INTRODUCTION
Rice (Oryza sativa L.) is one of most important food crops in China, with an annual cultivated area of 30 million hectares and a yield of 206 million tons (Deng et al., 2019). Due to the popularization and application of direct-seeding cultivation technology, the harm of rice root-knot nematode was becoming more and more serious (Jabbar et al., 2020). Among all the root-knot nematodes (RKNs) that harm rice,Meloidogyne graminicola is considered to be one of the most harmful plant parasitic nematodes in rice cultivation system (Jabbar et al., 2020; Onkendi et al., 2013; Pokharel et al., 2007; Mantelin et al., 2017). M. graminicola can infiltrate the roots, cause root galling, inhibit plant defense systems, manipulate the plant’s metabolic system, and establish giant cells for its nutrition(Jabbar et al., 2020; Luo et al., 2020). Therefore, over time, the plant loses its vitality, which eventually leads to a significant yield loss (Bridge et al., 2005; Jabbar et al., 2020). M. graminicola is widely distributed in the tropical and subtropical regions of China, India, Bangladesh, Thailand, the United States, and other countries (Singh 2010). In China, it was recorded for the first time on Alliumtistulosum in Hainan province (Zhao et al., 2001). In recent years, the distribution of this nematode has expanded from Hainan Province to Guangdong, Guangxi, Fujian, Yunnan, Hunan, Hubei, Anhui, Sichuan, Jiangxi, Henan and other provinces. A total of about 1 million hectares of rice were found to be infected, with a high incidence(Du 2003; Liu et al., 2011; Luo et al., 2020).Several studies have shown that M. graminicola infections usually result in a 10-20% reduction in rice yield, and in severe cases, a 50-72% reduction in rice yield (Khan and Ahamad 2009; Jabbar et al., 2020; Luo et al., 2020).Thus, M. graminicola has becomea major threat to rice production(Huang et al., 2018; Khan 2019; Jabbar et al., 2020).
Mitochondrial DNA (mtDNA) has become an important genetic marker for the study of nematode molecular phylogeography due to its matrilineal inheritance, fast evolutionary rate, and lack of recombination (Derycke et al., 2005; Saccone et al., 2000).Recent studies in nematology have employed mitochondrial cytochrome oxidase subunit I (COI) as a molecular marker to analyze the intra specific genetic structure of closely related Xiphinema species (Gutierrez-Gutierrez et al., 2011).Sun et al. (2005) and Deng et al. (2016) used barcoding techniques of mitochondrial COII-LrRNA gene fragments to analyze the differences between Meloidogyne spp. and Rotylenchulusreniformispopulations,and found that M. incongnita , M. javanica ,M. arenaria and R. reniformis could be identified by mtDNA-PCR. Rashidifard (2019) studied the molecular characteristics of 37 Meloidogyne populations from four provinces in South Africa and showed that COII-16S could accurately identify differentM . enterolobii populations. Additionally, the characterization of COII and 16S rRNA has been proved to be useful for the identification of different Meloidogyne species from different geographical regions of the world (OnkendiandMoleleki 2013). Janssen et al. (2016) proved that analysis based on mitochondrial haplotypes can reveal the evolution and genetic variation of root nematodes, and pointed out that the barcode region Nad5 can reliably identify the major lineages of tropical root-knot nematodes.Based on the mtCOI gene, four main plant-parasitic Aphelenchoides species were successfully diagnosed, and the multiple origins of the parasitic genus were proved (Sánchez-Monge et al. 2017).Ye et al. (2007) constructed 19Bursaphelenchus spp. phylogenetic trees using sequences including COI genes and analysed the phylogenetic relationships among species in this genus.Genetic diversity of the root-knot nematode M. enterolobii in mulberry has been analyzed by Shao et al. (2020) and observed that the high level of gene flow, a high genetic variation and a small genetic distance among M. enterolobii populations. A comprehensive phylogenetic analysis of several hundred COI and ITS rRNA gene sequences from the Heterodera avenae group showed that COI haplotypes corresponded to certain pathotypes of cereal cyst nematodes. Therefore, compared to other molecular markers,mitochondrial DNA markers emerged as a valuable tool in the study of genetic diversity, population differentiation, and evolutionary relationship between closely related nematode species.
To date, there are many studies focused on the genetic diversity of RKN populations on different crops, but no work has been done on the genetic diversity of M. graminicola in China. Therefore, the genetic diversity, genetic differentiation and origin of M. graminicolacollected from10provinces of China based on the COI gene were analysed. It will provide a theoretical basis to reveal the historical dynamics of M. graminicola populations and develop efficient management strategies in China.