Phenotypic plasticity, genetic structure, and systematic position of Fisher, 1960 (Acanthocephala: Neoechinorhynchidae) a parasite of emydid turtles from the Nearctic and Neotropical regions.

The taxonomy of the 10 recognized species associated with emydid turtles is complex due to the morphological conservatism. In the present study, specimens of from northern and southeastern Mexico exhibit great phenotypic plasticity on its diagnostic characteristics. We sequenced three molecular markers: the internal transcribed spacers ITS1, ITS2 and 5.8S gene, the D2 + D3 domains of the large subunit from nuclear DNA and cytochrome oxidase subunit I () from mitochondrial DNA. Sequences of the nuclear molecular markers were aligned and compared with other congeneric species associated with emydids available in GenBank. Phylogenetic analyses supported the polyphyly of . The species from emydids formed a clade, which was subdivided into five subclades that correspond with each species analysed (, , , and ). To understand better the genetic structure of a haplotype network was inferred with 29 sequences, revealing the presence of 13 haplotypes, two of which were shared and 11 were unique. The high values of fixation index, (0.4227–0.8925) detected between the two populations from southeastern and the two from northern Mexico indicated low genetic flow among the populations. Our data suggest that the species associated with emydid turtles diversified in the eastern USA and that of expanded its distribution range reached southeastern Mexico.