Genetic diversity is an important part of rice germplasm resource utilization. A total of 141 local rice landraces from seven cities in Yunnan were analyzed for their genetic diversity in our experiment, and the results showed that the rice landraces have rich genetic diversity, which are the same as the previous studies [28, 29]. Although the genetic diversity of rice populations in the seven localities was high (He = 0.63), the population diversity level was not evenly distributed, showing a clear spatial distribution and a significantly negative correlation with geographical longitude. This shows that the longitude gradient has an impact on the genetic diversity of Yunnan rice populations. There was study reported that when populations were divided according to latitude and longitude, the variance of the genetic diversity within populations was small, indicating that latitude and longitude are important factors affecting the genetic diversity of populations . In addition, some people found that the biological genetic diversity throughout the Mediterranean basin is significantly correlated with longitude . They also declared that it is difficult to use a single factor to explain the genetic diversity across Mediterranean areas, which have complex environments with specific climate types, nonetheless, longitude was an important factor . Some studies have shown that factors such as temperature and altitude have varying degrees of influence on genetic diversity because of the complex terrain of Yunnan, but no specific analysis has been made from the perspective of longitude . This study found that there is a significant correlation between rice genetic diversity and longitude in Yunnan. Thus, we provide a new way to study rice genetic diversity and new ideas for rice germplasm innovation and genetic diversity protection.
Genetic differentiation indicates the degree of variation within a population . This study showed that the genetic variation of the rice population in Yunnan is driven mostly by variation within populations (94%). The Fst was low (0.037), which is consistent with the findings of previous studies. William  used SSR markers to assess the genetic diversity of rice genotypes and found that 70% of the genetic variation originates from variation within populations. Tu  also used SSR markers to analyze the genetic diversity of 60 rice germplasms in Yunnan and reported similar results. Wright  reported that self-pollinated rice has high genetic differentiation within populations, while genetic differentiation among populations is limited. We found that Yunnan rice populations experienced a high degree of gene flow (Nm) (6.505) and showed obvious distance isolation. In addition, geographically close populations showed a high degree of gene flow, which was limited to geographically separated populations, which are similar to the results of rice populations in Sri Lanka . Some studies have reported that the frequency of landrace seed exchange is more than ten times that of foreign exchanges among farmers in Yunnan, these frequent exchange of seeds between neighboring regions may affect gene flow [36, 37]. Thus, we speculated that seed exchange between farmers and regions may be the reason for the formation of high gene flow in geographically close regions.
As one of the basic characteristics of species and populations, genetic structure is the result of multiple factors such as selection, mutation, genetic drift, migration, and gene flow . Structural analysis in this study showed that the genetic structure of rice populations includes components associated with two main groups: japonica and indica rice (Fig. 3a, K=2). Using SSR markers, Chen  analyzed the population structure of rice germplasm resources from 16 cities in Yunnan, and the results showed that 908 rice germplasms could be divided into two major groups (japonica and indica), which was consistent with our results. However, Yunnan is divided into three types of rice planting areas, i.e., an indica area, an indica-japonica overlapping area and a japonica area, due to the unique geography and climatic . When the K value in the structure analysis was 3, we found that two genetic components were typical japonica rice and typical indica but that a third component was not typical indica or japonica rice (Fig. 3b, K=3). PCoA also showed that there were three subgroups, two of which included typical japonica or indica, and the third subgroup was an intermediate type (Fig. 4). In addition, we found that most rice populations (except Yuxi) included all three components (Figure 3b), indicating that there was large genetic differentiation within populations and further confirming that there was high gene exchange within populations.
Rice blast control is an important part of improving the yield and quality of rice . The results of this study showed that rice populations have a rich genetic diversity and that there is a positive correlation between genetic diversity and rice blast resistance (Fig. 5a). Zhu  found that genetic diversity was a significant factor in the control of rice blast. Tu  and He  also reported that the higher the genetic diversity of rice varieties was, the higher the resistance of the varieties to rice blast. Therefore, it is highly important to determine the relationship between the genetic diversity of rice landraces and the rice blast resistance of genotypes. Applying these important germplasm resources to rice production is a future endeavor.