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

Climatic factors, but not geographic distance, promote genetic structure and differentiation of Cleistogenes squarrosa (Trin.) Keng populations

Climate can shape plant genetic diversity and genetic structure, and genetic diversity and genetic structure can reflect the adaptation of plants to climate change. We used rbcl and trnL-trnF sequences to analyze the genetic diversity and genetic structure of C. squarrosa under the influence of different environmental factors in Inner Mongolia grassland. The results showed that the genetic diversity of this species was low. (The trnL-trnF sequences have higher genetic diversity than rbcl sequences.) C. squarrosa had low genetic diversity compared to other prairie plants, but had a more pronounced genetic structure. The haplotype network diagram of the combined sequences could be divided into two categories, and the results of the NJ, MP, and ML trees also showed that the haplotypes were divided into two branches. The results of genetic structure analysis showed that that the populations located in the desert steppe fall into exactly one cluster, and the populations located in the typical steppe fall into exactly another cluster. The neutrality tests were all negative and the mismatch distribution also showed a single peak across the population, suggesting that C. squarrosa had undergone population expansion and was well adapted to the local environment. The results of the mantel test showed that climate had a greater influence on the genetic distance of C. squarrosa, with annual precipitation having a higher influence than mean annual temperature. This study provided basic genetic information on the genetic structure of C. squarrosa and contributes to the study of genetic adaptation mechanisms in grassland plants.

A comparative analysis of chloroplast genomes revealed the chloroplast heteroplasmy of Artemisia annua

Artemisia annua L. is the main source of artemisinin, an antimalarial drug. High diversity of morphological characteristics and artemisinin contents of A. annua has affected the stable production of artemisinin while efficient discrimination method of A. annua strains is not available. The complete chloroplast (cp) genomes of 38 A. annua strains were assembled and analyzed in this study. Phylogenetic analysis of Artemisia species showed that distinct intraspecific divergence occurred in A. annua strains. A total of 38 A. annua strains were divided into two distinct lineages, one lineage containing widely-distributed strains and the other lineage only containing strains from northern China. The A. annua cp genomes ranged from 150, 953 to 150, 974 bp and contained 131 genes, and no presence or absence variation of genes was observed. The IRs and SC junctions were located in rps19 and ycf1, respectively, without IR contraction observed. Rich sequence polymorphisms were observed among A. annua strains, and a total of 60 polymorphic sites representing 14 haplotypes were identified which unfolding the cpDNA heteroplasmy of A. annua. In conclusion, this study provided valuable resource for A. annua strains identification and provided new insights into the evolutionary characteristics of A. annua.

Pueraria montana Population Structure and Genetic Diversity Based on Chloroplast Genome Data

Despite having a generally conserved structure, chloroplast genome data have been helpful for plant population genetics and evolution research. To mine Pueraria montana chloroplast genome variation architecture and phylogeny, we investigated the chloroplast variation architecture of 104 P. montana accessions from across China. P. montana's chloroplast genome showed high diversity levels, with 1674 variations, including 1118 single nucleotide polymorphisms and 556 indels. The intergenic spacers, psbZ-trnS and ccsA-ndhD, are the two mutation hotspot regions in the P. montana chloroplast genome. Phylogenetic analysis based on the chloroplast genome dataset supported four P. montana clades. P. montana variations were conserved among and within clades, which showed high gene flow levels. Most P. montana clades were estimated to have diverged at 3.82-5.17 million years ago. Moreover, the East Asian summer monsoon and South Asian summer monsoon may have accelerated population divergence. Our results show that chloroplast genome sequences were highly variable and can be used as molecular markers to assess genetic variation and relationships in P. montana.

Comparative chloroplast genome analysis of seven extant Citrullus species insight into genetic variation, phylogenetic relationships, and selective pressure

Citrullus ecirrhosus, Citrullus rehmii, and Citrullus naudinianus are three important related wild species of watermelon in the genus Citrullus, and their morphological differences are clear, however, their chloroplast genome differences remain unknown. This study is the first to assemble, analyze, and publish the complete chloroplast genomes of C. ecirrhosus, C. rehmii, and C. naudinianus. A comparative analysis was then conducted among the complete chloroplast genomes of seven extant Citrullus species, and the results demonstrated that the average genome sizes of Citrullus is 157,005 bp, a total of 130-133 annotated genes were identified, including 8 rRNA, 37 tRNA and 85-88 protein-encoding genes. Their gene content, order, and genome structure were similar. However, noncoding regions were more divergent than coding regions, and rps16-trnQ was a hypervariable fragment. Thirty-four polymorphic SSRs, 1,271 SNPs and 234 INDELs were identified. Phylogenetic trees revealed a clear phylogenetic relationship of Citrullus species, and the developed molecular markers (SNPs and rps16-trnQ) could be used for taxonomy in Citrullus. Three genes (atpB, clpP1, and rpoC2) were identified to undergo selection and would promote the environmental adaptation of Citrullus.

Maternal Donor and Genetic Variation of Lagerstroemia indica Cultivars

Lagerstroemia indica L. is a well-known ornamental plant with large pyramidal racemes, long flower duration, and diverse colors and cultivars. It has been cultivated for nearly 1600 years and is essential for investigating the germplasm and assessing genetic variation to support international cultivar identification and breeding programs. In this study, 20 common Lagerstroemia indica cultivars from different varietal groups and flower morphologies, as well as multiple wild relative species, were analyzed to investigate the maternal donor of Lagerstroemia indica cultivars and to discover the genetic variation and relationships among cultivars based on plastome and nuclear ribosomal DNA (nrDNA) sequences. A total of 47 single nucleotide polymorphisms (SNPs) and 24 insertion/deletions (indels) were identified in the 20 L. indica cultivars' plastome and 25 SNPs were identified in the nrDNA. Phylogenetic analysis based on the plastome sequences showed that all the cultivars formed a clade with the species of L. indica, indicating that L. indica was the maternal donor of the cultivars. Population structure and PCA analyses supported two clades of cultivars, which exhibited significant genetic differences according to the plastome dataset. The results of the nrDNA supported that all 20 cultivars were divided into three clades and most of the cultivars had at least two genetic backgrounds and higher gene flow. Our results suggest that the plastome and nrDNA sequences can be used as molecular markers for assessing the genetic variation and relationships of L. indica cultivars.

Variations in genetic diversity in cultivated Pistacia chinensis

Identification of the evolution history and genetic diversity of a species is important in the utilization of novel genetic variation in this species, as well as for its conservation. Pistacia chinensis is an important biodiesel tree crop in China, due to the high oil content of its fruit. The aim of this study was to uncover the genetic structure of P. chinensis and to investigate the influence of intraspecific gene flow on the process of domestication and the diversification of varieties. We investigated the genetic structure of P. chinensis, as well as evolution and introgression in the subpopulations, through analysis of the plastid and nuclear genomes of 39 P. chinensis individuals from across China. High levels of variation were detected in the P. chinensis plastome, and 460 intraspecific polymorphic sites, 104 indels and three small inversions were identified. Phylogenetic analysis and population structure using the plastome dataset supported five clades of P. chinensis. Population structure analysis based on the nuclear SNPs showed two groups, clearly clustered together, and more than a third of the total individuals were classified as hybrids. Discordance between the plastid and nuclear genomes suggested that hybridization events may have occurred between highly divergent samples in the P. chinensis subclades. Most of the species in the P. chinensis subclade diverged between the late Miocene and the mid-Pliocene. The processes of domestication and cultivation have decreased the genetic diversity of P. chinensis. The extensive variability and structuring of the P. chinensis plastid together with the nuclear genomic variation detected in this study suggests that much unexploited genetic diversity is available for improvement in this recently domesticated species.

Comparative analysis of chloroplast genomes reveals phylogenetic relationships and intraspecific variation in the medicinal plant Isodon rubescens

Isodon rubescens (Hemsley) H. Hara (Lamiaceae) is a traditional Chinese medicine plant that has been used to treat various human diseases and conditions such as inflammation, respiratory and gastrointestinal bacterial infections, and malignant tumors. However, the contents of the main active components of I. rubescens from different origins differ significantly, which greatly affected its quality. Therefore, a molecular method to identify and classify I. rubescens is needed. Here, we report the DNA sequence of the chloroplast genome of I. rubescens collected from Lushan, Henan province. The genome is 152,642 bp in length and has a conserved structure that includes a pair of IR regions (25,726 bp), a LSC region (83,527 bp) and a SSC region (17,663 bp). The chloroplast genome contains 113 unique genes, four rRNA genes, 30 tRNA genes, and 79 protein-coding genes, 23 of which contain introns. The protein-coding genes account for a total of 24,412 codons, and most of them are A/T biased usage. We identified 32 simple sequence repeats (SSRs) and 48 long repeats. Furthermore, we developed valuable chloroplast molecular resources by comparing chloroplast genomes from three Isodon species, and both mVISTA and DnaSP analyses showed that rps16-trnQ, trnS-trnG, and ndhC-trnM are candidate regions that will allow the identification of intraspecific differences within I. rubescens. Also 14 candidate fragments can be used to identify interspecific differences between species in Isodon. A phylogenetic analysis of the complete chloroplast genomes of 24 species in subfamily Nepetoideae was performed using the maximum likelihood method, and shows that I. rubescens clustered closer to I. serra than I. lophanthoides. Interestingly, our analysis showed that I. rubescens (MW018469.1) from Xianyang, Shaanxi Province (IR-X), is closer to I. serra than to the other two I. rubescens accessions. These results strongly indicate that intraspecific diversity is present in I. rubescens. Therefore, our results provide further insight into the phylogenetic relationships and interspecific diversity of species in the genus Isodon.