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.