Phylogeny of Palaearctic <i>Azygia</i> spp. (Trematoda: Azygiidae) and description of <i>Azygia shedkoae</i> sp. nov.

The genus <i>Azygia</i> Looss, 1899 comprises plagiorchiid digeneans of freshwater fishes distributed in the northern hemisphere. Its taxonomic boundaries are contestable, and identification of some reported material is uncertain. We performed a phylogenetic analysis of Palaearctic representatives of <i>Azygia</i> based on five molecular markers and described a new species of this genus from Amur pike <i>Esox reichertii</i> Dybowski, 1869 from the Russian Far East. We found that <i>Azygia</i> was diverged into three phylogenetic lineages with unclear relationships. Lineage 1 and Lineage 2 were represented by Palaearctic species of <i>Azygia</i>, while Lineage 3 comprised Nearctic azygiids, particularly Nearctic species of <i>Azygia</i> and representatives of the genera <i>Leuceruthrus</i> Marshall &amp; Gilbert, 1905 or <i>Proterometra</i> Horsfall, 1934. The new species, <i>Azygia shedkoae</i> sp. nov., belonged to Lineage 2 together with <i>Azygia lucii</i> (Müller, 1776), <i>Azygia robusta</i> Odhner, 1911, and <i>Azygia sibirica</i> Vainutis, Voronova &amp; Zhokhov, 2023. Morphologically, the new species is close to <i>A. robusta</i> but differs from it in the length of the forebody, the length of post-testicular space, and the length of uterine fields as well as genetically. Our data demonstrate a deep molecular differentiation of the azygiid lineages apparently resulting from some complex long-term evolutionary processes. We found no arguments in favour of the recently proposed synonymization of <i>Azygia</i> and <i>Leuceruthrus</i>. At the same time, our results appear to indicate that <i>Azygia</i> should be divided into several distinct genera. http://zoobank.org/urn:lsid:zoobank.org:pub:5E964870-1DF5-438D-94AE-5D9C37EDBECD