Conservation genetics and geographic patterns of genetic variation of endangered shrub Ammopiptanthus (Fabaceae) in northwestern China

A reference-grade genome of the xerophyte Ammopiptanthus mongolicus sheds light on its evolution history in legumes and drought-tolerance mechanisms

Plants that grow in extreme environments represent unique sources of stress-resistance genes and mechanisms. Ammopiptanthus mongolicus (Leguminosae) is a xerophytic evergreen broadleaf shrub native to semi-arid and desert regions; however, its drought-tolerance mechanisms remain poorly understood. Here, we report the assembly of a reference-grade genome for A. mongolicus, describe its evolutionary history within the legume family, and examine its drought-tolerance mechanisms. The assembled genome is 843.07 Mb in length, with 98.7% of the sequences successfully anchored to the nine chromosomes of A. mongolicus. The genome is predicted to contain 47 611 protein-coding genes, and 70.71% of the genome is composed of repetitive sequences; these are dominated by transposable elements, particularly long-terminal-repeat retrotransposons. Evolutionary analyses revealed two whole-genome duplication (WGD) events at 130 and 58 million years ago (mya) that are shared by the genus Ammopiptanthus and other legumes, but no species-specific WGDs were found within this genus. Ancestral genome reconstruction revealed that the A. mongolicus genome has undergone fewer rearrangements than other genomes in the legume family, confirming its status as a "relict plant". Transcriptomic analyses demonstrated that genes involved in cuticular wax biosynthesis and transport are highly expressed, both under normal conditions and in response to polyethylene glycol-induced dehydration. Significant induction of genes related to ethylene biosynthesis and signaling was also observed in leaves under dehydration stress, suggesting that enhanced ethylene response and formation of thick waxy cuticles are two major mechanisms of drought tolerance in A. mongolicus. Ectopic expression of AmERF2, an ethylene response factor unique to A. mongolicus, can markedly increase the drought tolerance of transgenic Arabidopsis thaliana plants, demonstrating the potential for application of A. mongolicus genes in crop improvement.

Conservation Priorities and Demographic History of Saussurea involucrata in the Tianshan Mountains and Altai Mountains

Rare and vulnerable endemic plants represent different evolutionary units that occur at different times, and protecting these species is a key issue in biological protection. Understanding the impact of the history of endangered plant populations on their genetic diversity helps to reveal evolutionary history and is crucial for guiding conservation efforts. Saussurea involucrata, a perennial alpine species mainly distributed in the Tianshan Mountains, is famous for its medicinal value but has become endangered due to over-exploitation. In the present study, we employed both nuclear and chloroplast DNA sequences to investigate the genetic distribution pattern and evolutionary history of S. involucrata. A total of 270 individuals covering nine S. involucrata populations were sampled for the amplification and sequencing of nrDNA Internal Transcribed Spacer (ITS) and chloroplast trnL-trnF, matK and ndhF-rpl32 sequences. Via calculation, we identified 7 nuclear and 12 plastid haplotypes. Among the nine populations, GL and BA were characterized by high haplotype diversity, whereas BG revealed the lowest haplotype diversity. Molecular dating estimations suggest that divergence among S. involucrata populations occurred around 0.75 Ma, coinciding with the uplift of Tianshan Mountains. Our results reveal that both isolation-by-distance (IBD) and isolation-by-resistance (IBR) have promoted genetic differentiation among populations of S. involucrata. The results from the ecological niche modeling analyses show a more suitable habitat for S. involucrata in the past than at present, indicating a historical distribution contraction of the species. This study provides new insight into understanding the genetic differentiation of S. involucrata, as well as the theoretical basis for conserving this species.

Parallel adaptation prompted core-periphery divergence of Ammopiptanthus mongolicus

Range expansion requires peripheral populations to shift adaptive optima to breach range boundaries. Opportunities for range expansion can be assessed by investigating the associations of core-periphery environmental and genetic differences. This study investigates differences in the core-periphery adaptation of Ammopiptanthus mongolicus, a broad-leaved evergreen shrub species in a relatively homogeneous temperate Asian desert environment, to explore the environmental factors that limit the expansion of desert plants. Temperate deserts are characterized by severe drought, a large diurnal temperature range, and seasonality. Long-standing adaptation to the harsh desert environment may confine the genetic diversity of A. mongolicus, despite its distribution over a wide range of longitude, latitude, and altitude. Since range edges defined by climate niches may have different genetic responses to environmental extremes, we compared genome-wide polymorphisms between nine environmental core populations and ten fragmented peripheral populations to determine the "adaptive peripheral" populations. At least four adaptive peripheral populations had similar genetic-environmental association patterns. High elevations, summer drought, and winter cold were the three main determinants of converging these four adaptive peripheral populations. Elevation mainly caused similar local climates among different geographic regions. Altitudinal adaptation resulting from integrated environmental-genetic responses was a breakthrough in breaching niche boundaries. These peripheral populations are also located in relatively humid and warmer environments. Relaxation of the drought and cold constraints facilitated the genetic divergence of these peripheral populations from the core population's adaptive legacy. We conclude that pleiotropic selection synchronized adaptative divergence to cold and drought vs. warm and humid environments between the core and peripheral populations. Such parallel adaptation of peripheral populations relies on selection under a background of abundant new variants derived from the core population's standing genetic variation, i.e., integration of genetic surfing and local adaptation.

Population Genetic Structure and Biodiversity Conservation of a Relict and Medicinal Subshrub Capparis spinosa in Arid Central Asia

As a Tertiary Tethyan relict, Capparis spinosa is a typical wind-preventing and sand-fixing deciduous subshrub in arid central Asia. Due to its medicinal and energy value, this species is at risk of potential threat from human overexploitation, habitat destruction and resource depletion. In this study, our purpose was to evaluate the conservation strategies of C. spinosa according to its genetic structure characteristics and genetic diversity pattern among 37 natural distributional populations. Based on genomic SNP data generated from dd-RAD sequencing, genetic diversity analysis, principal component analysis, maximum likelihood phylogenetic trees and ADMIXTURE clustering, the significant population structure and differentiation were explored. The results showed the following: (1) Six distinct lineages were identified corresponding to geographic locations, and various levels of genetic diversity existed among the lineages for the natural habitat heterogeneity or human interferences; (2) The lineage divergences were influenced by isolation by distances, vicariance and restricted gene flow under complex topographic and climatic conditions. Finally, for the preservation of the genetic integrity of C. spinosa, we suggest that conservation units should be established corresponding to different geographic groups, and that attention should be paid to isolated and peripheral populations that are experiencing biodiversity loss. Simultaneously, monitoring and reducing anthropogenic disturbances in addition to rationally and sustainably utilizing wild resources would be beneficial to guarantee population resilience and evolutionary potential of this xerophyte in response to future environmental changes.

Spatial Genetic Structure of Prunus mongolica in Arid Northwestern China Based on RAD Sequencing Data

The extensive range of sand deserts, gravel deserts, and recent human activities have shaped habitat fragmentation of relict and endangered plants in arid northwestern China. Prunus mongolica is a relict and endangered shrub that is mainly distributed in the study area. In the present study, population genomics was integrated with a species distribution model (SDM) to investigate the spatial genetic diversity and structure of P. mongolica populations in response to habitat fragmentation and create a proposal for the conservation of this endangered species. The results showed that the northern marginal populations were the first isolated from other populations. The SDM suggested that these marginal populations had low levels of habitat suitability during the glacial period. They could not obtain migration corridors, and thus possessed low levels of gene flow connection with other populations. Additionally, several populations underwent secondarily geographical isolation from other central populations, which preserved particular genetic lineages. Genetic diversity was higher in southern populations than in northern ones. It was concluded that long-term geographical isolation after historical habitat fragmentation promoted the divergence of marginal populations and refugial populations along mountains from other populations. The southern populations could have persisted in their distribution ranges and harbored higher levels of genetic diversity than the northern populations, whose distribution ranges fluctuated in response to paleoclimatic changes. We propose that the marginal populations of P. mongolica should be well considered in conservation management.

Isolation-by-environment as a driver of genetic differentiation among populations of the only broad-leaved evergreen shrub Ammopiptanthus mongolicus in Asian temperate deserts

Whether the effect of migration-selection-drift equilibrium on population structure is governed by spatial or environmental differences is usually elucidated by isolation-by-distance (IBD), isolation-by-environment (IBE), and isolation-by-resistance (IBR) tests. The population structure of Ammopiptanthus mongolicus, a broad-leaved evergreen psammophyte in eastern Central Asia, was previously thought to follow an isolation by distance pattern. However, recent studies have emphasized the effects of environmental factors on its growth and distribution, suggesting an important influence of local adaptation on the genetic structure of the species. Using inter-simple sequence repeat (ISSR) markers, we verified the previously inferred low intra-population variation and high inter-population differentiation. However, in contrast to previous studies, the results of partial Mantel tests and a maximum likelihood population effects mixed model (MLPE) suggested that local climate differences, rather than geographic distances or resistance distances, are the main factor affecting population differentiation. Further analysis with removal of multicollinear climatic variables and univariate MLPE found that summer and winter precipitation were crucial for shaping the current population genetic structure. Since local precipitation is related to the regeneration, colonization, and overwintering survival of A. mongolicus, its influence on demographic change may explain its effect on the population genetic structure. In addition, precipitation is related to terrain despite westward decreases, which explains the independence of genetic difference and geographic distance. The identified role of IBE suggests that collecting germplasm resources from genetically differentiated populations could be a more effective strategy to preserve the overall genetic diversity of the species than the establishment of corridors to enhance gene flow among populations.

Intraspecific divergences and phylogeography of Panzerina lanata (Lamiaceae) in northwest China

Climatic fluctuations during the Quaternary significantly affect many species in their intraspecific divergence and population structure across northwest China. In order to investigate the impact of climate change on herbaceous plants, we studied Panzerina lanata (Lamiaceae), a widely distributed species. Sequences of two chloroplast DNA (cpDNA) intergenic spacers (trnH-psbA and rpoB-trnC) and a nuclear ribosomal region (nrDNA, ITS) were generated from 27 populations of Panzerina lanata and resulted in the identification of seven chloroplast haplotypes and thirty-two nuclear haplotypes. We applied AMOVA, neutrality test and mismatch distribution analysis to estimate genetic differentiation and demographic characteristics. The divergence times of the seven cpDNA haplotypes were estimated using BEAST. Our results revealed high levels of genetic diversity (cpDNA: Hcp = 0.6691, H _T = 0.673; nrDNA: Hnr = 0.5668, H _T = 0.577). High level of genetic differentiation (G _ST = 0.950) among populations was observed in the cpDNA sequences, while the genetic differentiation values (G _ST = 0.348) were low in nuclear sequences. AMOVA results revealed major genetic variation among the three groups: northern, central, and eastern group. However, the genetic differentiation in ITS data was not found. The species distribution modeling and demographic analysis indicated that P. lanata had not experienced recent range expansion. The occurrence of divergence between seven cpDNA haplotypes, probably during Pleistocene, coincides with aridification and expansion of the desert across northwest China that resulted in species diversification and habitat fragmentation. In addition, we discovered that the deserts and the Helan Mountains acted as effective geographic barriers that promoting the intraspecific diversity of P. lanata.

Evolutionary response to the Qinghai-Tibetan Plateau uplift: phylogeny and biogeography of Ammopiptanthus and tribe Thermopsideae (Fabaceae)

Previous works resolved diverse phylogenetic positions for genera of the Fabaceae tribe Thermopsideae, without a thoroughly biogeography study. Based on sequence data from nuclear ITS and four cpDNA regions (matK, rbcL, trnH-psbA, trnL-trnF) mainly sourced from GenBank, the phylogeny of tribe Thermopsideae was inferred. Our analyses support the genera of Thermopsideae, with the exclusion of Pickeringia, being merged into a monophyletic Sophoreae. Genera of Sophoreae were assigned into the Thermopsoid clade and Sophoroid clade. Monophyly of Anagyris, Baptisia and Piptanthus were supported in the Thermopsoid clade. However, the genera Thermopsis and Sophora were resolved to be polyphyly, which require comprehensive taxonomic revisions. Interestingly, Ammopiptanthus, consisting of A. mongolicus and A. nanus, nested within the Sophoroid clade, with Salweenia as its sister. Ammopiptanthus and Salweenia have a disjunct distribution in the deserts of northwestern China and the Hengduan Mountains, respectively. Divergence age was estimated based on the ITS phylogenetic analysis. Emergence of the common ancestor of Ammopiptanthus and Salweenia, divergence between these two genera and the split of Ammopiptanthus species occurred at approximately 26.96 Ma, 4.74 Ma and 2.04 Ma, respectively, which may be in response to the second, third and fourth main uplifts of the Qinghai-Tibetan Plateau, respectively.

Evolutionary History of a Desert Shrub Ephedra przewalskii (Ephedraceae): Allopatric Divergence and Range Shifts in Northwestern China

Based on two chloroplast DNA sequences, psbA-trnH and trnT-trnF, phylogeographical patterns of a desert shrub, Ephedra przewalskii, were examined across most of its geographic range in northwestern China. A total of sixteen haplotypes were detected. There was a common haplotype in each basin, that was haplotype A in Tarim Basin, haplotype G in Junggar Basin, and haplotype M in Qaidam Basin. Genetic variance mainly occurred among populations, geographic regions, and eleven geographic groups subdivided by SAMOVA analysis. E. przewalskii likely had a smaller and more fragmented geographic range during the Last Glacial Maximum, which was determined based on ecological niche modelling. Three groups of E. przewalskii populations were detected to have experience range expansion, and this was based on significant values of Fu’s F_S, Tajima’s D, and unimodel mismatch distributions. The cold and dry climate during the glacial period of the Quaternary is postulated to have been a driver for significant genetic isolation and divergence among populations or groups in E. przewalskii, whereas the warmer and wetter climate during the interglacial period is speculated to have provided favourable conditions for range expansion of the species.

Divergent Population Genetic Structure of the Endangered Helianthemum (Cistaceae) and Its Implication to Conservation in Northwestern China

Population genetic studies provide a foundation for conservation planning, especially for endangered species. Three chloroplast SSRs (mtrnSf-trnGr, mtrnL2-trnF, and mtrnL5-trnL3) and the internal transcribed spacer were used to examine the population structure of Helianthemum in northwestern China. A total of 15 populations of the genus were collected. Nine chloroplast haplotypes and two nuclear genotypes were detected. Both the nuclear and chloroplast data showed two lineages in Helianthemum songaricum, respectively, distributed in Yili Valley and western Ordos Plateau. A total of 66.81% (p < 0.001) of the genetic variation was supported by this lineage split. A Mantel test showed a significant correlation between genetic distance and geographical distance (r = 0.937, p < 0.001). Based on genetic analyses, cpSSRs data support strong genetic divergence between regions. We speculate that the climate change during the late Tertiary and early Quaternary isolated H. songaricum into their current distribution, resulting in interruption of gene flow, leading to isolation and genetic divergence between the two regions. Meanwhile, possible selfing would increase genetic drift in small fragmented populations, that might account for the observed genetic divergence in both regions. Given the loss of genetic diversity and genetic divergence in small populations of Helianthemum in northwestern China immediate conservation management steps should be taken on the species.