4.2 Genetic differentiation and genetic structure
Genetic
structure of a species is effectively the sum of genetic differentiation
among and within populations (Hamrick & Godt, 1989). Overall, genetic
structure occurring among populations results from the evolutionary
history of the species in question; natural selection; genomic factors
(e.g., mutations, reorganization, and genetic drift); and biological
characteristics, including gene flow, mating system, mode of
reproduction, and seed dispersal mechanisms (Slatkin, 1987; Zhen, 2010).
Genetic differentiation is primarily controlled by aspects of gene flow,
such as its rate and directionality (Hamrick & Godt, 1989). In plants,
gene flow occurs primarily via the transmission of pollen and seeds
during sexual reproduction. However, for clonal species, such asP. villosa , asexual propagules often have limited dispersal
distance, and this restricts gene flow among populations (Xia, Li, &
Li, 2002).
For P. villosa , we inferred that more than 56% of the genetic
variation existed within populations, with average pairwiseF ST of 0.35841 for all 43 population and gene
flow (N m) of 0.5761 (Table 4). According to Wright’s (1978)
theory, genetic differentiation among populations might be large whenF ST > 0.25, but this can be
mitigated by gene flow of N m > 1, which can reduce
the effects of genetic drift and prevent genetic differentiation among
populations. In P. villosa , we found highF ST but limited gene flow, which should yield
high rates of between population differentiation. However, we found
higher rates of within population differentiation for the species. This
differs from findings in other studies of P. villosa that
revealed greater genetic variation among populations, such as in Li &
Ge (2001), who studied genetic diversity in P. villosa using and
seven populations, of which two were from Shihuimiao Ecological Station
and five were from the Shilongmiao Ecological Station. Similarly, Wang
et al. (1999) assessed the genetic diversity of four populations ofP. villosa from mobile and fixed sand dunes in the Shihuimiao
Ecological Station (two populations) and the Shilongmiao Ecological
Station (two populations) and found greater genetic variation among
populations. Nevertheless, population genetic structure was a
comprehensive result of a variety of factors that were related to the
evolutionary origins and modes of dispersal and sexual reproduction,
which all might be unique to each population within a species (Wang, Ge,
& Dong, 1999). Accordingly, we inferred this inconsistency may be
related to the numbers of populations and geographic location. Our
sampling, which included a larger number of populations and a greater
portion of the geographic range ofP.
villosa , might have yielded results that more robustly detect patterns
among unique populations. In addition, the efficiency and accuracy of
gene identification are also related to the number of polymorphic bands,
and the results obtained by different experimental methods are
different.
In this study, our SAMOVA analysis revealed two well-defined groups
corresponding to Group1 and Group2 (Table S3). These groups are also
consistent with our analyses using UPGMA, STRUCTURE, SplitsTree, and
PCoA (Figure 2-5). Within populations of Groups 1 and 2, the average
genetic variation was 64% with an average pairwiseF ST value of 0.43865. The gene flow (N m)
in Groups 1and 2 was 0.9996, 0.6605, respectively (Table 4). In
addition, the UPGMA tree revealed that populations with closer
geographic distances did not always cluster together, and we observed a
similar pattern in the results from STRUCTURE and SplitsTree. Similarly,
the PCoA showed that individuals from the same population did not always
group together, further indicating that, while gene flow may be limited
in P. villosa , it occurs over long distances more often than
between adjacent populations. This may suggest that some critical
dispersal vector, such as birds, is yet-unknown for the species.
Overall, P. villosa has undergone considerable genetic divergence
and has a high level of genetic structure based on the combined results
from our population genetics analyses.
Based on a Mantel test, we found that there was significant correlation
between genetic distance and geographic distance, indicating that
geographic distance is an important factor affecting the genetic
structure of P. villosa . Therefore, we inferred that genetic
structure might have been resulted mainly from geographic isolation
imposed by mountains (e.g., Yin Mountains; Helan Mountains) and large
deserts in northwestern China (e.g. Tengger Desert; Mu Us Sandy Land) as
well as range contraction and population fragmentation induced by
climatic oscillations (e.g., Gymnocarpos przewalskii Maxim.;Helianthemum songaricum Schrenk) (Liu, 1995; Su, Zhang, &
Sanderson, 2011; Ma, Zhang, & Sanderson, 2012; Meng, Gao, Huang, &
Zhang, 2014). In addition, founder effects and population bottlenecks
might have also contributed to the genetic structures of the species
(Birky, Fuerst, & Maruyama, 1989; Liu et al., 2015).
4.3 Demographic historyofP. villosa
The
genetic diversity within the Group 1, identified according to SAMOVA,
was lower than that of the populations in Group 2 (Table 3). Based on
this, extant populations of this species originated from the genetic
stock of Group 2, as geographic areas with both high genetic diversity
and frequency of dominant genes usually represent centers of origins for
source populations (Vavilov, 1926). However, our study design and
results cannot discern the exact center of origin for the species nor
the main migrational patterns of P. villosa , and accomplishing
this will require additional molecular data and informatics approaches.
Climate oscillation during the Quaternary has often been hypothesized to
be an important factor in influencing the current geographical
distribution and demographic history of plant species (Hewitt, 2004; Su
& Zhang, 2013). One widely utilized approach to comparing past and
future distributions of plant species and determining the primary
environment factors driving them is via ENM (e.g., Nabout, Magalhães,
Ma, & Da, 2016; Bai et al., 2017; Huang et al., 2017; Noulèkoun, Chude,
Zenebe, & Birhane, 2017; Swanti, Kusum, Dhruval, & Rajkanti, 2018;
Wei, Wang, Hou, Wang, & Wu, 2018). Based on the neutral test, we found
that the species did not expand its range during the Quaternary.
Although these results were not statistically significant (Table 3), our
ENMs, in general, show that the range of P. villosa contracted
from the LGM to the present (Figure 6-7). Specifically, our models show
that the range of P. villosa was the most extensive during the
LIG period and included the northeast edge of the Qinghai-Tibet Plateau,
Tarim Basin, Tianshan Mountains, Inner Mongolia Plateau, and the western
regions of DaXinggan Ling. The range became limited to the Inner
Mongolia Plateau, Ordos Plateau, and the Yinshan-Helanshan area during
the LGM. The contraction of the range is likely the result of glaciation
and climatic shifts within the Tianshan Mountains and Tarim Basin, where
temperatures dropped significantly as glaciation developed on a large
scale in the Northern Hemisphere during the early-Middle Pleistocene
(Williams, Dunkerley, De, Dekker, Kershaw, & Stokes, 1993; Yi et al.,
2004; Shi, Cui, & Su, 2005; Lehmkuhl & Owen, 2005; Xu et al., 2010;
Meng, Gao, Huang, & Zhang, 2014). Nevertheless, it is surprising that
the species range did not rebound as temperatures grew warmer following
the LGM. This may be because of the onset of extreme aridity within the
region during the Quaternary period, as this is widely-known to have
played a significant role in determining the geographic distribution and
evolutionary history of many plant species (Meng & Zhang, 2011; Su,
Zhang, & Sanderson, 2011; Su & Zhang, 2013). For example, in a
previous study of Helianthemum songaricum (Cistaceae), which
occurs in Northern China and adjacent desert areas of central Asia (Yang
& Gilbert, 2007), the worsening of the dry climate restricted the
distribution range, and acceptable habitats for the species gradually
became reduced and isolated (Su, Zhang, & Sanderson, 2011). Besides, it
needs to explain that AFLP dataset used in this manuscript do not reveal
too many informative sites, so more markers should be selected to
discuss the evolutionary history of P. villosa in the future.