The population genomic analyses of chloroplast genomes shed new insights on the complicated ploidy and evolutionary history in Fragaria

Comparative chloroplast genomics provides insights into the phylogenetic relationships and evolutionary history for Actinidia species

Actinidia species are fruit trees with various functions, such as providing edible fruit, serving as ornamental plants, and having medicinal benefits. However, the taxonomy of Actinidia species is controversial due to widespread hybridization, the history of divergence and polyploid speciation among Actinidia species also remains unclear. In this study, we conducted comparative analyses of the chloroplast genomes and ploidy among multiple Actinidia species. The genes clpP, infA, ndhD, ndhK, and rpl20 were absent from these chloroplast genomes. The ycf2 and rpl20 genes in the Actinidia species were under positive selection. Several regions (rps16-trnQ-UUG, trnS-GCU-trnR-UCU, ndhC-trnV-UAC, rbcL-accD, rps12-psbB, trnN-GUU-ndhF, ycf1-trnN-GUU, and trnH-GUG-psbA) and genes (ycf1, ycf2, accD, rpl20) exhibited high variability, which could potentially serve as molecular markers in species delineation and other phylogenetic studies. Through divergence time estimation, the Actinidia genus originated 23 million years ago (Ma), and experienced a tetraploidization event in ~ 20 Ma. Subsequently, Actinidia has undergone extensive diploidization. Our findings will provide valuable information in species identification, breeding programs, and conservation efforts for Actinidia species.

The Complete Chloroplast Genome of Erythrina variegata L. (Papilionoideae, Fabaceae)

Erythrina variegata L. 1754, a thorny deciduous tree of Fabaceae, contains various chemical compounds such as alkaloids, flavonoids, and triterpenoids and exhibits anti-depressant, anti-inflammatory, and antidiabetic activities. However, genomic data of E. variegata are limited. In this study, the complete chloroplast genome of E. variegata was sequenced and characterized using Illumina sequencing platform. The chloroplast genome of E. variegata was 152,351 bp in length and consisted of a large single copy (82,907 bp), a small single copy (26,309 bp), and two inverted repeat regions (16,826 bp). There were 79 protein-coding genes, 30 transfer RNA genes, and four ribosomal RNA genes. Comparative analysis revealed high conservation of chloroplast genomes among Erythrina species regarding genome size, structure, and gene content. The phylogenetic study also indicated a close relationship between E. variagata and E. sanwicensis. This study provides initial plastome data for further genomic studies examining E. variegata and related species in Fabaceae.

The complete chloroplast genome sequence of Melothria scabra (Cucurbitaceae)

Melothria scabra has gradually become an economically important plant worldwide. The complete chloroplast genome of M. scabra has a length of 156,744 bp, contains a large single-copy (LSC) region (86,387 bp), a small single-copy (SSC) region (18,055 bp), and two inverted repeats (IRs) with the same length of 26,151 bp. In total, 126 genes were detected, including 83 protein-encoding genes, 35 transfer RNA (tRNA) genes, and eight ribosomal RNA (rRNA) genes. For phylogenetic analysis, M. scabra has a closer genetic relationship with Cucumis sativus and Citrullus lanatus. The complete chloroplast genome sequence of M. scabra would promote the germplasm exploration, phylogenetic relationships, and molecular biology researches in Melothria.

Characterization of the organellar genomes of Mazzaella laminarioides and Mazzaella membranacea (Gigartinaceae, Rhodophyta)

Mazzaella, a genus with no genomic resources available, has extensive distribution in the cold waters of the Pacific, where they represent ecologically and economically important species. In this study, we aimed to sequence, assemble, and annotate the complete mitochondrial and chloroplast genomes from two Mazzaella spp. and characterize the intraspecific variation among them. We report for the first time seven whole organellar genomes (mitochondria: OR915856, OR947465, OR947466, OR947467, OR947468, OR947469, OR947470; chloroplast: OR881974, OR909680, OR909681, OR909682, OR909683, OR909684, OR909685) obtained through high-throughput sequencing for six M. laminarioides sampled from three Chilean regions and one M. membranacea. Sequenced Mazzaella mitogenomes have identical gene number, gene order, and genome structure. The same results were observed for assembled plastomes. A total of 52 genes were identified in mitogenomes, and a total of 235 genes were identified in plastomes. Although the M. membranacea plastome included a full-length pbsA gene, in all M. laminarioides samples, the pbsA gene was split in three open reading frames (ORFs). Within M. laminarioides, we observed important plastome lineage-specific variations, such as the pseudogenization of the two hypothetical protein-coding genes, ycf23 and ycf45. Nonsense mutations in the ycf23 and ycf45 genes were only detected in the northern lineage. These results are consistent with phylogenetic reconstructions and divergence time estimation using concatenated coding sequences that not only support the monophyly of M. laminarioides but also underscore that the three M. laminarioides lineages are in an advanced stage of divergence. These new results open the question of the existence of still undisclosed species in M. laminarioides.

Advances in genomics and genome editing for improving strawberry (Fragaria ×ananassa)

The cultivated strawberry, Fragaria ×ananassa, is a recently domesticated fruit species of economic interest worldwide. As such, there is significant interest in continuous varietal improvement. Genomics-assisted improvement, including the use of DNA markers and genomic selection have facilitated significant improvements of numerous key traits during strawberry breeding. CRISPR/Cas-mediated genome editing allows targeted mutations and precision nucleotide substitutions in the target genome, revolutionizing functional genomics and crop improvement. Genome editing is beginning to gain traction in the more challenging polyploid crops, including allo-octoploid strawberry. The release of high-quality reference genomes and comprehensive subgenome-specific genotyping and gene expression profiling data in octoploid strawberry will lead to a surge in trait discovery and modification by using CRISPR/Cas. Genome editing has already been successfully applied for modification of several strawberry genes, including anthocyanin content, fruit firmness and tolerance to post-harvest disease. However, reports on many other important breeding characteristics associated with fruit quality and production are still lacking, indicating a need for streamlined genome editing approaches and tools in Fragaria ×ananassa. In this review, we present an overview of the latest advancements in knowledge and breeding efforts involving CRISPR/Cas genome editing for the enhancement of strawberry varieties. Furthermore, we explore potential applications of this technology for improving other Rosaceous plant species.

Comparative Analysis of Chloroplast Pan-Genomes and Transcriptomics Reveals Cold Adaptation in Medicago sativa

Alfalfa (Medicago sativa) is a perennial forage legume that is widely distributed all over the world; therefore, it has an extremely complex genetic background. Though population structure and phylogenetic studies have been conducted on a large group of alfalfa nuclear genomes, information about the chloroplast genomes is still lacking. Chloroplast genomes are generally considered to be conservative and play an important role in population diversity analysis and species adaptation in plants. Here, 231 complete alfalfa chloroplast genomes were successfully assembled from 359 alfalfa resequencing data, on the basis of which the alfalfa chloroplast pan-genome was constructed. We investigated the genetic variations of the alfalfa chloroplast genome through comparative genomic, genetic diversity, phylogenetic, population genetic structure, and haplotype analysis. Meanwhile, the expression of alfalfa chloroplast genes under cold stress was explored through transcriptome analysis. As a result, chloroplast genomes of 231 alfalfa lack an IR region, and the size of the chloroplast genome ranges from 125,192 bp to 126,105 bp. Using population structure, haplotypes, and construction of a phylogenetic tree, it was found that alfalfa populations could be divided into four groups, and multiple highly variable regions were found in the alfalfa chloroplast genome. Transcriptome analysis showed that tRNA genes were significantly up-regulated in the cold-sensitive varieties, while rps7, rpl32, and ndhB were down-regulated, and the editing efficiency of ycf1, ycf2, and ndhF was decreased in the cold-tolerant varieties, which may be due to the fact that chloroplasts store nutrients through photosynthesis to resist cold. The huge number of genetic variants in this study provide powerful resources for molecular markers.

Phased gap-free genome assembly of octoploid cultivated strawberry illustrates the genetic and epigenetic divergence among subgenomes

The genetic and epigenetic mechanisms underlying the coexistence and coordination of the four diverged subgenomes (ABCD) in octoploid strawberries (Fragaria × ananassa) remains poorly understood. In this study, we have assembled a haplotype-phased gap-free octoploid genome for the strawberry, which allowed us to uncover the sequence, structure, and epigenetic divergences among the subgenomes. The diploid progenitors of the octoploid strawberry, apart from subgenome A (Fragaria vesca), have been a subject of public controversy. Phylogenomic analyses revealed a close relationship between diploid species Fragaria iinumae and subgenomes B, C, and D. Subgenome A, closely related to F. vesca, retains the highest number of genes, exhibits the lowest content of transposable elements (TEs), experiences the strongest purifying selection, shows the lowest DNA methylation levels, and displays the highest expression level compared to the other three subgenomes. Transcriptome and DNA methylome analyses revealed that subgenome A-biased genes were enriched in fruit development biological processes. In contrast, although subgenomes B, C, and D contain equivalent amounts of repetitive sequences, they exhibit diverged methylation levels, particularly for TEs located near genes. Taken together, our findings provide valuable insights into the evolutionary patterns of subgenome structure, divergence and epigenetic dynamics in octoploid strawberries, which could be utilized in strawberry genetics and breeding research.