DNA-barcode and endophallus morphology delimit congruent species in a systematic revision of the oxyporine rove beetles of Russia (Coleoptera: Staphylinidae: Oxyporinae)

<p>Rigorous species delimitation is a challenge in biology and systematics in particular. In insects, male genitalia traditionally, and the barcoding region of the CO1 gene recently, are the main markers to identify species, even though a standalone use of CO1 for that is often criticized. In our systematic revision of the mycophagous and in other ways peculiar oxyporine<i> </i>rove beetles of Russia, the legacy alpha-taxonomy could not be improved by traditional investigation of genitalia as they are unusually character-poor in this group. Using phylogenetic inference and ancestral state reconstruction, we demonstrate that CO1 and endophallus are useful markers for species delimitation in <i>Oxyporus</i>. We also show that many morphological traits previously used for species delimitation in Oxyporus are in fact highly variable and thus inconsistent. We hypothesize that in <i>Oxyporus</i> diversification of the endophallic structures is driven by intense intra- and inter-species interactions of multiple individuals co-occurring in narrow spaces inside fungal bodies during mating. Our results encourage broader use of both markers, especially easy-to-generate DNA barcodes, for the desired alpha-taxonomical work in Oxyporinae globally. The revision revealed 10 species of <i>Oxyporus </i>in the fauna of the Russian Federation; eliminated erroneous species records; established two new synonyms, <i>Oxyporus </i>(<i>Oxyporus</i>)<i> basicornis </i>Cameron, 1930 = <i>O. </i>(<i>O.</i>)<i> aequicollis</i> Bernhauer, 1935, <b>syn.nov. </b>= <i>O.</i> (<i>O.</i>)<i> parvus</i> Lee et al., 2020, <b>syn. nov.; </b>and raised the hypothesis that <i>O. </i>(<i>O.</i>)<i> aokii </i>Dvořák, 1956, <i>O. </i>(<i>O.</i>)<i> basiventris</i> Jarrige, 1948 and <i>O. </i>(<i>O.</i>)<i> kobayashii</i> Hayashi, 2015 are conspecific with <i>O. </i>(<i>O.</i>)<i> maxillosus </i>Fabricius, 1793. Lectotypes are designated for <i>O. (O.) basicornis</i> Cameron, 1930, <i>O. (O.) germanus </i>Sharp, 1889<i>, O. </i>(<i>O.</i>)<i> niger </i>Sharp, 1889, and <i>O. </i>(<i>O</i>.)<i> triangulus </i>Sharp, 1889. Comprehensive taxonomic treatment and an identification key are provided for all species. </p>