CONCLUSION AND DISCUSSION
In this study, we examined the genetic diversity, population structure, and relationship between geographic distribution and genetic distance of the M. graminicola population using the mtDNA gene. The genetic diversity of the M. graminicola population was first studied in this study using mitochondrial genes in China. High level of genetic variation (Fst = 0.593) and little gene exchange (Nm = 0.333) were observed in M. graminicola populations. In addition, the overall genetic variation in M. graminicola may primarily due to variation within geographic groups,and no substantial relationship between genetic distance and geographic distribution was observed. This study further enriched the phylogenetic information of M. graminicola and provided the basic evidence for the inherent genetic factors of damage from M. graminicola .
Genetic diversity is not only the basis of biodiversity, but also the driving force of species evolution. The reduction or loss of genetic diversity poses a huge threat to populations or species living in a constantly changing environment (Hedrén 2004). Haplotype polymorphisms (Hd) and nucleotide polymorphisms (π) are commonly used to measure the genetic diversity of species or populations (Hao et al., 2014). The 54 nematode populations used in this study had a total of 15 haplotypes with a Hd of 0.646, indicating high haplotype diversity in M. graminicola populations in China. However, the total nucleotide diversity was very low (π=0.00682). The high haplotype diversity and low nucleotide diversity implied that M. graminicola populations had a bottleneck, followed by rapid population growth. The high haplotype diversity observed among these populations, with substantial nucleotide similarity might have resulted from the accumulation of mutations (Shao et al., 2020). As many invertebrates with large maternal active populations and robust reproductive capacities are known for having high haplotype diversity and low nucleotide diversity (Grantand Bowen 1998; Lavery et al., 2008). According to Tajima’s D and Fu’s FS analyses, allM. graminicola populations in the present research might have experienced population expansion during evolution.
The genetic differentiation index (Fst) and gene flow (Nm) are two important indicators that reflect genetic differences and gene exchange among populations (Rousset 1997). The coefficient of genetic differentiation (Fst) is usually used to measure the degree of differentiation between different populations. In the range of 0 to 1, the larger the Fst value, the higher the degree of differentiation between populations (Wright, 1978). Based on the present research (Table 3), the Fst values (0.593) among all M. graminicola populations were greater than 0.25, which showed that there was a high genetic differentiation among all populations. The gene flow values (0.33) were all less than 1, and gene communication between populations was blocked, which may be because these groups are relatively stable in space and time, resulting in less gene exchange among them. On the other hand, over time, the accumulation of mutations may lead to high levels of genetic differentiation among populations, which may be related to the isolation of geographical distance and the weaker migration ability and slower migration speed of nematode populations.
Deng et al. (2016) analyzed the genetic diversity of R.reniformisin China based on the sequence of COII gene and found a high variation among different R. reniformis populations based on theCOII-LrRNA sequence, which helps them to adapt to changes in the environment. The result is consistent with the results of our study. Wang (2015) investigated the genetic structure of H. glycines in China using COI genes. The Fst value and Nm value of theH.glycines population was 0.27442 and 1.322, respectively. The results indicate some genetic differentiation between populations in general, but also a high level of gene exchange. This finding differs from our observation in this study, and it might be due to the different nematodes species. The Fst value (0.0169) and a high level of gene flow (7.02) were detected among the 19 M. enterolobii populations, and high genetic variation within each population and a small genetic distance among populations were observedbased on the diversity analysis of mitochondrial COI gene.These results are totally inconsistent with our study, probably because the time of M. enterolobii andM. graminicola in China is different, and the mode of transmission may be different, though both are root knot nematodes.
The phylogenetic results revealed that the M. graminicolapopulations in China were divided into three groups: Cluster1 SC, Cluster2 YP and Cluster3 CR (Fig. 1). The M. graminicola strains of French selected from NCBI were not clustered separately, but with Cluster 2 and Cluster 3 of M. graminicola from China, respectively. It showed that there were relatively few genetic differences between the M. graminicola populations in France and the Chinese populations, and there were no genetic differences due to geographic differences. It was also possible that the Chinese M. graminicola populations had the same origin as the M. graminicola populations from French.Additionally, the haplotype analysis indicated that the Hap8 was shared with all M. Graminicola groups in China and that the other haplotypes were evolved from Hap8. Furthermore, the high genetic variation and low gene exchange among the populations as well as the absence of a relationship between haplotype and geographic region, further supported the hypothesis that the different M. graminicola populations isolated from China originated from different places (Yu 2009). The Hap8 was splited into three major groups, with no clustering among haplotypes from the same geographical group. These findings added to the evidence that there was litter genetic flow among M. graminicola populations, resulting in high genetic diversity. The haplotypes of Henan Province were separated from other haplotypes by a significant genetic distance. The genetic distance between them varied significantly, which might be related to the rotation of rice with wheat.M. graminicola was firstly spread from the main rice-producing areas to the wheat and rice rotation areas in 2020 (Liu et al., 2021) and wheat was often planted in Henan Province. The population in Henan Province is the northernmost population in China and the different climate, planting mode and host may affect the infection and development of M. graminicola , and then contribute different genetic variation in this population.
In the present research, no significant correlation ship was observed between genetic distance and geographic distance (Fig 4). This might be a result of the effects of natural irrigation and long-range seed transporting. Based on the Mitochondrial COI gene, Shao et al. (2020) analysed the genetic diversity of the M. enterolobiipopulations in China. The findings also reveal that the genetic distance of M. enterolobii populations does not match their geographical distance.Similarly, the genetic differentiation of H. schachtiiwas found to be less influenced by geographical distance when studying the population genetic structure of the sugar beet cyst nematode in French (Plantard and Porte , 2003).Therefore, we hypothesize that the J2 of M. graminicola would passively be transported to a long distance as a result of human agricultural operations or natural factors such as wind, rain, and water. Then it would affect the genetic structure of M. graminicola populations in China, resulting in a weak correlation between genetic distance and geographic distance.
The genetic diversity of the mitochondrial genes of M. graminicola populations was reported for the first time in China. The genetic diversity of M. graminicola populations in China were found to be small and gene exchange were hindered among them. The present study provided a theoretical basis for the management of theM. graminicola and wouldbe helpful in increasing the production of rice in the future.