Phylogenetic relationships of Posthodiplostomum sp. metacercarial morphotype (Diplostomidae) infecting four Pampean rivulid genera.

Morphologically similar diplostomid metacercariae were found infecting four species of Pampean rivulid fishes - Austrolebias bellottii, Amatolebias patriciae, Argolebias nigripinnis, and Titanolebias elongatus - from temporary freshwater ponds in the Pampean region of Argentina. Although these larvae occur in different host species and localities, comparative morphological analyses revealed extensive overlap in diagnostic characters. To assess whether this morphological similarity reflected conspecificity, molecular analyses based on partial 28S rDNA and ITS2 rDNA sequences were performed. Phylogenetic reconstructions recovered all metacercariae from rivulid hosts as a single, well-supported lineage, consistent with their assignment to a single lineage Posthodiplostomum species. Genetic distances among ITS2 sequences were zero, indicating complete sequence identity across hosts and localities. Infection parameters varied among host species and populations, with Austrolebias bellottii and Titanolebias elongatus exhibiting the highest values. Differences in infection levels could reflect variation in local environmental conditions, habitat characteristics, and the availability of avian definitive hosts, potentially influencing parasite transmission dynamics. This study provides integrative morphological and molecular evidence consistent with all metacercariae representing a single Posthodiplostomum lineage infecting multiple Pampean rivulid species and contributes to a better understanding of parasite diversity and transmission patterns in South American temporary ponds.