4.2 Genetic differentiation and genetic structure in C. hainanense
PCA analysis showed that PC1, PC2, and PC3 contributed 10.04%, 6.85%, and 5.07%, respectively. The principal component analysis’s clustering results may differ from those of the other group analyses. In PCA, the Wuzhishan population is genetically distant from other populations and forms 1 cluster separately. In addition, the cluster structure of principal component analysis, K-value selection of cluster structure, and phylogenetic tree analysis of all samples resulted in the same clusters. The fact that PCA showed the Wuzhishan population as genetically distant from other populations, forming a separate cluster, suggests that there may be unique genetic features in this population that were captured by the PCA. This could indicate that the Wuzhishan population has experienced different evolutionary pressures, gene flow patterns, or demographic histories compared to the other populations in the study. It is also important to note that the PCA clustering results may differ from other group analyses. This is because PCA is focused on capturing the maximum amount of variance in the data, while other methods, such as phylogenetic tree analysis or population structure analysis using Bayesian clustering approaches (e.g., STRUCTURE), might focus on different aspects of the data. These other methods may consider more specific evolutionary models or genetic relationships, leading to different interpretations of the population structure. Despite the differences in clustering results between PCA and other group analyses, the study found that the cluster structure of the PCA, K-value selection of the cluster structure, and phylogenetic tree analysis all resulted in the same clusters. This consistency across different analytical methods provides strong evidence for the observed population structure, lending credibility to the findings. The PCA results highlight the genetic distinctiveness of the Wuzhishan population, which may have important implications for the conservation and management of C. hainanense . Understanding the factors contributing to the genetic differentiation between populations can inform targeted conservation efforts aimed at preserving the unique genetic diversity found in each population. Further research on the specific genetic differences between the Wuzhishan population and others, as well as the potential environmental or ecological factors driving these differences, would provide valuable insights into the species’ evolutionary history and inform future conservation strategies.
All supported the division of 6 clusters into two clusters, so it was reasonable to divide 35 C. hainanense samples from 6 clusters into two clusters of 1 cluster (Diaoyu Mountain and Baisha Ridge) and 2 clusters (Wuzhishan, Huishan, Bawang Ridge, and Tsimshatsui Ridge). This division into two primary clusters could be a result of various factors, such as historical gene flow, geographical isolation, or habitat fragmentation. The two clusters may have experienced different evolutionary pressures, environmental conditions, or demographic histories that have shaped their genetic makeup. The consistency of this clustering pattern across multiple analytical methods (PCA, K-value selection, and phylogenetic tree analysis) strengthens the evidence for the observed population structure. This consistent finding implies that the division of the six populations into two primary clusters is a robust and meaningful representation of the genetic relationships among these C. hainanense samples.
Differences in population genetic structure are an important expression of genetic diversity. A species’ evolutionary potential and ability to withstand adverse environments depends not only on the level of genetic variation within the species but also on the genetic structure of the species (McCauley et al., 2014; Hamrick et al., 2011; Qu et al., 2004). Our results indicate that the six populations on Hainan Island are divided into two broad taxa, consistent with the principal component analysis results. A population’s genetic differentiation index (Fst) is an important parameter to measure the degree of genetic differentiation among populations. It can explain the factors that affect the genetic differentiation of populations (Zhou et al., 2022). The genetic differentiation coefficient among the populations showed that the Fst values among the six C. hainanense populations were between -0.09648-0.076729. There is a moderate degree of genetic differentiation (0.05 < Fst < 0.15) between the two populations of Baishaling (BSL) and Jianfengling (JFL). Furthermore, the genetic differentiation among other populations is low, and the differentiation is not noticeable (Fst < 0.05). Therefore, the genetic differentiation among C. hainanense populations is weak. The existing gene flow may originate from the genetic exchange between their common ancestor populations and be brought into other populations by other factors such as human factors, animal carrying, or geological factors. In addition, the topography may also affect gene flow in this species. The terrain of Hainan Island is high in the middle and low in the surrounding areas. It rises and descends to the periphery step by step. The cascade structure is prominent, and the terraces, hills, plains, and mountains form a ring-shaped layered landform (Chen et al., 2022).
The samples taken in this study were from Wuzhishan, Diaoluoshan, Jianfengling, and their surrounding forests. Geographically, the Jianfengling and Baishaling populations are far apart (>100km). However, the genetic distance between the two populations is relatively small (Fst = 0.076729), possibly due to the number of sampling populations of C. hainanense being too small. The population distribution of C. hainanense is relatively concentrated under the influence of habitat fragmentation, so the evidence is not sufficient. Therefore, the next step is to expand the scope of the sampling population for subsequent research and analysis. Limited sampling: As the study itself points out, the number of sampled populations of C. hainanense may be too small to accurately represent the full extent of genetic diversity and differentiation within the species. Additional sampling from other populations, particularly those that may be geographically intermediate between Jianfengling and Baishaling, could help provide a more comprehensive understanding of the genetic relationships among populations. The small genetic distance between the Jianfengling and Baishaling populations may be due to historical gene flow that occurred before the populations became geographically isolated. Over time, barriers such as geographical features, habitat fragmentation, or human activity may have restricted the movement of individuals between populations, but the genetic legacy of past gene flow could still be apparent in the current populations.C. hainanense may be capable of long-distance seed dispersal through mechanisms such as wind, water, or animal-mediated dispersal. Even if the frequency of long-distance dispersal events is low, they can still contribute to maintaining gene flow between geographically distant populations and reducing genetic differentiation. Human activities, such as the movement of seeds or plants for horticulture, agriculture, or reforestation efforts, may have inadvertently facilitated gene flow between the Jianfengling and Baishaling populations. This could result in the observed small genetic distance despite the large geographical separation between the populations. Small or isolated populations are more susceptible to founder effects and genetic drift, which can reduce genetic diversity and lead to increased genetic differentiation between populations. However, these processes can sometimes produce counterintuitive patterns, such as closely related populations that are geographically distant, or vice versa. To better understand the factors influencing the genetic structure of C. hainanense , further research is needed. This could include increasing the number of sampled populations and individuals, examining historical patterns of gene flow, and investigating potential mechanisms of seed dispersal. Ultimately, a more comprehensive understanding of the species’ genetic diversity and population structure can inform conservation strategies and help ensure the long-term survival of C. hainanense .