Structural variations in papaya genomes

Gender specific SNP markers in Coscinium fenestratum (Gaertn.) Colebr. for resource augmentation

BACKGROUND

Unregulated extraction of highly traded medicinal plant species results in drastic decline of the natural resources and alters viable sex ratio of populations. Conservation and long-term survival of such species, require gender specific restoration programs to ensure reproductive success. However, it is often difficult to differentiate sex of individuals before reaching reproductive maturity. C. fenestratum is one of the medicinally important and overexploited dioecious woody liana, with a reproductive maturity of 15 years. Currently, no information is available to identify sex of C. fenestratum in seedling stage while augmenting the resources. Thus, the current study envisages to utilize transcriptomics approach for gender differentiation which is imperative for undertaking viable resource augmentation programmes.

METHODS AND RESULTS

Gender specific SNPs with probable role in sexual reproduction/sex determination was located using comparative transcriptomics approach (sampling male and female individuals), alongside gene ontology and annotation. Nine sets of primers were synthesized from 7 transcripts (involved in sexual reproduction/other biological process) containing multiple SNP variants. Out of the nine primer pairs, only one SNP locus with no available information of its role in reproduction, showed consistent and accurate results (males-heterozygous and females-homozygous), in the analyzed 40 matured individuals of known sexes. Thus validated the efficiency of this SNP marker in differentiating male and female individuals.

CONCLUSIONS

The study could identify SNPs linked to the loci with apparent role in gender differentiation. This SNP marker can be used for early sexing of seedlings for in-situ conservation and resource augmentation of C. fenestratum in Kerala, India.

k-mer-Based Genome-Wide Association Studies in Plants: Advances, Challenges, and Perspectives

Genome-wide association studies (GWAS) have allowed the discovery of marker-trait associations in crops over recent decades. However, their power is hampered by a number of limitations, with the key one among them being an overreliance on single-nucleotide polymorphisms (SNPs) as molecular markers. Indeed, SNPs represent only one type of genetic variation and are usually derived from alignment to a single genome assembly that may be poorly representative of the population under study. To overcome this, k-mer-based GWAS approaches have recently been developed. k-mer-based GWAS provide a universal way to assess variation due to SNPs, insertions/deletions, and structural variations without having to specifically detect and genotype these variants. In addition, k-mer-based analyses can be used in species that lack a reference genome. However, the use of k-mers for GWAS presents challenges such as data size and complexity, lack of standard tools, and potential detection of false associations. Nevertheless, efforts are being made to overcome these challenges and a general analysis workflow has started to emerge. We identify the priorities for k-mer-based GWAS in years to come, notably in the development of user-friendly programs for their analysis and approaches for linking significant k-mers to sequence variation.

Evolution of sex in crops: recurrent scrap and rebuild

Sexuality is the main strategy for maintaining genetic diversity within a species. In flowering plants (angiosperms), sexuality is derived from ancestral hermaphroditism and multiple sexualities can be expressed in an individual. The mechanisms conferring chromosomal sex determination in plants (or dioecy) have been studied for over a century by both biologists and agricultural scientists, given the importance of this field for crop cultivation and breeding. Despite extensive research, the sex determining gene(s) in plants had not been identified until recently. In this review, we dissect plant sex evolution and determining systems, with a focus on crop species. We introduced classic studies with theoretical, genetic, and cytogenic approaches, as well as more recent research using advanced molecular and genomic techniques. Plants have undergone very frequent transitions into, and out of, dioecy. Although only a few sex determinants have been identified in plants, an integrative viewpoint on their evolutionary trends suggests that recurrent neofunctionalization events are potentially common, in a "scrap and (re)build" cycle. We also discuss the potential association between crop domestication and transitions in sexual systems. We focus on the contribution of duplication events, which are particularly frequent in plant taxa, as a trigger for the creation of new sexual systems.

Spectrum and Density of Gamma and X-ray Induced Mutations in a Non-Model Rice Cultivar

Physical mutagens are a powerful tool used for genetic research and breeding for over eight decades. Yet, when compared to chemical mutagens, data sets on the effect of different mutagens and dosages on the spectrum and density of induced mutations remain lacking. To address this, we investigated the landscape of mutations induced by gamma and X-ray radiation in the most widely cultivated crop species: rice. A mutant population of a tropical upland rice, Oryza sativa L., was generated and propagated via self-fertilization for seven generations. Five dosages ranging from 75 Gy to 600 Gy in both X-ray and gamma-irradiated material were applied. In the process of a forward genetic screens, 11 unique rice mutant lines showing phenotypic variation were selected for mutation analysis via whole-genome sequencing. Thousands of candidate mutations were recovered in each mutant with single base substitutions being the most common, followed by small indels and structural variants. Higher dosages resulted in a higher accumulation of mutations in gamma-irradiated material, but not in X-ray-treated plants. The in vivo role of all annotated rice genes is yet to be directly investigated. The ability to induce a high density of single nucleotide and structural variants through mutagenesis will likely remain an important approach for functional genomics and breeding.

Comprehensive analysis of structural variants in chickens using PacBio sequencing

Structural variants (SVs) are one of the main sources of genetic variants and have a greater impact on phenotype evolution, disease susceptibility, and environmental adaptations than single nucleotide polymorphisms (SNPs). However, SVs remain challenging to accurately type, with several detection methods showing different limitations. Here, we explored SVs from 10 different chickens using PacBio technology and detected 49,501 high-confidence SVs. The results showed that the PacBio long-read detected more SVs than Illumina short-read technology genomes owing to some SV sites on chromosomes, which are related to chicken growth and development. During chicken domestication, some SVs beneficial to the breed or without any effect on the genomic function of the breed were retained, whereas deleterious SVs were generally eliminated. This study could facilitate the analysis of the genetic characteristics of different chickens and provide a better understanding of their phenotypic characteristics at the SV level, based on the long-read sequencing method. This study enriches our knowledge of SVs in chickens and improves our understanding of chicken genomic diversity.

Whole-genome resequencing reveals genomic footprints of Italian sweet and hot pepper heirlooms giving insight into genes underlying key agronomic and qualitative traits

Background

Pepper is a major crop species of the Solanaceae family, largely appreciated for its high nutritional and healthy contribution to human diets. In the Mediterranean basin, the favorable pedoclimatic conditions enhanced the selection of several diversified landraces cultivated pepper (Capsicum annuum), for whom Italy can be considered a main pole of diversification. Hence, a survey of traditional C. annuum genetic resources is essential for deep understanding of such diversity and for applications in genomics assisted breeding. Here, we report whole-genome resequencing analyses of two sweet and two pungent genotypes highly diffused in South Italy and representative of the variability for shape, colour and nutritional properties.

Results

The four genomes were reconstructed at a chromosomal scale using a reference-guided approach, based on a dataset of 2.6 billion paired-end reads, corresponding to 20× genome coverage and a mapping rate above 99% for a final genomes size of approximately 3 Gb. After five iterations of variant calling, a total of 29,258,818 single nucleotide polymorphisms (SNPs) and 1,879,112 InDels, were identified. Substantial differences were observed among the four genomes based on geographical origin, with chromosomes 9 and 11 showing more polymorphisms in the accessions with higher fruit weight and absence of pungency. Among the identified variants, a small private indel (T - > TA) shared between sweet and big fruits accessions induces a frameshift with the generation of a new stop codon in a gene annotated as extensin, whereas two private SNPs within hot types were identified in 1-aminocyclopropane-1-carboxylate oxidase (ACO), a key gene involved in fruit ripening. The estimation of repetitive elements highlights a preponderant presence of Long Terminal Repeats (LTRs), the majority of which belonged to Gypsy superfamily. By comparing the four genomes with publicly available references including ‘CM334’ and Zunla-1 highlight the presence of 49,475 shared gene families.

Conclusions

The new genomic sequences aim to enrich the whole genome information of pepper local varieties, providing a valuable tool for precision gene mapping, marker discovery, comparative studies. Such knowledge widens the frontiers to understand the selection history of Italian pepper landraces toward the recognition of specificity local agri-food products marks.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-022-01039-9.

Genomic characterization of a rare Carica papaya X chromosome mutant reveals a candidate monodehydroascorbate reductase 4 gene involved in all-hermaphrodite phenomenon

Sex form is one of the most important characteristics in papaya cultivation in which hermaphrodite is the preferable form. Self-pollination of H*-TSS No.7, an inbred line derived from a rare X chromosome mutant SR*, produced all-hermaphrodite progeny. The recessive lethal allele controlling the all-hermaphrodite phenomenon was proposed to be the recessive Germination suppressor (gs) locus. This study employed next-generation sequencing technology and genome comparison to identify the candidate Gs gene. One specific gene, monodehydroascorbate reductase 4 (MDAR4) harboring a unique polymorphic 3 bp deletion in H*-TSS No.7 was identified. The function of MDAR4 is known to be involved in the hydrogen peroxide (H₂O₂) scavenging pathway and is associated with seed germination. Furthermore, MDAR4 showed higher expression in the imbibed seeds than that in the dry seeds indicating its potential role in the seed germination. Perhaps this is the very first report providing the evidences that MDAR4 is the candidate of Gs locus in H*-TSS No.7. In addition, Gs allele-specific markers were developed which would be facilitated for breeding all-hermaphrodite lines.