LEVERAGING MORPHOLOGICAL AND PHYLOGENOMIC METHODS TO ELUCIDATE THE EVOLUTION OF DIXIDAE

Dixidae is an obscure family of Diptera with highly cryptic morphologies and a confused evolutionary history. An intrafamily systematic review is thus far unattempted and existing generic concepts are currently untested through molecular methods. 2261 Nuclear orthologs and 13 mitochondrial coding sequences were leveraged to create ML phylogenies for this family. Phylogenies created from both datasets are highly concordant with one another. The results herein support the existing phylogenetic placements of the three most robust genera (Dixa, Dixella, Nothodixa) but two were found to be highly nested (Asiodixa nested within Dixella and Meringodixa nested within Dixa). Cytochrome oxidase I, the current standard barcoding gene, reveals extensive topological differences between both nuclear and mitochondrial datasets, suggesting it is an ineffective proxy for the intrafamily history of Dixidae. An entirely new genus of heretofore undescribed Dixidae is proposed. The robust genetic resources leveraged here will allow for future identification and classification of Dixidae specimens.

Meringodixa Nowell (1951) is a genus of meniscus midges (Diptera:Dixidae) with only one described Nearctic species (M. chalonensis). Adult terminalia of Meringodixa are well characterized but female terminalia and larval forms are undescribed. Larval specimens with morphologies highly congruous to M. chalonensis were associated with identified adult males of four species of Dixa, D. arge, D. lobata, D. melanderi, and D. xavia. The taxonomic placements of these specimens alongside others from two undescribed species were evaluated through a ML phylogeny created with COI sequences using Sangro v0.1, a new Nextflow-based phylogenetics pipeline. All specimens form a monophyletic clade representing Meringodixa, and D. arge, D. lobata, D. melanderi, and D. xavia were reclassified as Meringodixa. Figures and descriptions of larval forms and female terminalia of M. arge, M. lobata, M. “MM” sp. nov., M. tahoensis sp. nov., and M. xavia and an identification key for larval forms are provided herein. Larval forms of Meringodixa are strongly apomorphic and easily distinguishable between species while female terminalia are more difficult to diagnose. Results here will be combined with redescriptions of male terminalia to achieve a generic revision of Meringodixa.