Genetic Identity, Diversity, and Population Structure of CIP's Sweetpotato (I. batatas) Germplasm Collection

Genotyping Genebank Collections: Strategic Approaches and Considerations for Optimal Collection Management

The maintenance of plant germplasm and its genetic diversity is critical to preserving and making it available for food security, so this invaluable diversity is not permanently lost due to population growth and development, climate change, or changing needs from the growers and/or the marketplace. There are numerous genebanks worldwide that serve to preserve valuable plant germplasm for humankind's future and to serve as a resource for research, breeding, and training. The United States Department of Agriculture (USDA) National Plant Germplasm System (NPGS) and the Consultative Group for International Agricultural Research (CGIAR) both have a network of plant germplasm collections scattered across varying geographical locations preserving genetic resources for the future. Besides the USDA and CGIAR, there are germplasm collections established in many countries across the world that also aim to preserve crop and plant collections. Due to the advancement of technology, genotyping and sequencing whole genomes of plant germplasm collections is now feasible. Data from genotyping can help define genetic diversity within a collection, identify genetic gaps, reveal genetic redundancies and verify uniqueness, enable the comparison of collections of the same crop across genebanks (rationalization), and determine errors or mix-ups in genetic identity that may have occurred in a germplasm collection. Large-scale projects, such as genotyping germplasm collections, require strategic planning and the development of best practices. This article details strategies and best practices to consider when genotyping whole collections, considerations for the identity verification of germplasm and determining genetic replicates, quality management systems (QMS)/QC genotyping, and some use cases.

Genetic diversity and population structure studies of West African sweetpotato [Ipomoea batatas (L.) Lam] collection using DArTseq

BACKGROUND

Sweetpotato is a vegetatively propagated crop cultivated worldwide, predominantly in developing countries, valued for its adaptability, short growth cycle, and high productivity per unit land area. In most sub-Saharan African (SSA) countries, it is widely grown by smallholder farmers. Niger, Nigeria, and Benin have a huge diversity of sweetpotato accessions whose potential has not fully been explored to date. Diversity Arrays Technology (DArTseq), a Genotyping by Sequencing (GBS) method, has been developed and enables genotyping with high-density single nucleotide polymorphisms (SNPs) in different crop species. The aim of this study was to assess the genetic diversity and population structure of the West African sweetpotato collection using Diversity Arrays Technology through Genotyping by Sequencing (GBS).

RESULTS

29,523 Diversity Arrays Technology (DArTseq) single nucleotide polymorphism markers were used to genotype 271 sweetpotato accessions. Genetic diversity analysis revealed an average polymorphic information content (PIC) value of 0.39, a minor allele frequency of 0.26, and an observed heterozygosity of 10%. The highest value of polymorphic information content (PIC) (0.41) was observed in chromosomes 4, while the highest proportion of heterozygous (He) (0.18) was observed in chromosomes 11. Molecular diversity revealed high values of polymorphic sites (Ps), theta (θ), and nucleotide diversity (π) with 0.973, 0.158, and 0.086, respectively, which indicated high genetic variation. The pairs of genetic distances revealed a range from 0.08 to 0.47 with an overall average of 0.34. Population structure analysis divided the 271 accessions into four populations (population 1 was characterised by a mixture of accessions from all countries; population 2, mostly comprised of Nigerian breeding lines; population 3 contained exclusively landraces from Benin; and population 4 was composed by only landraces from West African countries) at K = 4, and analysis of molecular variance (AMOVA) based on PhiPT values showed that most of the variation was explained when accessions were categorized based on population structure at K = 4 (25.25%) and based on cluster analysis (19.43%). Genetic distance showed that group 4 (which constituted by landraces of Niger and Benin) was genetically distant (0.428) from groups 2 (formed by 75% of breeding lines of Nigeria), while group 1 was the closest (0.182) to group 2.

CONCLUSIONS

This study employed 7,591 DArTseq-based SNP markers, revealing extensive polymorphism and variation within and between populations. Variability among countries of origin (11.42%) exceeded that based on biological status (9.13%) and storage root flesh colour (7.90%), emphasizing the impact of migration on genetic diversity. Population structure analysis using principal component analysis (PCA), Neighbor-Joining (NJ) tree, and STRUCTURE at K = 4 grouped 271 accessions into distinct clusters, irrespective of their geographic origins, indicating widespread genetic exchange. Group 4, dominated by landraces (95%), showed significant genetic differentiation (Nei's Gst = 0.428) from Group 2, mainly comprising breeding lines, suggesting their potential as heterotic groups for breeding initiatives like HEBS or ABS.

Refining polyploid breeding in sweet potato through allele dosage enhancement

Allele dosage plays a key role in the phenotypic variation of polyploids. Here we present a genome-wide variation map of hexaploid sweet potato that captures allele dosage information, constructed from deep sequencing of 294 hexaploid accessions. Genome-wide association studies identified quantitative trait loci with dosage effects on 23 agronomic traits. Our analyses reveal that sweet potato breeding has progressively increased the dosage of favourable alleles to enhance trait performance. Notably, the Mesoamerican gene pool has evolved towards higher dosages of favourable alleles at multiple loci, which have been increasingly introgressed into modern Chinese cultivars. We substantiated the breeding-driven dosage accumulation through transgenic validation of IbEXPA4, an expansin gene influencing tuberous root weight. In addition, we explored causative sequence variations that alter the expression of the Orange gene, which regulates flesh colour. Our findings illuminate the breeding history of sweet potato and establish a foundation for leveraging allele dosages in polyploid breeding practices.

SNP loci identification and KASP marker development system for genetic diversity, population structure, and fingerprinting in sweetpotato (Ipomoea batatas L.)

Sweetpotato (Ipomoea batatas L.), an important food and industrial crop in the world, has a highly heterozygous hexaploid genome, making the development of single nucleotide polymorphism (SNP) markers challenging. Identifying SNP loci and developing practical SNP markers are crucial for genomic and genetic research on sweetpotato. A restriction site-associated DNA sequencing analysis of 60 sweetpotato accessions in this study yielded about 7.97 million SNPs. Notably, 954 candidate SNPs were obtained from 21,681 high-quality SNPs. Based on their stability and polymorphism, 274 kompetitive allele specific PCR (KASP) markers were then developed and uniformly distributed on chromosomes. The 274 KASP markers were used to genotype 93 sweetpotato accessions to evaluate their utility for assessing germplasm and analyzing genetic diversity and population structures. These markers had respective mean values of 0.24, 0.34, 0.31, and 0.25 for minor allele frequency, heterozygosity, gene diversity, and polymorphic information content (PIC). Their genetic pedigree led to the division of all accessions into three primary clusters, which were found to be both interrelated and independent. Finally, 74 KASP markers with PIC values greater than 0.35 were selected as core markers. These markers were used to construct the DNA fingerprints of 93 sweetpotato accessions and were able to differentiate between all accessions. To the best of our knowledge, this is the first attempt at the development and application of KASP markers in sweetpotato. However, due to sweetpotato's polyploidy, heterozygosity and the complex genome, the KASP marker conversion rate in this study was relatively low. To improve the KASP marker conversion rate, and accuracies in SNP discovery and marker validation, further studies including more accessions from underrepresented regions are needed in sweetpotato.

Genome-wide identification of sweet potato U-Box E3 ubiquitin ligases and roles of IbPUB52 in negative regulation of drought stress

The plant U-box (PUB) proteins, a family of ubiquitin ligases (E3) enzymes, are pivotal in orchestrating many biological processes and facilitating plant responses to environmental stressors. Despite their critical roles, exploring the PUB gene family's characteristics and functional diversity in sweet potato (Ipomoea batatas (L.) Lam.) has been notably limited. There were 81 IbPUB genes identified within the sweet potato genome, and they were categorized into eight distinct groups based on domain architecture, revealing a non-uniform distribution across the 15 chromosomes of I. batatas. The investigation of cis-acting elements has shed light on the potential of PUBs to participate in a wide array of biological processes, particularly emphasizing their role in mediating responses to abiotic stresses. Transcriptome profiles revealed that IbPUB genes displayed a wide range of expression levels among different tissues and were regulated by salt or drought stress. IbPUB52 has emerged as a gene of significant interest due to its induction by salt and drought stresses. Localization studies have confirmed the presence of IbPUB52 in both the nucleus and the cytoplasm, and its ubiquitination activity has been validated through rigorous in vitro and in vivo assays. Intriguingly, the heterogeneous expression of IbPUB52 in Arabidopsis resulted in decreased drought tolerance. The virus-induced gene silencing (VIGS) of IbPUB52 in sweet potatoes led to enhanced resistance to drought. This evidence suggests that IbPUB52 negatively regulates the drought tolerance of plants. The findings of this study are instrumental in advancing our comprehension of the functional dynamics of PUB E3 ubiquitin ligases in sweet potatoes.

Promiscuous potato: elucidating genetic identity and the complex genetic relationships of a cultivated potato germplasm collection

A total of 3,860 accessions from the global in trust clonal potato germplasm collection w3ere genotyped with the Illumina Infinium SolCAP V2 12K potato SNP array to evaluate genetic diversity and population structure within the potato germplasm collection. Diploid, triploid, tetraploid, and pentaploid accessions were included representing the cultivated potato taxa. Heterozygosity ranged from 9.7% to 66.6% increasing with ploidy level with an average heterozygosity of 33.5%. Identity, relatedness, and ancestry were evaluated using hierarchal clustering and model-based Bayesian admixture analyses. Errors in genetic identity were revealed in a side-by-side comparison of in vitro clonal material with the original mother plants revealing mistakes putatively occurring during decades of processing and handling. A phylogeny was constructed to evaluate inter- and intraspecific relationships which together with a STRUCTURE analysis supported both commonly used treatments of potato taxonomy. Accessions generally clustered based on taxonomic and ploidy classifications with some exceptions but did not consistently cluster by geographic origin. STRUCTURE analysis identified putative hybrids and suggested six genetic clusters in the cultivated potato collection with extensive gene flow occurring among the potato populations, implying most populations readily shared alleles and that introgression is common in potato. Solanum tuberosum subsp. andigena (ADG) and S. curtilobum (CUR) displayed significant admixture. ADG likely has extensive admixture due to its broad geographic distribution. Solanum phureja (PHU), Solanum chaucha (CHA)/Solanum stenotomum subsp. stenotomum (STN), and Solanum tuberosum subsp. tuberosum (TBR) populations had less admixture from an accession/population perspective relative to the species evaluated. A core and mini core subset from the genebank material was also constructed. SNP genotyping was also carried out on 745 accessions from the Seed Savers potato collection which confirmed no genetic duplication between the two potato collections, suggesting that the collections hold very different genetic resources of potato. The Infinium SNP Potato Array is a powerful tool that can provide diversity assessments, fingerprint genebank accessions for quality management programs, use in research and breeding, and provide insights into the complex genetic structure and hybrid origin of the diversity present in potato genetic resource collections.

Genetic diversity, population structure, and selection of breeder germplasm subsets from the USDA sweetpotato (Ipomoea batatas) collection

Sweetpotato (Ipomoea batatas) is the sixth most important food crop and plays a critical role in maintaining food security worldwide. Support for sweetpotato improvement research in breeding and genetics programs, and maintenance of sweetpotato germplasm collections is essential for preserving food security for future generations. Germplasm collections seek to preserve phenotypic and genotypic diversity through accession characterization. However, due to its genetic complexity, high heterogeneity, polyploid genome, phenotypic plasticity, and high flower production variability, sweetpotato genetic characterization is challenging. Here, we characterize the genetic diversity and population structure of 604 accessions from the sweetpotato germplasm collection maintained by the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), Plant Genetic Resources Conservation Unit (PGRCU) in Griffin, Georgia, United States. Using the genotyping-by-sequencing platform (GBSpoly) and bioinformatic pipelines (ngsComposer and GBSapp), a total of 102,870 polymorphic SNPs with hexaploid dosage calls were identified from the 604 accessions. Discriminant analysis of principal components (DAPC) and Bayesian clustering identified six unique genetic groupings across seven broad geographic regions. Genetic diversity analyses using the hexaploid data set revealed ample genetic diversity among the analyzed collection in concordance with previous analyses. Following population structure and diversity analyses, breeder germplasm subsets of 24, 48, 96, and 384 accessions were established using K-means clustering with manual selection to maintain phenotypic and genotypic diversity. The genetic characterization of the PGRCU sweetpotato germplasm collection and breeder germplasm subsets developed in this study provide the foundation for future association studies and serve as precursors toward phenotyping studies aimed at linking genotype with phenotype.

The world's largest potato cryobank at the International Potato Center (CIP) - Status quo, protocol improvement through large-scale experiments and long-term viability monitoring

Long-term conservation of Plant Genetic Resources (PGR) is a key priority for guaranteeing food security and sustainability of agricultural systems for current and future generations. The need for the secure conservation of genetic resources collections ex situ is critical, due to rapid and extreme climatic changes which are threatening and reducing biodiversity in their natural environments. The International Potato Center (CIP) conserves one of the most complete and diverse genetic resources collections of potato, with more than 7500 accessions composed of 4900 cultivated potato and 2600 potato wild relative accessions. The clonal conservation of cultivated potato, principally landraces, through in vitro or field collections is indispensable to maintain fixed allelic states, yet it is costly and labor-intensive. Cryopreservation, the conservation of biological samples in liquid nitrogen (-196°C), is considered the most reliable and cost-efficient long-term ex-situ conservation method for clonal crops. Over the last decade, CIP has built one of the largest potato cryobanks worldwide, cyopreserving more than 4000 cultivated potato accessions which represents 84% of the total cultivated potato collection currently conserved at CIP. In approximately, four years the entire potato collection will be cryopreserved. The development of an applied, robust cryopreservation protocol for potato, serves as a model for other clonally maintained crop collections. The CIP cryobank designs experiments with a high number of genetically diverse genotypes (70-100 accessions, seven cultivated species), to obtain reliable results that can be extrapolated over the collection as genotypes can often respond variably to the same applied conditions. Unlike most published reports on cryopreservation of plants, these large-scale experiments on potato are unique as they examine the acclimatization process of in vitro plants prior to, as well as during cryopreservation on up to ten times the number of genotypes conventionally reported in the published literature. As a result, an operational cryopreservation protocol for potato has advanced that works well across diverse potato accessions, not only with reduced processing time and costs, but also with an increased average full-plant recovery rate from 58% to 73% (+LN) for routine cryopreservation. The present article describes the composition of CIP's cryobank, the cryopreservation protocol, methodology for the dynamic improvement of the operational protocol, as well as data collected on regeneration from long term cryopreserved potatoes.

Exploring and exploiting genetics and genomics for sweetpotato improvement: Status and perspectives

Sweetpotato (Ipomoea batatas (L.) Lam.) is one of the most important root crops cultivated worldwide. Because of its adaptability, high yield potential, and nutritional value, sweetpotato has become an important food crop, particularly in developing countries. To ensure adequate crop yields to meet increasing demand, it is essential to enhance the tolerance of sweetpotato to environmental stresses and other yield-limiting factors. The highly heterozygous hexaploid genome of I. batatas complicates genetic studies and limits improvement of sweetpotato through traditional breeding. However, application of next-generation sequencing and high-throughput genotyping and phenotyping technologies to sweetpotato genetics and genomics research has provided new tools and resources for crop improvement. In this review, we discuss the genomics resources that are available for sweetpotato, including the current reference genome, databases, and available bioinformatics tools. We systematically review the current state of knowledge on the polyploid genetics of sweetpotato, including studies of its origin and germplasm diversity and the associated mapping of important agricultural traits. We then outline the conventional and molecular breeding approaches that have been applied to sweetpotato. Finally, we discuss future goals for genetic studies of sweetpotato and crop improvement via breeding in combination with state-of-the-art multi-omics approaches such as genomic selection and gene editing. These approaches will advance and accelerate genetic improvement of this important root crop and facilitate its sustainable global production.