Identification of new polymorphic positions in rDNA sequences of the "intermediate" Fasciola forms.

Fascioliasis is a foodborne zoonotic disease generally caused by the parasitic flukes Fasciola gigantica and Fasciola hepatica in class Trematoda. An "intermediate" Fasciola forms between F. gigantica and F. hepatica has been shown to exist. However, the relationships among F. gigantica, F. hepatica, and "intermediate" Fasciola forms remain unclear. In this study, we found five new polymorphic positions in 18S and 28S rDNAs sequences of "intermediate" Fasciola forms. According to the high-throughput sequencing results, all known 16 polymorphic positions of "intermediate" Fasciola forms show a clear and consistent tendency for F. gigantica or F. hepatica, and the percentages of the most frequently occurring bases were different in specimens. In the three ITS sequence fragments, hybrid-type base combinations of the polymorphic positions were detected, and the percentages of the most frequent base combinations were different in specimens too. In addition, interestingly, the newly detected ITS-802 position was not a traditional polymorphic position in "intermediate" Fasciola forms, and the bases in ITS-802 position are not same as the allele bases of F. gigantica or F. hepatica. Our results will be helpful to investigations into the molecular taxonomy, population genetics, and ecology of F. gigantica, F. hepatica, and "intermediate" Fasciola forms.