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.