Infidum similis Travassos, 1916 (Dicrocoeliidae: Leipertrematinae) was found in the gall bladder of Leptophis diplotropis Günther, 1872 from El Podrido, Acapulco, Guerrero, Mexico. A phylogenetic analysis based on partial sequences of the 28S ribosomal RNA using maximum likelihood (ML) and Bayesian inference (BI) analyses was carried out to assess its phylogenetic position within suborder Xiphidiata, alongside members of the superfamilies Gorgoderoidea and Plagiorchoidea. The phylogenetic trees showed that the genus is most-closely related to the Plagiorchoidea rather than to the Gorgoderoidea, in keeping with previous taxonomic designations. Phylogenies obtained from ML and BI analysis of the 28S rDNA gene revealed a well supported clade in which Choledocystus hepaticus (Lutz, 1928) Sullivan, 1977 is sister to I. similis. On the other hand, a tree obtained using a partial sequence of the cytochrome c oxidase subunit 1 (cox1) mtDNA gene (ML and BI analysis), with species supposed to be closely related to I. similis according to 28S, does not support this relatedness. Based on the independence of Infidum from the subfamily Leipertrematinae Yamaguti, 1958, our results clearly demonstrated that the genus corresponds to a different family and with species closely related to C. hepaticus within Plagiorchoidea. New data are presented about the tegumental surface of I. similis by scanning electron microscopy as well as the estimation of its haploid genome size using Feulgen Image Analysis Densitometry of sperm nuclei as part of the characterization of this species. This is the first genome size estimated for a member of Plagiorchiida, and these data will provide a new source of knowledge on helminth diversity and evolutionary studies. This constitutes the first host record, and new geographical distribution, for this species in Mexico.

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