Population Structure of and Conservation Strategies for Wild Pyrus ussuriensis Maxim. in China

Study on the Genetic Diversity Characteristics of the Endemic Plant Rhododendron bailiense in Guizhou, China Based on SNP Molecular Markers

Rhododendron bailiense was identified as a new species in 2013, with approximately 150 individuals existing globally, found only in Dafang County and Panzhou City, Guizhou Province, China. Despite its discovery, the genetic diversity and population structure of this species remain poorly understood, hindering efforts to collect and conserve wild germplasm resources. In this study, double digest restriction-site associated DNA sequencing was conducted on 26 samples from two populations of R. bailiense to identify single nucleotide polymorphism (SNP) loci. Using these data, the research explores the genetic diversity and structure of R. bailiense populations and infers their population dynamics and evolutionary history. The results indicate that R. bailiense has a moderate level of genetic diversity (π = 0.2489, H o  = 0.2039, H e  = 0.2331). Genetic differentiation between populations is relatively high (55.94%), with a genetic differentiation coefficient (F ST ) of 0.1907. This suggests that R. bailiense historically might have been a large population, which, due to geological historical events, became fragmented into the two existing populations. The Panzhou population demonstrates a heterozygote selection advantage. Conversely, the Dafang population faces the risk of inbreeding depression, further exacerbated by its limited gene flow. Consequently, in situ conservation is recommended for the Panzhou population, while ex-situ conservation is suggested for the Dafang population. Additionally, research on breeding techniques is necessary to expand the population size while maintaining high genetic diversity.

Genetic Variation Analysis and Development of KASP Marker for Leaf Area and Hight in Southern-Type Populus deltoides

Populus deltoides holds significant ecological and economic importance and is a crucial gene donor for the world's staple poplar varieties. To select and breed P. deltoides with improved agronomic traits, nine growth and leaf traits were examined in 375 different genotypes, assessing their genetic diversity and performing correlation and comprehensive ranking analyses. Phenotyping results were then utilized to screen a total of 2,009,263 SNP (single nucleotide polymorphism) loci significantly associated with the nine phenotypic traits. A total of 45 SNP loci exhibited significant associations with growth traits based on a general linear model (GLM) analysis. By analyzing the Linkage disequilibrium (LD) block of five SNP loci with significant leaf area and height, we identified five candidate genes related to leaf area and height. Three of the five SNP loci were successfully validated using KASP (kompetitive allele-specific PCR) assays. One loci Chr08_16007979 was closely linked with leaf area, and two loci Chr05_12148738, and Chr05_17106547 were closely linked with height. The developed functional KASP markers offer valuable insights for subsequent further marker-assisted breeding and genetic improvement studies in southern-type poplars.

Genotyping-by-sequencing provides new genetic and taxonomic insights in the critical group of Centaurea tenorei

Centaurea L. is one of the most widespread, differentiated, and critical genera of Asteraceae in the Euro-Mediterranean area, with more than 100 currently recognized species inhabiting the region. The controversial C. tenorei group, narrowly endemic to the Peninsula of Sorrento (Campania region, southern Italy), includes three weakly differentiated microspecies: C. tenorei Guss. ex Lacaita, C. montaltensis (Fiori) Peruzzi and C. lacaitae Peruzzi. However, their taxonomic distinctiveness and relationships with close or sympatric species are still unclear. In particular, the existence in several localities of individuals with intermediate morphology suggests inadequate taxonomic assessment within the group or hybridization and introgression with other species. In this study we aimed at defining population structure in this complex. With this objective, we sampled the three currently accepted species from their loci classici (i.e., the localities in which the taxa were originally described) and from other localities throughout the range, including populations of difficult identification occurring where the ranges of different taxa overlap. We employed a panel of SNPs obtained via genotyping-by-sequencing for investigations on genetic structure, admixture and ploidy inference, the latter also compared with chromosome counts. Our results showed that Centaurea tenorei s.l. is consistently tetraploid, contradicting the current taxonomy that was also based on ploidy level. Population structure analyses indicated the presence of four to seven clusters, most of which with clear evidence of admixture. Furthermore, contrarily to what previously supposed, we demonstrated a remarkable contribution of C. deusta, more that of C. cineraria in the genetic make-up of C. tenorei. However, we found a population of C. cineraria outside its ecological range, probably driven by climate change, which could be responsible in the future of further hybridization phenomena.

Unravelling the genetic diversity and population structure of common walnut in the Iranian Plateau

BACKGROUND

Common walnut (Juglans regia L.) has a long cultivation history, given its highly valuable wood and rich nutritious nuts. The Iranian Plateau has been considered as one of the last glaciation refugia and a centre of origin and domestication for the common walnut. However, a prerequisite to conserve or utilize the genetic resources of J. regia in the plateau is a comprehensive evaluation of the genetic diversity that is conspicuously lacking. In this regard, we used 31 polymorphic simple sequence repeat (SSR) markers to delineate the genetic variation and population structure of 508 J. regia individuals among 27 populations from the Iranian Plateau.

RESULTS

The SSR markers expressed a high level of genetic diversity (H_O = 0.438, and H_E = 0.437). Genetic differentiation among the populations was moderate (F_ST = 0.124), and genetic variation within the populations (79%) significantly surpassed among populations (21%). The gene flow (N_m = 1.840) may have remarkably influenced the population genetic structure of J. regia, which can be attributed to anthropological activities and wind dispersal of pollen. The STRUCTURE analysis divided the 27 populations into two main clusters. Comparing the neighbor-joining and principal coordinate analysis dendrograms and the Bayesian STRUCTURE analysis revealed the general agreement between the population subdivisions and the genetic relationships among the populations. However, a few geographically close populations dispersed into different clusters. Further, the low genetic diversity of the Sulaymaniyah (SMR) population of Iraq necessitates urgent conservation by propagation and seedling management or tissue culture methods; additionally, we recommend the indispensable preservation of the Gonabad (RGR) and Arak (AKR) populations in Iran.

CONCLUSIONS

These results reflected consistent high geographical affinity of the accession across the plateau. Our findings suggest that gene flow is a driving factor influencing the genetic structure of J. regia populations, whereas ecological and geological variables did not act as strong barriers. Moreover, the data reported herein provide new insights into the population structure of J. regia germplasm, which will help conserve genetic resources for the future, hence improving walnut breeding programs' efficiency.

Management of genetic erosion: The (successful) case study of the pear (Pyrus communis L.) germplasm of the Lazio region (Italy)

Pyrus communis L. is an important temperate fruit with high nutritional and economic value. Italy, as the largest pear producer in the EU and second in the world, has a particularly rich germplasm characterized by hundreds of local varieties. The Lazio Region was the first Italian region to start programs focused on safeguarding varieties at risk of extinction and has started a massive census of fruit varieties by combining molecular data and productive-territorial information. In this study, 311 pear accessions collected from the five provinces of the Lazio region were genetically characterized by the means of nine simple sequence repeat (SSR) markers, eight of which were recommended by the European Cooperative Programme for Plant Genetic Resources (ECPGR). The SSR analysis revealed 250 unique genotypes and 14 cases of synonymies, namely, accessions with different names but identical molecular profiles (100% genetic similarity). The microsatellite set showed a high polymorphism information content (PIC; mean PIC=0.77) and an exceptionally high discrimination power (DP = 0.99), making it particularly efficient both for the study of genetic diversity and for the prediction of the degree of ploidy. Notably, 20% of the accessions displayed triallelic profiles (i.e., accessions having ≥2 loci with a third allele), while the remaining accessions were diploids. These results were further confirmed by flow cytometry data analysis. Standardization of the molecular analyses at the national and international levels and harmonization of the SSR sets used for germplasm characterization are of paramount importance for producing molecular profiles useful for registration and variety maintenance.

Genetic Diversity of Juglans mandshurica Populations in Northeast China Based on SSR Markers

Juglans mandshurica is a native tree species in Northeast China. Due to habitat destruction and human disturbance, its population size has sharply decreased. Currently, information on molecular markers of J. mandshurica is limited and cannot meet the needs of germplasm resource evaluation and molecular marker-assisted breeding of J. mandshurica. Based on transcriptomic data from three tissues (leaves, bark, and fruit pericarp), we developed expressed sequence tag-simple sequence repeats (EST-SSRs) for J. mandshurica, and 15 polymorphic EST-SSR primers were initially selected. The average number of alleles (Na), expected heterozygosity (He), and the polymorphic information content (PIC) at different loci were 18.27, 0.670, and 0.797, respectively. Population genetic diversity analysis revealed that the average Na, He, and Shannon information indices (I) for 15 J. mandshurica populations were 6.993, 0.670, and 1.455, respectively. Among them, population Hunchun exhibited the highest genetic diversity (Na = 7.933, He = 0.723, and I = 1.617), while population Heihe exhibited the lowest genetic diversity (Na = 4.200, He = 0.605, and I = 1.158). STRUCTURE analysis, neighbor-joining method cluster analysis, and principal coordinate analysis showed that the 343 individuals of J. mandshurica from 15 populations were clustered into three categories. Category 1 (green) had 147 individuals from eight populations in Qingyuan, Caohekou, Jian, Ningan, Yongji, Baishishan, Helong, and Maoershan; category 2 (blue) had 81 individuals from three populations in Hulin, Boli, and Sanchazi; and category 3 (red) had 115 individuals from four populations in Heihe, Hunchun, Fangzheng, and Liangshui. Analysis of molecular variance (AMOVA) showed that genetic variations among and within individuals accounted for 16.22% and 21.10% of the total genetic variation, respectively, indicating that genetic variations within populations were greater than genetic variations among populations. The average genetic differentiation coefficient (Fst) and gene flow (Nm) between different populations were 0.109 and 4.063, respectively, implying moderate levels of genetic differentiation and gene flow. Based on the genetic diversity characteristics of different populations, we proposed various genetic conservation strategies for J. mandshurica.

Population structure and genetic diversity of watermelon (Citrullus lanatus) based on SNP of chloroplast genome

Citrullus amarus (citronmelon) is an important crop with resistance to many diseases. The chloroplast genome is important in studying the genetic evolution of plants. The C. amarus chloroplast genome was first reported in this study using a novel assembly method based on whole genome sequencing. We identified 82 SNP sites in chloroplast genome with 313 watermelon materials. The 82 SNPs could effectively divide the natural watermelon population into four groups: C. lanatus subsp. lanatus, C. lanatus subsp. mucosospermus, C. lanatus subsp. vulgaris (ecologically from the Americas) and C. lanatus subsp. vulgaris (ecologically from Asia), with decreasing genetic diversity (π) (6.6 × 10^-5, 2.4 × 10^-5, 9.8 × 10^-6 and 5.41 × 10^-6, respectively). The single fruit weight, soluble solids, fruit color and 1000-seed weight of C. lanatus subsp. lanatus were significantly different from those of the other three groups. These results indicate that the complete chloroplast genome can be used in studying population genetics of watermelon, which is helpful for classification among intra species subgroups and identification of core germplasm resources.

Genetic diversity and population structure of black cottonwood (Populus deltoides) revealed using simple sequence repeat markers

Background

Black cottonwood (Populus deltoides) is one of the keystone forest tree species, and has become the main breeding parents in poplar hybrid breeding. However, the genetic diversity and population structure of the introduced resources are not fully understood.

Results

In the present study, five loci containing null alleles were excluded and 15 pairs of SSR (simple sequence repeat) primers were used to analyze the genetic diversity and population structure of 384 individuals from six provenances (Missouri, Iowa, Washington, Louisiana, and Tennessee (USA), and Quebec in Canada) of P. deltoides. Ultimately, 108 alleles (N_a) were detected; the expected heterozygosity (H_e) per locus ranged from 0.070 to 0.905, and the average polymorphic information content (PIC) was 0.535. The provenance ‘Was’ had a relatively low genetic diversity, while ‘Que’, ‘Lou’, and ‘Ten’ provenances had high genetic diversity, with Shannon’s information index (I) above 1.0. The mean coefficient of genetic differentiation (F_st) and gene flow (N_m) were 0.129 and 1.931, respectively. Analysis of molecular variance (AMOVA) showed that 84.88% of the genetic variation originated from individuals. Based on principal coordinate analysis (PCoA) and STRUCTURE cluster analysis, individuals distributed in the Mississippi River Basin were roughly classified as one group, while those distributed in the St. Lawrence River Basin and Columbia River Basin were classified as another group. The cluster analysis based on the population level showed that provenance ‘Iow’ had a small gene flow and high degree of genetic differentiation compared with the other provenances, and was classified into one group. There was a significant relationship between genetic distance and geographical distance.

Conclusions

P. deltoides resources have high genetic diversity and there is a moderate level of genetic differentiation among provenances. Geographical isolation and natural conditions may be the main factors causing genetic differences among individuals. Individuals reflecting population genetic information can be selected to build a core germplasm bank. Meanwhile, the results could provide theoretical support for the scientific management and efficient utilization of P. deltoides genetic resources, and promote the development of molecular marker-assisted breeding of poplar.

Biochemical characterization and expression analysis of lignification in two pear (Pyrus ussuriensis Maxim.) varieties with contrasting stone cell content

As lignified stone cells reduce fruit quality, we investigated lignin deposition, phenolic metabolites, and expression of lignin biosynthetic genes during fruit development to elucidate the molecular mechanism of stone cell lignification using histological, biochemical, and transcriptional data from two Ussurian pear varieties (Jianba and Nanguo) with contrasting stone cell content. Lignin content and distribution coincided with stone cell accumulation. As per LC-MS analysis, Jianba exhibited higher levels of lignin monomers and hydroxycinnamates than Nanguo, consistently with lignin amount in each case. However, flavonoid content was much higher in Nanguo. Transcriptional data showed that most monolignol biosynthesis-related genes were particularly upregulated in Jianba during lignin accumulation; especially CCR and LAC, two monolignol biosynthesis-specific genes, were substantially upregulated in Jianba fruits at critical stages. Therefore, differences in stone cell content between "Jianba" and "Nanguo" may result from differential expression of lignin synthase genes located downstream of the lignin biosynthesis pathway. Taken together, our data may provide a deeper understanding of the molecular mechanism for stone cell lignification in pear fruit.

Elucidating the contribution of wild related species on autochthonous pear germplasm: A case study from Mount Etna

The pear (genus Pyrus) is one of the most ancient and widely cultivated tree fruit crops in temperate climates. The Mount Etna area claims a large number of pear varieties differentiated due to a long history of cultivation and environmental variability, making this area particularly suitable for genetic studies. Ninety-five pear individuals were genotyped using the simple sequence repeat (SSR) methodology interrogating both the nuclear (nDNA) and chloroplast DNA (cpDNA) to combine an investigation of maternal inheritance of chloroplast SSRs (cpSSRs) with the high informativity of nuclear SSRs (nSSRs). The germplasm was selected ad hoc to include wild genotypes, local varieties, and national and international cultivated varieties. The objectives of this study were as follows: (i) estimate the level of differentiation within local varieties; (ii) elucidate the phylogenetic relationships between the cultivated genotypes and wild accessions; and (iii) estimate the potential genetic flow and the relationship among the germplasms in our analysis. Eight nSSRs detected a total of 136 alleles with an average minor allelic frequency and observed heterozygosity of 0.29 and 0.65, respectively, whereas cpSSRs allowed identification of eight haplotypes (S4 Table). These results shed light on the genetic relatedness between Italian varieties and wild genotypes. Among the wild species, compared with P. amygdaliformis, few P. pyraster genotypes exhibited higher genetic similarity to local pear varieties. Our analysis revealed the presence of genetic stratification with a 'wild' subpopulation characterizing the genetic makeup of wild species and the international cultivated varieties exhibiting the predominance of the 'cultivated' subpopulation.

Inferring Genetic Variation and Demographic History of Michelia yunnanensis Franch. (Magnoliaceae) from Chloroplast DNA Sequences and Microsatellite Markers

Michelia yunnanensis Franch., is a traditional ornamental, aromatic, and medicinal shrub that endemic to Yunnan Province in southwest China. Although the species has a large distribution pattern and is abundant in Yunnan Province, the populations are dramatically declining because of overexploitation and habitat destruction. Studies on the genetic variation and demography of endemic species are necessary to develop effective conservation and management strategies. To generate such knowledge, we used 3 pairs of universal cpDNA markers and 10 pairs of microsatellite markers to assess the genetic diversity, genetic structure, and demographic history of 7 M. yunnanensis populations. We calculated a total of 88 alleles for 10 polymorphic loci and 10 haplotypes for a combined 2,089 bp of cpDNA. M. yunnanensis populations showed high genetic diversity (Ho = 0.551 for nuclear markers and Hd = 0.471 for cpDNA markers) and low genetic differentiation (F_ST = 0.058). Geographical structure was not found among M. yunnanensis populations. Genetic distance and geographic distance were not correlated (P > 0.05), which indicated that geographic isolation is not the primary cause of the low genetic differentiation of M. yunnanensis. Additionally, M. yunnanensis populations contracted ~20,000-30,000 years ago, and no recent expansion occurred in current populations. Results indicated that the high genetic diversity of the species and within its populations holds promise for effective genetic resource management and sustainable utilization. Thus, we suggest that the conservation and management of M. yunnanensis should address exotic overexploitation and habitat destruction.

Characterization and Phylogenetic Analysis of Ancient Italian Landraces of Pear

Pear is one of the oldest fruit tree crops and the third most important temperate fruit species. Its domestication took place independently in the Far East (China) and in the Caucasus region. While the origin of Eastern Asian cultivars is clear, that of European cultivars is still in doubt. Italy has a wealth of local varieties and genetic resources safeguarded by several public and private collections to face the erosion caused by the introduction of improved varieties in specialized orchards. The objectives of the present study were: (i) to characterize the existing germplasm through nuclear (SSR) and (ii) to clarify the genetic divergence between local and cultivated populations through chloroplast DNA (cpDNA) markers in order to provide insights into phylogenetic relationships of Pyrus spp. For this reason, 95 entries from five different germplasm collections, including nine European, Mediterranean and Eastern Asian species, were analyzed, and the intergenic accD-psaI sequences were compared to the worldwide distributed dataset encompassing a total of 298 sequences from 26 different Pyrus species. The nine nuclear SSRs were able to identify a total of 179 alleles, with a loci polymorphism P = 0.89. Most of the variation (97%) was found within groups. Five accessions from different sources were confirmed to be the same. Eight out of 20 accessions of unknown origin were identified, and six synonyms were detected. Locus NH030a was found to be monomorphic in all the cultivated accessions and in reference species interfertile with P. communis, leading to hypothesize selection pressures for adaptation to cultivation. The cpDNA sequences of the 95 accessions were represented by 14 haplotypes, six of which (derived from P. communis, P. cossonii and P. ussuriensis) are recorded here for the first time and may suggest the ancient origin of some local varieties. The network analysis of the 298 cpDNA sequences allowed two different haplogroups, Eastern and Western Eurasia, to be defined, supporting recent views of a clear division between Occidental and Oriental species. By combining the results from nuclear and uniparental markers, it was possible to better define many unknown accessions.

Genetic structure and diversity of the wild Ussurian pear in East Asia

The Ussurian pear is the most important cultivated pear in the northern part of China. Cultivated Ussurian pears are considered to have derived from Pyrus ussuriensis Maxim. which is native to the northeast of China. In Japan, two varieties of P. ussuriensis, P. ussuriensis var. aromatica and var. hondoensis are native to the northern area and the central area of the main island respectively. In order to reveal the origin of Pyrus ussuriensis var. aromatica distributed in the northern area of main island of Japan, more than 40 explorations have been performed in Japan and in China, and more than 30 natural habitats were recognized. These natural habitats are at risk of extinction because of human development and forest degradation caused by climate change. Population structure and genetic diversity of P. ussuriensis in China and P. ussuriensis var. aromatica in Japan have been investigated using both morphological and molecular markers in order to define appropriate conservation units, and to provide a good focus for conservation management. Distant evolutionary relationships between P. ussuriensis Maxim. in China and P. ussuriensis var. aromatica in Japan inferred from population genetic structure and phylogenetic analysis are also discussed.