Structure of the Ambrosia Beetle (Coleoptera: Curculionidae) Mycangia Revealed Through Micro-Computed Tomography

Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae and Platypodinae) rely on a symbiosis with fungi for their nutrition. Symbiotic fungi are preserved and transported in specialized storage structures called mycangia. Although pivotal in the symbiosis, mycangia have been notoriously difficult to study, given their minute size and membranous structure. We compared the application of novel visualization methods for the study of mycangia, namely micro-computed tomography (micro-CT) and laser ablation tomography (LATscan) with traditional paraffin sectioning. Micro-CT scanning has shown the greatest promise in new organ discovery, while sectioning remains the only method with sufficient resolution for cellular visualization. All three common types of mycangia (oral, mesonotal, and pronotal) were successfully visualized and presented for different species of ambrosia beetles: Ambrosiodmus minor (Stebbing) 1909, Euplatypus compositus (Say) 1823, Premnobius cavipennis Eichhoff 1878, Scolytoplatypus raja Blandford 1893, Xylosandrus crassiusculus (Motschulsky) 1866 and X. amputatus (Blandford) 1894. A reconstruction of the mycangium and the surrounding musculature in X. amputatus is also presented. The advantages of micro-CT compared to the previously commonly used microtome sectioning include the easy visualization and recording of three-dimensional structures, their position in reference to other internal structures, the ability to distinguish natural aberrations from technical artifacts, and the unprecedented visualizations of the anatomic context of mycangia enabled by the integrated software

Metagenome skimming of insect specimen pools: potential for comparative genomics

Metagenomic analyses are challenging in metazoans, but high-copy number and repeat regions can be assembled from lowcoverage sequencing by “genome skimming,” which is applied here as a new way of characterizing metagenomes obtained in an ecological or taxonomic context. Illumina shotgun sequencing on two pools of Coleoptera (beetles) of approximately 200 species each were assembled into tens of thousands of scaffolds. Repeated low-coverage sequencing recovered similar scaffold sets consistently, although approximately 70% of scaffolds could not be identified against existing genome databases. Identifiable scaffolds included mitochondrial DNA, conserved sequences with hits to expressed sequence tag and protein databases, and knownrepeatelementsof high and low complexity, includingnumerous copies ofrRNAandhistone genes.Assemblies of histones captured a diversity of gene order and primary sequence in Coleoptera. Scaffolds with similarity to multiple sites in available coleopteran genome sequences for Dendroctonus and Tribolium revealed high specificity of scaffolds to either of these genomes, in particular for high-copy number repeats. Numerous “clusters” of scaffolds mapped to the same genomic site revealed intraand/or intergenomic variation within a metagenome pool. In addition to effect of taxonomic composition of the metagenomes, the number of mapped scaffolds also revealed structural differences between the two reference genomes, although the significance of this striking finding remains unclear. Finally, apparently exogenous sequences were recovered, including potential food plants, fungal pathogens, and bacterial symbionts. The “metagenome skimming” approach is useful for capturing the genomic diversity of poorly studied, species-rich lineages and opens new prospects in environmental genomic

One becomes two: second species of the Euwallacea fornicatus (Coleoptera: Curculionidae: Scolytinae) species complex is established on two Hawaiian Islands

The cryptic species that make up the Euwallacea fornicatus species complex can be readily distinguished via their DNA sequences. Until recently, it was believed that the Hawaiian Islands had been invaded by only one of these cryptic species, E. perbrevis (tea shot hole borer; TSHB). However, following the 2016 deposition of a DNA sequence in the public repository GenBank, it became evident that another species, E. fornicatus (polyphagous shot hole borer; PSHB), had been detected in macadamia orchards on Hawaiʻi Island (the Big Island). We surveyed the two most-populous islands of Hawaiʻi, Big Island and Oʻahu, and herein confirm that populations of TSHB and PSHB are established on both. Beetles were collected using a variety of techniques in macadamia orchards and natural areas. Individual specimens were identified to species using a high-resolution melt assay, described herein and validated by subsequent sequencing of specimens. It remains unclear how long each species has been present in the state, and while neither is currently recognized as causing serious economic or ecological damage in Hawaiʻi, the similarity of the newly-confirmed PSHB population to other damaging invasive PSHB populations around the world is discussed. Although the invasive PSHB populations in Hawaiʻi and California likely have different geographic origins within the beetle’s native range, they share identical Fusarium and Graphium fungal symbionts, neither of which have been isolated from PSHB in that native range