Itagaki Tadashi, Hayashi Kei, Ohari Yuma
Laboratory of Veterinary Parasitology, Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka 020-8550, Japan.
Laboratory of Parasitology, Faculty of Veterinary Medicine, Okayama University of Science, 1-3 Ikoinooka, Imabari 794-8555, Japan.
Infect Genet Evol. 2022 Apr;99:105248. doi: 10.1016/j.meegid.2022.105248. Epub 2022 Feb 17.
Parthenogenetic Fasciola is the causative agent of fascioliasis in animals and humans and is widely distributed in Asian countries, such as Japan, South Korea, China, Vietnam, Thailand, the Philippines, Myanmar, Bangladesh, Nepal, and India. Parthenogenetic Fasciola geographically originated from central and eastern China, where it exists between the habitats of Fasciola hepatica and Fasciola gigantica; it likely appeared thousands of years ago following hybridization between F. hepatica and F. gigantica. Parthenogenetic Fasciola consists of diploids and triploids that possess nuclear genome of both F. hepatica and F. gigantica and mitochondrial genome of either F. hepatica or F. gigantica. Maternal parents of parthenogenetic Fasciola are either F. hepatica having Fh-C4 haplotype or F. gigantica having Fg-C2 haplotype in mitochondrial NADH dehydrogenase subunit 1 (ND1) nucleotide sequences. Parthenogenetic Fasciola flukes with the Fh-C4 haplotype have spread from China to South Korea and Japan, whereas the flukes with the Fg-C2 haplotype have not only spread to Korea and Japan but also southward to Vietnam, Thailand, the Philippines, Myanmar, Bangladesh, Nepal, and India. Parthenogenetic Fasciola can be distinguished from F. hepatica and F. gigantica using combinational DNA sequence analysis of nuclear phosphoenolpyruvate carboxykinase (pepck) and DNA polymerase delta (pold) along with mitochondrial ND1 markers. The establishment of parthenogenetic Fasciola is expected as follows: parthenogenetic diploids with the Fh-C4 and Fg-C2 haplotypes first appeared based on single or multiple interspecific hybridization events; subsequently, parthenogenetic triploids emerged via backcross events between the maternal parthenogenetic diploid and either paternal bisexual F. hepatica or F. gigantica. Parthenogenetic Fasciola diploids and triploids then survived for thousands of years by clonal parthenogenetic reproduction, and generated descendants with ND1 haplotypes, which were derived from the Fh-C4 and Fg-C2 due to nucleotide substitution. Thus, the emergence of parthenogenetic Fasciola may be due to extremely uncommon and accidental events. Parthenogenetic Fasciola should be treated as a new asexual hybrid species.
孤雌生殖肝片吸虫是人和动物肝片吸虫病的病原体，广泛分布于亚洲国家，如日本、韩国、中国、越南、泰国、菲律宾、缅甸、孟加拉国、尼泊尔和印度。孤雌生殖肝片吸虫在地理上起源于中国中部和东部，存在于肝片吸虫和巨片吸虫的栖息地之间；它可能在数千年前肝片吸虫和巨片吸虫杂交后出现。孤雌生殖肝片吸虫由二倍体和三倍体组成，它们拥有肝片吸虫和巨片吸虫的核基因组以及肝片吸虫或巨片吸虫的线粒体基因组。孤雌生殖肝片吸虫的母本要么是线粒体烟酰胺腺嘌呤二核苷酸脱氢酶亚基1（ND1）核苷酸序列中具有Fh-C4单倍型的肝片吸虫，要么是具有Fg-C2单倍型的巨片吸虫。具有Fh-C4单倍型的孤雌生殖肝片吸虫已从中国传播到韩国和日本，而具有Fg-C2单倍型的吸虫不仅传播到韩国和日本，还向南传播到越南、泰国、菲律宾、缅甸、孟加拉国、尼泊尔和印度。利用核磷酸烯醇式丙酮酸羧激酶（pepck）和DNA聚合酶δ（pold）的组合DNA序列分析以及线粒体ND1标记，可以将孤雌生殖肝片吸虫与肝片吸虫和巨片吸虫区分开来。孤雌生殖肝片吸虫的形成过程预计如下：具有Fh-C4和Fg-C2单倍型的孤雌生殖二倍体首先基于单次或多次种间杂交事件出现；随后，孤雌生殖三倍体通过母本孤雌生殖二倍体与父本两性肝片吸虫或巨片吸虫之间的回交事件出现。孤雌生殖肝片吸虫的二倍体和三倍体随后通过克隆孤雌生殖繁殖存活了数千年，并产生了具有ND1单倍型的后代，这些单倍型由于核苷酸取代而源自Fh-C4和Fg-C2。因此，孤雌生殖肝片吸虫的出现可能是由于极其罕见和偶然的事件。孤雌生殖肝片吸虫应被视为一种新的无性杂交物种。
