Accurate identification of aspermic Fasciola forms in Japan remains difficult because of their morphological variations. In order to characterize the forms genetically, nucleotide sequences of ribosomal internal transcribed spacer (ITS1 and ITS2) and mitochondrial cytochrome c oxidase I (COI) and NADH dehydrogenase I (NDI) genes in 34 liver flukes from 16 prefectures in Japan were analysed. Two major forms represented by Fsp 1 and Fsp 2 had sequences identical to or closely resembling those of F. hepatica and F. gigantica, respectively, in all the 4 DNA markers and were mainly distributed in northern and eastern-western parts of Japan, respectively. Fsp 1 and Fsp 2 would have been introduced into Japan with infected cattle of 2 distinct lineages via the Korean Peninsula and spread through limited parts of Japan (northern and eastern-western parts) together with the movement of each cattle lineage. The Japanese form (Fsp 1/2), which showed heterozygosity in ribosomal DNA and Fsp 2 haplotype in mitochondrial DNA, may have originated in interspecific cross hybridization between paternal F. hepatica and maternal F. gigantica.

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