This study aimed to precisely discriminate Fasciola spp. based on DNA sequences of nuclear internal transcribed spacer 1 (ITS1) and mitochondrial nicotinamide adenine dinucleotide (NADH) dehydrogenase subunit 1 (nad1) gene. We collected 150 adult flukes from the bile ducts of cattle, buffaloes, sheep, and goats from six different regions of Bangladesh. Spermatogenic status was determined by analyzing stained seminal vesicles. The ITS1 types were analyzed using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The nad1 haplotypes were identified based on PCR and direct sequencing and analyzed phylogenetically by comparing with nad1 haplotypes of Fasciola spp. from other Asian countries. Of the 127 aspermic flukes, 98 were identified as Fg type in ITS1, whereas 29 were identified as Fh/Fg type, indicating a combination of ITS1 sequences of Fasciola hepatica and Fasciola gigantica. All the 127 aspermic flukes showed Fsp-NDI-Bd11 in nad1 haplotype with nucleotide sequences identical to aspermic Fasciola sp. from Asian countries. Further, 20 spermic flukes were identified as F. gigantica based on their spermatogenic status and Fg type in ITS1. F. gigantica population was thought to be introduced into Bangladesh considerably earlier than the aspermic Fasciola sp. because 11 haplotypes with high haplotype diversity were detected from the F. gigantica population. However, three flukes from Bangladesh could not be precisely identified, because their spermatogenic status, ITS1 types, and nad1 haplotypes were ambiguous. Therefore, developing a robust method to distinguish aspermic Fasciola sp. from other Fasciola species is necessary in the future.

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The well-known pathogens of fasciolosis, Fasciola hepatica (Fh) and Fasciola Gigantica (Fg), possess abundant mature sperms in their seminal vesicles, and thus, they reproduce bisexually. On the other hand, aspermic Fasciola flukes reported from Asian countries, which have no sperm in their seminal vesicles, probably reproduce parthenogenetically. The aim of this study was to reveal the origin of aspermic Fasciola flukes. The nuclear single copy markers, phosphoenolpyruvate carboxykinase and DNA polymerase delta, were employed for analysis of Fasciola species from China. The hybrid origin of aspermic Fasciola flukes was strongly suggested by the presence of the Fh/Fg type, which includes DNA fragments of both F. hepatica and F. gigantica. China can be regarded as the cradle of the interspecific hybridization because F. hepatica and F. gigantica were detected in the northern and southern parts of China, respectively, and hybrids flukes were distributed between the habitats of the two species. The Chinese origin was supported by the fact that a larger number of mitochondrial NADH dehydrogenase subunit 1 (nad1) haplotypes was detected in Chinese aspermic Fasciola populations than in aspermic populations from the neighbouring countries. Hereafter, 'aspermic' Fasciola flukes should be termed as 'hybrid' Fasciola flukes.

Nuclear gene markers, phosphoenolpyruvate carboxykinase (pepck) and DNA polymerase delta (pold), have been developed for precise discrimination of Fasciola flukes instead of internal transcribed spacer 1. In this study, these two genes of 730 Fasciola flukes from eight Asian countries were analyzed. The results were compared with their mitochondrial NADH dehydrogenase subunit 1 (nad1) lineages for obtaining a definitive evidence of the hybrid origin of aspermic Fasciola flukes. All the flukes categorized into the aspermic nad1 lineages possessed both the fragment patterns of F. hepatica and F. gigantica (mixed types) in pepck and/or pold. These findings provide clear evidence for the hybrid origin of aspermic Fasciola lineages and suggest that “aspermic Fasciola flukes” should hereafter be called “hybrid Fasciola flukes”.