Integrative taxonomic assessment of Cruzia sp. (Nematoda: Cosmocercoidea, Kathlaniidae) from the Argentine black and white tegu lizard: insights into the long-standing Kathlaniidae taxonomic puzzle

Kathlaniidae (Ascaridomorpha: Cosmocercoidea) represents a diverse family of intestinal nematodes that are parasitic in all classes of vertebrates worldwide, except Aves. However, our knowledge on the systematic and molecular phylogeny of this family is unclear and complicated. The phylogenetic relationships between its subfamilies Kathlaniinae, Oxyascaridinae and Cruziinae are still under debate. Moreover, the systematic position of the genus Cruzia (Cruziinae) within Kathlaniidae remains unclear. This work aimed to characterize, both morphologically and genetically, specimens of Cruzia sp. and evaluate the phylogenetic position of this genus within the Kathlaniidae, the phylogenetic relationships of this family with other taxa of Cosmocercoidea, and the status of Cruziinae as a subfamily. During a parasitological survey carried out in December 2024, specimens of Cruzia sp. were collected from the intestine of Salvator merianae (Squamata: Teiidae) from Corrientes (Argentina) and characterized using morphological (light and scanning electron microscopy) and molecular (sequencing of partial fragments of 18S rDNA, and COI mtDNA) methods. Phylogenies were reconstructed based on Bayesian inference. The morphological analysis indicated the presence of 15–16 teeth per row in the pharynx, equatorial vulva in females, and 11 pairs of cloacal papillae in males, arranged as follows: 3 precloacal, 3 paracloacal, 5 postcloacal, and a minute precloacal unpaired, and spicule length less than 1mm. Such a combination of morphological features seems to be unique when compared with that of other congeners, suggesting a possible new taxon. In the phylogenetic reconstructions based on 18S rDNA and COI mtDNA, Cruzia sp. formed an independent lineage, sister to a group formed by C. americana and C. tentaculata. Kathlaniidae was paraphyletic, because Cruziinae formed a separate lineage from other kathlaniids and closely related to the cosmocercid Cosmocerca (Cosmocercidae, Cosmococercoidea). Based on these results, we suggest that Cruziinae should be elevated to family.