree. Small-bodied angiosperm-feeding Monochamus group with the monotypic genus Microgoes Casey. The African Monochamus subgenera sampled are distantly related to the conifer-feeding clade. The multispecies coalescent delimitation methods BPP and STACEY delimit 17 conifer-feeding Monochamus species for a total of 18 species, and supports the retention of all current species. An interrogation with nuclear gene allele phasing reveals that unphased data can be unreliable for accurate delimitations and divergence times. The delimited species are discussed with integrative evidence, highlighting real-world challenges in recognizing the completion of speciation.

Automated summary

This study uses molecular data to classify plant-feeding beetle species, particularly Monochamus in coniferous forests, and finds that it is a monophyletic group consisting of multiple associated species. The analysis identifies 17 conifer-feeding populations within this genus, and supports the retention of all current species.