4.2 Diversity spatial structure of C. udensis diploids and tetraploids in the Hualongshan Mountains
Plants through clonal propagation can produce the descendant with the same genotype of parents and affect the genetic diversity and population spatial structure. For facultative clonal plants, it can supply the loss of genetic variation through sexual reproduction and increase the level of genetic diversity within population (Van Drunen & husband, 2018; Alonso Marcos et al., 2019). The genetic diversity of C. udensisand different ploidy populations in the Hualongshan Mountains was high (Table 4). This not only confirmed the facultative clonal characteristic of the species but also was consistent with previous studies (Wang et al., 2008; Wang et al., 2010; Wang et al., 2011; He et al., 2017).
The C. udensis breeding system is partially self-compatible and outcrossed through pollinators. Under ambient conditions, the rate of seed setting was 58%; however, by artificial xenogamy, the rate of seed setting was 82% (Bai et al., 2009). During the investigation and observation of the C. udensis breeding system, the fruit could be produced by no-clipping anther and bagging (including inflorescence and single flower) and there were seeds with in these fruits, which indicated that the self-pollination of C. udensis was fertile, although the seed setting rate was very low (Bai et al., 2009). Therefore, it was hypothesized that C. udensis facultative breeding system increased the survival fitness and improved the genetic diversity of the population.
Clonal growth typically leads to a decrease of genetic variation within populations (Ma et al., 2016; Jiang et al., 2018). However, increasing research has shown that the genetic diversity of clonal plant populations was not as low as anticipated (Pang et al., 2010; Pirog et al., 2017; Mandel et al., 2019). In this paper, both diploids and tetraploids had relatively high clonal diversity. Different clones had the capacity to occupy different microhabitats due to habitat heterogeneity under forest shade, clonal reproduction increased the survival of offspring and saved the resource consumption about sexual reproduction, impacted the fitness and the evolution of population (He et al., 2017). These might result in a high level of C. udensisclone diversity (Ma et al., 2019). Through morphological plasticity,C. udensis rhizomes could place their clonal ramets within favorable environments to achieve foraging behavior. Moreover, the rhizomes could renew their buds and preform photosynthetic products. These would provide material resources for seedling formation, clonal ramet growth, and the long-term survival of dormant buds, ensuring the success of clonal growth and improving the adaptability of C. udensis ’s genet diploids and tetraploids (Wang et al., 2008).
The lifecycles of clonal plants are generally quite long, where a high degree of clonal diversity can be maintained even under very low levels of seedling regeneration (Wang et al., 2008; Liu et al., 2016). Nevertheless, the clonal diversity of different ploidies of C. udensis in the Hualongshan Mountains (Table 4) was lower than the average values of other clonal plants (D = 0.62) (Kartzinel et al., 2015), which was affected by the relative contribution ratio of sexual reproduction and asexual reproduction of diploids and tetraploids in different habitats.
For the two different C. udensis ploidies, the diversity indices of the tetraploids were slightly higher than that of the diploids (Table 4). Being an autopolyploid, the tetraploids of C. udensis has a double genome relative to the diploids. The gene loss rate of tetraploids was half of the diploids. Thus, compared with the diploid, the genetic drift on the tetraploid was smaller. Consequently, the effective population size of tetraploid was relative larger than the diploid that resulting in the relative higher genetic diversity. Meanwhile, the genetic effective size within population of clonal plants mainly depends on the genet number not on the ramet number (Zhang & Zhang, 2006). In this study, the tetraploid has more genets than the diploid (Table 4), which might be another reason for the higher level of genetic diversity in the tetraploid of C. udensis .
As descendants of diploids, tetraploids might occupy myriad microhabitats of various areas with different genets coming from multiple genotypes during the initial establishment of populations (Table 4). Subsequently, this ploidy will propagate offspring through rhizome buds. The current individuals within tetraploid population are likely to be clone progeny, or the progeny of clone progeny from the original genet (Alix et al., 2017; Van Drunen & Husband, 2018; Wang et al., 2019). Compared with the high-altitude habitats of the diploids, the tetraploids occupied low altitude habitats with higher temperatures and increased precipitation. Other environment factors (e.g., light and nutrients) of tetraploid surroundings were also different from those of diploids. When in different habitats, the tetraploids of C. udensis presented higher clonal and genetic diversity to adapt to heterogeneous environments during the evolutionary process. These results supported the views proposed by Wang (Wang et al., 2008), but were different from the viewpoints of He et al. (2017). The clonal diversity, clonal spatial structures, and genetic diversity of clonal plants were influenced by sample scale, sample strategy, sample point layout, and individual sample selection (Van Drunen et al., 2015; Pirog et al., 2017; Wang et al., 2018a, b).
There was significant genetic differentiation between the diploids and tetraploids of C. udensis (Table 5), which might be closely related to its habitat distribution, breeding strategy, and life history. Through field investigations, we found that C. udensiswas often distributed under the arbor and shrub forests of high-altitude areas, and highly dependent on the humid environment. In the Hualongshan Mountains, the diploids distribute across shady southern slopes at an altitude of 2400 meters (peak of 2917 meters), while the tetraploids distribute under the arbor forests of the 40° northern slopes at an altitude of from 1880-2000 meters.
The geographical distance between the two ploidy populations was 20 kilometers. Compared with the tetraploids, the growth cycles of the diploids were shorter due to the relatively higher altitude and lower temperature. The flowering time of the diploids was later than that of the tetraploids by about two weeks, whereas the fruits of the diploids matured in early August compared with those of the tetraploids, which matured in early September (Bai et al., 2009). The growth cycle of the diploids was about 45 days shorter than that of the tetraploids. The geographical isolation of the two ploidies and the asynchronous phenological period would effectively block gene exchange between the diploids and tetraploids, with limited wind or insect pollination to a certain extent, which enhanced the differentiation between the two ploidies.
Most of the C. udensis diploid and tetraploid genotypes were localized; thus, obvious clonal differentiation was observed between the two ploidies, which was consistent with the results of Wang et al. (2008) and He et al. (2017). The genotype distribution patterns of the different C. udensis ploidies in the Hualongshan Mountains resulted from the adaptation of the diploids and tetraploids to different habitats. Under the different selection pressures, various genotypes were fixed within the diploids and tetraploids, respectively, through the formation of different clones to adapt to diverse environments, whereafter the clone differentiation between the diploids and tetraploids were gradually manifest.