Rediscovering a Neotropical anuran fluke: a morphological and phylogenetic study of Rauschiella proxima (Trematoda: Plagiorchioidea).

Species of the genus Rauschiella are trematodes of frogs and snakes in the Americas. The taxonomy of the group is complex, and most of the 18 currently valid species are known only from the original description. Moreover, genetic data are available only for two North American species (Rauschiella tineri and Rauschiella poncedeleoni). In this context, integrative taxonomy studies are necessary for Rauschiella spp. found in South America. Herein, during a long-term herpetological and helminthological study conducted in Selvíria, Mato Grosso do Sul, Brazil, between 2016 and 2019, 296 anurans from 17 species were necropsied. A plagiorchioid trematode found in the intestine of 7/12 Leptodactylus macrosternum, 10/106 Leptodactylus podicipinus, and 13/20 Pseudis platensis was subject to morphological and molecular characterisation. Samples of the trematodes were studied by optical and scanning electron microscopies and identified as Rauschiella proxima (Freitas, 1941), here reported in new anuran hosts and geographical area. Sequences of the nuclear gene 28S rDNA (1148 bp) were generated and subjected to phylogenetic analysis. Our isolates of R. proxima from Brazil grouped in a well-supported clade with R. poncedeleoni and R. tineri, and genetic divergences to these species were low (0.45% and 0.54%, respectively), supporting the congeneric status among them. However, the addition of a South American representative of Rauschiella and the construction of more comprehensive phylogenetic analyses, including 78 plagiorchioid species from 15 families, did not result in advances concerning the familial level of classification of this genus, which remains as incertae sedis in the superfamily Plagiorchoidea.