1. Introduction
Desert ecosystems are among the most dominant ecosystems which play an essential part in terrestrial ecosystems [
1]. The natural environment of desert ecosystems is exceedingly fragile, and its formation and development are the consequence of arid climate, surface processes, and the evolution of vegetation [
2,
3]. Desertification is a major issue in arid and semi-arid areas, threatening approximately 41 percent of the world surface area and more than 38 percent of the population [
4,
5]. Global Desertification Vulnerability Index (GDVI) showed that deserts and surrounding regions are at high risk of desertification. Meanwhile, the Representative Concentration Pathways (RCPs) predicted an increased risk of desertification mainly in China and northern India [
6,
7]. As one of the most widespread ecosystems in China, desert ecosystems are also sensitive to global change [
8]. On the other hand, complex natural environment has nurtured a rich diversity of flora and fauna, and its organisms are unique compared to other ecosystems and occupy an important position in the composition of global biodiversity [
9].
The Tarim Basin, located in south Xinjiang Province, is the largest inland basin in China. It nourishes the world 's second largest mobile desert, the Taklimakan Desert [
10,
11]. Tarim Basin is 1,500 km long from east to west and 600 km wide from north to south, covering an area of about 530,000 km
2 and surrounded by altitudes ranging from 800-1,300 m. It is characterized by a typical continental arid climate with low precipitation and high evaporation [
12]. Due to the extreme harshness of ecological environment, the process of aridification in the Taklimakan Desert and its climate changes have a significant impact on the population structure of species [
13,
14]. Currently, research in the Tarim Basin and the hinterland of the desert mostly focused on plants, especially cherished and endangered plants [
15,
16], and has concentrated on the evolutionary history and phylogenetic processes [
17,
18,
19]. However, the research on the genetic diversity, structure and differentiation of animal populations in the hinterland and surrounding areas of the Taklimakan Desert is relatively lacking. Zhang et al. [
20] revealed the genetic mechanism of ovis and environmental adaptability of local sheep breeds in the Taklimakan Desert by analyzing the genome and transcriptome of sheep breeds with different agro-geographical characteristics, providing a theoretical basis for the development and conservation of sheep breed germplasm resources in extreme desert environments. The desert and semi-desert inhabited ungulate
Gazella subgutturosa, has declined sharply due to natural factors and habitat fragmentation. Studies on this species in Xinjiang showed that their population had a low level of genetic diversity and there was a certain genetic differentiation among the populations, addressing particularly urgent to strengthen the conservation of genetic resources of
G. subgutturosa in Xinjiang [
21].
As predominant species in desert ecosystems, reptiles have successfully survived in arid environments by improving morphological and physiological characteristics such as temperature regulation, water balance and movement processes [
22,
23].
Phrynocephalus forsythii, belonging Agamidae family, is an endemic species of the Tarim Basin and is a viviparous lizard that inhabits sparse scrub or Gobi area. Research on evolutionary history and phylogeny of
P. forsythii is relatively mature [
13,
24], while studies on its genetic structure and population differentiation is still rare.
The size of genetic diversity reflects the diversity of genetic factors and their combinations that determine biological traits. The richer the genetic diversity, the richer the morphological, behavioral, physiological and other characteristics of the population, and the stronger the adaptability to the environment. Therefore, the size of genetic diversity can be used as an indicator of population adaptability to its environment [
25]. Although
P. forsythii was not currently identified as endangered, it was demonstrated with potential high extinction risk under climate change [
26]. Baseline genetic data on genetic diversity and structure, therefore, is crucial for understanding the overall genetic status and providing accordant conservation strategies of the species. Our study specifically aimed to investigate the genetic diversity and population structure of
P. forsythii from 15 sites in the Tarim Basin using 21 highly polymorphic microsatellite markers. We also incorporated geography distance data and climatic factors in the analyses to preliminarily infer the potential influence on genetic structure. The results are expected to complement and enrich the basic genetic data of
P. forsythii in the Tarim Basin, providing scientific and theoretical basis for the conservation of the species, and contributing an important reference for the conservation and management of biodiversity in desert ecosystems.
4. Discussion
We used 21 microsatellite loci in the current study to investigate genetic structure and level of gene flow of the P. forsythii in the Tarim Basin region. We found significant genetic structure across the study region, with roughly consistent results from STRUCTURE and DAPC. Along with the results from IBD and RDA analyses, our study revealed generally low levels of gene flow in the study area, suggesting individual dispersal and migration may be restricted within populations.
The results of HWE test showed that 120 tests deviated from HWE after Bonferroni correction, and no deviation was found in populations with small sample sizes (XH, LT, JS). The HWE deviation mainly came from populations with relatively large sample sizes, which suggested that the effectiveness of HWE test was limited validity in populations with fewer samples. Similarly, the small sample size could be also the reason why the XH and LT populations showed significant differences in He. Micro-checker results demonstrated that there are no null alleles in the dataset. Therefore, the HWE deviation is likely to be caused by the Wahlund effect, with causal factors including inbreeding and possible genetic structure.
Regarding the genetic mechanism of inbreeding decline, researchers tend to support the dominant effect theory, which suggests that some recessive pure homozygotes have deleterious effects and inbreeding increases the proportion of these harmful homozygotes, which will cause the phenotypic performances to exhibit a decrease [
52]. Inbreeding coefficient was calculated based on HWE and reflected the relationship between
Ho and
He. Our
Fis results showed that the inbreeding coefficients of 15 populations were higher than 0, indicating that there was a lack of heterozygotes in the population, which was consistent with the results of heterozygous defects in HWE detection, suggesting that the populations may be in the state of inbreeding depression [
53]. Negative and a value closer to -1.0 of
IR means that there are fewer inbred individuals and less inbreeding in the population, while positive and closer to + 1.0 means more close relatives and higher inbreeding [
54]. The results showed that
IR was high (mean of 0.543) and there was no significant difference between populations, suggesting inbreeding may present in various populations. On the other hand, genetic structure is also a driver of Wahlund effect. In contrast to inbreeding, individuals in various groups within a large population with genetic structure are more likely to mate with unrelated individuals. If genetic structure is a major factor of Wahlund effect, then the high
Fis and
IR in our study cannot be explained by inbreeding. In fact, high
Fis values but not relating to inbreeding have also been reported in other species [
55,
56].Presumably, therefore, high
Fis and
IR values in our study might be likely the reflection of individual mating in a natural state of the species in the study region. This speculation, of course, needs further verification using
P. forsythii populations across its whole distribution region.
Genetic differentiation index is an important indicator of the degree of genetic differentiation among populations [
57]. The greater the degree of differentiation, the more pronounced the genetic structure of the population.
FST and
DEST of the 15 populations indicated that there were obvious genetic differences among the populations, and the differences were mainly from ALE. The pairing comparison between this population and other populations was almost always significant, and the results showed consistency. the LT population was discovered as the least distinct from other 14 populations, possibly due to its small sample size. Both the IBD test and the RDA analysis of the four distance-related variables and the genetic variation of the population indicated that there was a significant isolation effect of distance in the study area, and that geographical distance was significantly and positively correlated with genetic distance within 100 km. Therefore, it is suggested that the population gradually accumulates genetic variation during the process of dispersal. In addition, the dispersal ability of the
P. forsythii cannot be neglected when considering the geographical isolation effect. Based on previous studies on the relationship between genetic structure and dispersal in vertebrates, it is generally assumed that geographical factors limit species dispersal and thus limit gene flow [
58]. The dispersal capabilities of
P. forsythii might be crucial to explain the observed genetic structure. At present, however, detailed knowledge on the dispersal distance and home range of the species is still in the blank. Further research needs to confirm the threshold dispersal distance and whether the species are capable of fulfill long-distance movement.
Although the results of the RDA permutation test were non-significant, the mean daily temperature difference (Bio2) was higher than the rest climate factors, suggesting that environmental temperature changes may have a relatively larger effect on the population genetic variation. According to the daily air temperature data from 1960 to 2012 provided by the National Climate Center, the Meteorological Administration of China (
http://cmdp.ncc.cma.gov.cn/cn/index.htm), May to August in the Tarim Basin is the hottest season in a year. The monthly average temperature exceeds 20 °C, and the extreme high temperature exceeds 40 °C. Abbas et al. [
59] also monitored the temporal and spatial variation data of surface air temperature (SAT) in the Tarim Basin from 1961 to 2015, and found that the overall warming rate in the Taklimakan Desert was 0.25 °C per decade and the SAT growth rate in Northwest China was higher than the global average. Obviously, changes of ambient temperature have a regulatory impact on behavior of Poikilothermy. For example,
P. przewalskii and
P. frontalis would cool their bodies by adjusting the gaping of mouth, gasping or urination, and cooling their bodies through evaporating heat when needed [
60].
Eremias multiocellata can evade the lethal effects of high temperature through behavioral regulation methods such as hiding in the shade of
Elaeagnus angustifolia, under the leaves of dead trees in soil ditches, in the grass or climbing up to
Nitraria tangutorum [
61,
62]. Besides, studies have shown that climate warming increases the number of times that poikilothermic animal shuttle between moving into sunlight and drilling into caves to regulate body temperature, which resulted in shorter predation time, reduced chances of mating, and increased probability of being preyed on by predators, leading to increasing maternal mortality and reducing reproductive output [
63,
64]. Similarly, for avoiding negative effects of extreme high temperature, we speculated that
P. forsythii would take behavioral regulations (shorten daytime activities and reduce movement range for example), resulting in decreased effective mating and possibly reduced gene flow which further leads to declined genetic diversity and exacerbates the degree of population differentiation caused by habitat fragmentation.
5. Conclusions
In our study,
P. forsythii have some degree of genetic differentiation, with low levels of gene flow between populations, and individual dispersal and migration likely to be restricted to within populations, showing a clear genetic structure overall. Geographical distance and environmental temperature may also be important factors affecting the observed genetic structure. In concordance with this, Qi et.al. [
65] also found a relatively low genetic diversity of the species in the same region and suggested the high temperature being the main reason. Sinervo et al. [
26] developed an eco-physiological model for predicting extinction risk under climate change for 20
Phrynocephalus lizards and revealed 12 of them, including
P. forsythii, being at high extinction risk due to exceeded thermal limits. Additionally, by building a climate refugia map using locations of Nature Reserves and extinction areas of China, the author demonstrated that current distribution sites of
P. forsythii were beyond the refugia map [
26]. Hence, we call for strengthening conservations of
P. forsythii. Construction of reserves in the Tarim Basin, for example, would be an effective strategy for its protection.