Hybrid origins of cultivated potatoes

Defining autopolyploidy: Cytology, genetics, and taxonomy

Autopolyploidy is taxonomically defined as the presence of more than two copies of each genome within an organism or species, where the genomes present must all originate within the same species. Alternatively, "genetic" or "cytological" autopolyploidy is defined by polysomic inheritance: random pairing and segregation of the four (or more) homologous chromosomes present, with no preferential pairing partners. In this review, we provide an overview of methods used to categorize species as taxonomic and cytological autopolyploids, including both modern and obsolete cytological methods, marker-segregation-based and genomics methods. Subsequently, we also investigated how frequently polysomic inheritance has been reliably documented in autopolyploids. Pure or predominantly polysomic inheritance was documented in 39 of 43 putative autopolyploid species where inheritance data was available (91%) and in seven of eight synthetic autopolyploids, with several cases of more mixed inheritance within species. We found no clear cases of autopolyploids with disomic inheritance, which was likely a function of our search methodology. Interestingly, we found seven species with purely polysomic inheritance and another five species with partial or predominant polysomic inheritance that appear to be taxonomic allopolyploids. Our results suggest that observations of polysomic inheritance can lead to relabeling of taxonomically allopolyploid species as autopolyploid and highlight the need for further cytogenetic and genomic investigation into polyploid origins and inheritance types.

Potato soup: analysis of cultivated potato gene bank populations reveals high diversity and little structure

Cultivated potatoes are incredibly diverse, ranging from diploid to pentaploid and encompass four different species. They are adapted to disparate environments and conditions and carry unique alleles for resistance to pests and pathogens. Describing how diversity is partitioned within and among these populations is essential to understanding the potato genome and effectively utilizing landraces in breeding. This task is complicated by the difficulty of making comparisons across cytotypes and extensive admixture within section petota. We genotyped 730 accessions from the US Potato genebank including wild diploids and cultivated diploids and tetraploids using Genotype-by-sequencing. This data set allowed us to interrogate population structure and diversity as well as generate core subsets which will support breeders in efficiently screening genebank material for biotic and abiotic stress resistance alleles. We found that even controlling for ploidy, tetraploid material exhibited higher observed and expected heterozygosity than diploid accessions. In particular group chilotanum material was the most heterozygous and the only taxa not to exhibit any inbreeding. This may in part be because group chilotanum has a history of introgression not just from wild species, but landraces as well. All group chilotanum, exhibits introgression from group andigenum except clones from Southern South America near its origin, where the two groups are not highly differentiated. Moving north, we do not observe evidence for the same level of admixture back into group andigenum. This suggests that extensive history of admixture is a particular characteristic of chilotanum.

Comparative Analysis of the Genetic Diversity of Chilean Cultivated Potato Based on a Molecular Study of Authentic Herbarium Specimens and Present-Day Gene Bank Accessions

At the end of the 1920s, Vavilov organized several potato-collecting missions in South and Central America. Vavilov and his colleagues, Juzepczuk and Bukasov, participated in these expeditions and worked on gathered material, designated two centers of potato varietal riches and diversity-the Peru-Bolivia high-mountain center and the southern coast of Chile. The WIR Herbarium holds authentic specimens of many taxa described by Russian taxonomists. Here, a set of 20 plastid DNA-specific markers was applied for 49 authentic herbarium specimens of Solanum tuberosum L. from the WIR Herbarium to analyze the genetic diversity of the landrace population collected by Juzepczuk in 1928 in southern-central Chile. Two plastid DNA types, T and A, and two chlorotypes were identified in herbarium specimens, with a clear predominance (96%) of chlorotype cpT_III. In addition, we analyzed 46 living Chilean accessions from the VIR field potato gene bank that were collected after the appearance of Phytophthora infestans in Chile. These living accessions were differentiated into four chlorotypes. Finding a D-type cytoplasm in living Chilean accessions that possess two new chlorotypes indicates a replacement of native cultivars and introgression from the wild Mexican species S. demissum that was actively used in breeding as a source of race-specific resistance to late blight.

5S Ribosomal DNA of Genus Solanum: Molecular Organization, Evolution, and Taxonomy

The Solanum genus, being one of the largest among high plants, is distributed worldwide and comprises about 1,200 species. The genus includes numerous agronomically important species such as Solanum tuberosum (potato), Solanum lycopersicum (tomato), and Solanum melongena (eggplant) as well as medical and ornamental plants. The huge Solanum genus is a convenient model for research in the field of molecular evolution and structural and functional genomics. Clear knowledge of evolutionary relationships in the Solanum genus is required to increase the effectiveness of breeding programs, but the phylogeny of the genus is still not fully understood. The rapidly evolving intergenic spacer region (IGS) of 5S rDNA has been successfully used for inferring interspecific relationships in several groups of angiosperms. Here, combining cloning and sequencing with bioinformatic analysis of genomic data available in the SRA database, we evaluate the molecular organization and diversity of IGS for 184 accessions, representing 137 species of the Solanum genus. It was found that the main mechanisms of IGS molecular evolution was step-wise accumulation of single base substitution or short indels, and that long indels and multiple base substitutions, which arose repeatedly during evolution, were mostly not conserved and eliminated. The reason for this negative selection seems to be association between indels/multiple base substitutions and pseudogenization of 5S rDNA. Comparison of IGS sequences allowed us to reconstruct the phylogeny of the Solanum genus. The obtained dendrograms are mainly congruent with published data: same major and minor clades were found. However, relationships between these clades and position of some species (S. cochoae, S. clivorum, S. macrocarpon, and S. spirale) were different from those of previous results and require further clarification. Our results show that 5S IGS represents a convenient molecular marker for phylogenetic studies on the Solanum genus. In particular, the simultaneous presence of several structural variants of rDNA in the genome enables the detection of reticular evolution, especially in the largest and economically most important sect. Petota. The origin of several polyploid species should be reconsidered.

Genetic Diversity and Population Structure of Potato Germplasm in RDA-Genebank: Utilization for Breeding and Conservation

Potato (Solanum tuberosum L.) is an important staple food and economic crop in many countries. It is of critical importance to understand the genetic diversity and population structure for effective collection, conservation, and utilization of potato germplasm. Thus, the objective of the present study was to investigate the genetic diversity and population structure of potato germplasm conserved in the National Agrobiodiversity Center (NAC) of South Korea to provide basic data for future preservation and breeding of potato genetic resources. A total of 24 simple sequence repeat (SSR) markers were used to assess the genetic diversity and population structure of 482 potato accessions. A total of 257 alleles were detected, with an average of 10.71 alleles per locus. Analysis of molecular variance showed that 97% of allelic diversity was attributed to individual accessions within the population, while only 3% was distributed among populations. Results of genetic structure analysis based on STRUCTURE and discriminant analysis of principal components revealed that 482 potato accessions could be divided into two main subpopulations. Accessions of subpopulation 1 mainly belonged to cultivars and breeding lines. Accessions of subpopulations 2 basically corresponded to wild relatives of potatoes. Results of this study provide useful information for potato improvement and conservation programs, although further studies are needed for a more accurate evaluation of genetic diversity and phenotypic traits of potatoes.

Late Blight Resistance Evaluation and Genome-Wide Assessment of Genetic Diversity in Wild and Cultivated Potato Species

Late blight, caused by the oomycete Phytophthora infestans, is the most devastating disease in potato-producing regions of the world. Cultivation of resistant varieties is the most effective and environmentally friendly way to control potato late blight disease, and identification of germplasms with late blight resistance and clarification their genetic relationship would promote the development of the resistant varieties. In this study, a diverse population of 189 genotypes with potential late blight resistance, consisting of 20 wild species and cultivated Solanum tuberosum Andigenum group and Chilotanum group, was screened for the presence of late blight resistance by performing challenge inoculation with four Phytophthora infestans isolates including one 13_A2 isolate, CN152. Ten elite resources with broad-spectrum resistance and 127 with isolate-specific resistance against P. infestans were identified. To improve the available gene pool for future potato breeding programs, the population was genotyped using 30 simple sequence repeat (SSR) markers covering the entire potato genome. A total of 173 alleles were detected with an average of 5.77 alleles per locus. Structure analysis discriminated the 189 potato genotypes into five populations based on taxonomic classification and genetic origin with some deviations. There was no obvious clustering by country of origin, ploidy level, EBN (endosperm balance number) value, or nuclear clade. Analysis of molecular variance showed 10.08% genetic variation existed among populations. The genetic differentiation (Fst) ranged from 0.0937 to 0.1764, and the nucleotide diversity (π) was 0.2269 across populations with the range from 0.1942 to 0.2489. Further genotyping of 20K SNP array confirmed the classification of SSRs and could uncover the genetic relationships of Solanum germplasms. Our results indicate that there exits abundant genetic variation in wild and cultivated potato germplasms, while the cultivated S. tuberosum Chilotanum group has lower genetic diversity. The phenotypic and genetic information obtained in this study provide a useful guide for hybrid combination and resistance introgression from wild gene pool into cultivated species for cultivar improvement, as well as for germplasm conservation efforts and resistance gene mining.

Single Nucleotide Polymorphism (SNP) markers associated with high folate content in wild potato species

Micronutrient deficiency, also known as the hidden hunger, affects over two billion people worldwide. Potato is the third most consumed food crops in the world, and is therefore a fundamental element of food security for millions of people. Increasing the amount of micronutrients in food crop could help alleviate worldwide micronutrient malnutrition. In the present study, we report on the identification of single nucleotide polymorphism (SNP) markers associated with folate, an essential micronutrient in the human diet. A high folate diploid clone Fol 1.6 from the wild potato relative Solanum boliviense (PI 597736) was crossed with a low/medium folate diploid S. tuberosum clone USW4self#3. The resulting F1 progeny was intermated to generate an F2 population, and tubers from 94 F2 individuals were harvested for folate analysis and SNP genotyping using a SolCap 12K Potato SNP array. Folate content in the progeny ranged from 304 to 2,952 ng g-1 dry weight. 6,759 high quality SNPs containing 4,174 (62%) polymorphic and 2,585 (38%) monomorphic SNPs were used to investigate marker-trait association. Association analysis was performed using two different approaches: survey SNP-trait association (SSTA) and SNP-trait association (STA). A total of 497 significant SNPs were identified, 489 by SSTA analysis and 43 by STA analysis. Markers identified by SSTA were located on all twelve chromosomes while those identified by STA were confined to chromosomes 2, 4, and 6. Eighteen of the significant SNPs were located within or in close proximity to folate metabolism-related genes. Forty two SNPs were identical between SSTA and STA analyses. These SNPs have potential to be used in marker-assisted selection for breeding high folate potato varieties.

Short interspersed nuclear elements (SINEs) are abundant in Solanaceae and have a family-specific impact on gene structure and genome organization

Short interspersed nuclear elements (SINEs) are highly abundant non-autonomous retrotransposons that are widespread in plants. They are short in size, non-coding, show high sequence diversity, and are therefore mostly not or not correctly annotated in plant genome sequences. Hence, comparative studies on genomic SINE populations are rare. To explore the structural organization and impact of SINEs, we comparatively investigated the genome sequences of the Solanaceae species potato (Solanum tuberosum), tomato (Solanum lycopersicum), wild tomato (Solanum pennellii), and two pepper cultivars (Capsicum annuum). Based on 8.5 Gbp sequence data, we annotated 82 983 SINE copies belonging to 10 families and subfamilies on a base pair level. Solanaceae SINEs are dispersed over all chromosomes with enrichments in distal regions. Depending on the genome assemblies and gene predictions, 30% of all SINE copies are associated with genes, particularly frequent in introns and untranslated regions (UTRs). The close association with genes is family specific. More than 10% of all genes annotated in the Solanaceae species investigated contain at least one SINE insertion, and we found genes harbouring up to 16 SINE copies. We demonstrate the involvement of SINEs in gene and genome evolution including the donation of splice sites, start and stop codons and exons to genes, enlargement of introns and UTRs, generation of tandem-like duplications and transduction of adjacent sequence regions.

Homologues of potato chromosome 5 show variable collinearity in the euchromatin, but dramatic absence of sequence similarity in the pericentromeric heterochromatin

BACKGROUND

In flowering plants it has been shown that de novo genome assemblies of different species and genera show a significant drop in the proportion of alignable sequence. Within a plant species, however, it is assumed that different haplotypes of the same chromosome align well. In this paper we have compared three de novo assemblies of potato chromosome 5 and report on the sequence variation and the proportion of sequence that can be aligned.

RESULTS

For the diploid potato clone RH89-039-16 (RH) we produced two linkage phase controlled and haplotype-specific assemblies of chromosome 5 based on BAC-by-BAC sequencing, which were aligned to each other and compared to the 52 Mb chromosome 5 reference sequence of the doubled monoploid clone DM 1-3 516 R44 (DM). We identified 17.0 Mb of non-redundant sequence scaffolds derived from euchromatic regions of RH and 38.4 Mb from the pericentromeric heterochromatin. For 32.7 Mb of the RH sequences the correct position and order on chromosome 5 was determined, using genetic markers, fluorescence in situ hybridisation and alignment to the DM reference genome. This ordered fraction of the RH sequences is situated in the euchromatic arms and in the heterochromatin borders. In the euchromatic regions, the sequence collinearity between the three chromosomal homologs is good, but interruption of collinearity occurs at nine gene clusters. Towards and into the heterochromatin borders, absence of collinearity due to structural variation was more extensive and was caused by hemizygous and poorly aligning regions of up to 450 kb in length. In the most central heterochromatin, a total of 22.7 Mb sequence from both RH haplotypes remained unordered. These RH sequences have very few syntenic regions and represent a non-alignable region between the RH and DM heterochromatin haplotypes of chromosome 5.

CONCLUSIONS

Our results show that among homologous potato chromosomes large regions are present with dramatic loss of sequence collinearity. This stresses the need for more de novo reference assemblies in order to capture genome diversity in this crop. The discovery of three highly diverged pericentric heterochromatin haplotypes within one species is a novelty in plant genome analysis. The possible origin and cytogenetic implication of this heterochromatin haplotype diversity are discussed.

Implications of miR166 and miR159 induction to the basal response mechanisms of an andigena potato (Solanum tuberosum subsp. andigena) to salinity stress, predicted from network models in Arabidopsis

MicroRNA (miRNA) mediated changes in gene expression by post-transcriptional modulation of major regulatory transcription factors is a potent mechanism for integrating growth and stress-related responses. Exotic plants including many traditional varieties of Andean potatoes (Solanum tuberosum subsp. andigena) are known for better adaptation to marginal environments. Stress physiological studies confirmed earlier reports on the salinity tolerance potentials of certain andigena cultivars. Guided by the hypothesis that certain miRNAs play important roles in growth modulation under suboptimal conditions, we identified and characterized salinity stress-responsive miRNA-target gene pairs in the andigena cultivar Sullu by parallel analysis of noncoding and coding RNA transcriptomes. Inverse relationships were established by the reverse co-expression between two salinity stress-regulated miRNAs (miR166, miR159) and their target transcriptional regulators HD-ZIP-Phabulosa/Phavulota and Myb101, respectively. Based on heterologous models in Arabidopsis, the miR166-HD-ZIP-Phabulosa/Phavulota network appears to be involved in modulating growth perhaps by mediating vegetative dormancy, with linkages to defense-related pathways. The miR159-Myb101 network may be important for the modulation of vegetative growth while also controlling stress-induced premature transition to reproductive phase. We postulate that the induction of miR166 and miR159 under salinity stress represents important network hubs for balancing gene expression required for basal growth adjustments.

Diversity of potato genetic resources

A considerable number of highly diverse species exist in genus Solanum. Because they can adapt to a broad range of habitats, potato wild relatives are promising sources of desirable agricultural traits. Potato taxonomy is quite complex because of introgression, interspecific hybridization, auto- and allopolyploidy, sexual compatibility among many species, a mixture of sexual and asexual reproduction, possible recent species divergence, phenotypic plasticity, and the consequent high morphological similarity among species. Recent researchers using molecular tools have contributed to the identification of genes controlling several types of resistance as well as to the revision of taxonomical relationships among potato species. Historically, primitive forms of cultivated potato and its wild relatives have been used in breeding programs and there is still an enormous and unimaginable potential for discovering desirable characteristics, particularly in wild species Different methods have been developed to incorporate useful alleles from these wild species into the improved cultivars. Potato germplasm comprising of useful alleles for different breeding objectives is preserved in various gene banks worldwide. These materials, with their invaluable information, are accessible for research and breeding purposes. Precise identification of species base on the new taxonomy is essential for effective use of the germplasm collection.

Inter-SINE Amplified Polymorphism (ISAP) for rapid and robust plant genotyping

The unambiguous differentiation of crop genotypes is often laborious or expensive. A rapid, robust, and cost-efficient marker system is required for routine genotyping in plant breeding and marker-assisted selection. We describe the Inter-SINE Amplified Polymorphism (ISAP) system that is based on standard molecular methods resulting in genotype-specific fingerprints at high resolution. These markers are derived from Short Interspersed Nuclear Elements (SINEs) which are dispersed repetitive sequences present in most if not all plant genomes and can be efficiently extracted from plant genome sequences. The ISAP method was developed on potato as model plant but is also transferable to other plant species.

Ex-post analysis of landraces sympatric to a commercial variety in the center of origin of the potato failed to detect gene flow

The possible introduction of genetically modified potato in the Andean region raises concerns about the unintentional introduction of transgenes into the native potato germplasm because it is perceived to convey negative impacts on biodiversity. We investigated this question by an ex-post analysis of existing landraces resulting from natural hybridization between an unknown landrace and the fertile commercial variety ‘Yungay’. This variety can be regarded as exotic because it was bred in part from the southern Chilean germplasm of Solanum tuberosum Group Chilotanum. We sampled the landrace germplasm of 1,771 leaf samples comprising more than 400 different landraces from three regions where ‘Yungay’ and landraces have coexisted for 15–25 years in the Peruvian Andes. Simple sequence repeat (SSR) markers were used to identify putative hybrids based on allele sharing with those of ‘Yungay’. The exclusion procedure was iterative, starting with the SSR markers with highest discriminating capacity based on allele frequency of the variety ‘Yungay’ in our large database of 688 landraces by 24 SSR makers. With only 12 of the 24 SSR markers assayed, all of the samples could be rejected as possible hybrids with ‘Yungay’ as a parent. This result demonstrates that the unintentional introduction of a transgene, not under farmers’ selection, from a widely grown transgenic variety over a long period of time is unlikely to happen at a detectable scale. Our finding reinforces the prominent role of farmers in the selection and maintenance of landraces which, unlike hybrids, have specific characteristics that farmers appreciate.

Single copy nuclear gene analysis of polyploidy in wild potatoes (Solanum section Petota)

BACKGROUND

Recent genomic studies have drastically altered our knowledge of polyploid evolution. Wild potatoes (Solanum section Petota) are a highly diverse and economically important group of about 100 species widely distributed throughout the Americas. Thirty-six percent of the species in section Petota are polyploid or with diploid and polyploid cytotypes. However, the group is poorly understood at the genomic level and the series is ideal to study polyploid evolution. Two separate studies using the nuclear orthologs GBSSI and nitrate reductase confirmed prior hypotheses of polyploid origins in potato and have shown new origins not proposed before. These studies have been limited, however, by the use of few accessions per polyploid species and by low taxonomic resolution, providing clade-specific, but not species-specific origins within clades. The purpose of the present study is to use six nuclear orthologs, within 54 accessions of 11 polyploid species, 34 accessions of 29 diploid species of section Petota representing their putative progenitors, and two outgroups, to see if phenomena typical of other polyploid groups occur within wild potatoes, to include multiple origins, loss of alleles, or gain of new alleles.

RESULTS

Our results increase resolution within clades, giving better ideas of diploid progenitors, and show unexpected complexity of allele sharing within clades. While some species have little diversity among accessions and concur with the GBSSI and nitrate reductase results, such as S. agrimonifolium, S. colombianum, S. hjertingii, and S. moscopanum, the results give much better resolution of species-specific progenitors. Seven other species, however, show variant patterns of allele distributions suggesting multiple origins and allele loss. Complex three-genome origins are supported for S. hougasii, and S. schenckii, and one of the ten accessions of S. stoloniferum. A very unexpected shared presence of alleles occurs within one clade of S. verrucosum from Central America, and S. berthaultii from South America in six polyploid species S. demissum, S. hjertingii, S. hougasii, S. iopetalum, S. schenckii, and S. stoloniferum.

CONCLUSIONS

Our results document considerable genomic complexity of some wild potato polyploids. These can be explained by multiple hybrid origins and allele losses that provide a clear biological explanation for the taxonomic complexity in wild potato polyploids. These results are of theoretical and practical benefit to potato breeders, and add to a growing body of evidence showing considerable complexity in polyploid plants in general.

Organization and evolution of subtelomeric satellite repeats in the potato genome

Subtelomeric domains immediately adjacent to telomeres represent one of the most dynamic and rapidly evolving regions in eukaryotic genomes. A common feature associated with subtelomeric regions in different eukaryotes is the presence of long arrays of tandemly repeated satellite sequences. However, studies on molecular organization and evolution of subtelomeric repeats are rare. We isolated two subtelomeric repeats, CL14 and CL34, from potato (Solanum tuberosum). The CL14 and CL34 repeats are organized as independent long arrays, up to 1-3 Mb, of 182 bp and 339 bp monomers, respectively. The CL14 and CL34 repeat arrays are directly connected with the telomeric repeats at some chromosomal ends. The CL14 repeat was detected at the subtelomeric regions among highly diverged Solanum species, including tomato (Solanum lycopersicum). In contrast, CL34 was only found in potato and its closely related species. Interestingly, the CL34 repeat array was always proximal to the telomeres when both CL14 and CL34 were found at the same chromosomal end. In addition, the CL34 repeat family showed more sequence variability among monomers compared with the CL14 repeat family. We conclude that the CL34 repeat family emerged recently from the subtelomeric regions of potato chromosomes and is rapidly evolving. These results provide further evidence that subtelomeric domains are among the most dynamic regions in eukaryotic genomes.

Ecogeography of ploidy variation in cultivated potato (Solanum sect. Petota)

PREMISE OF THE STUDY

The taxonomy of cultivated potatoes has been highly controversial, with estimates of species numbers ranging from 3 to 17. Ploidy level has been one of the most important taxonomic characters to recognize cultivated potato species, containing diploid (2n = 2x = 24), triploid (2n = 3x = 36), tetraploid (2n = 4x = 48), and pentaploid (2n = 5x = 60) cultivars. We tested the environmental associations of different ploidy levels in cultivated potato species that traditionally have been recognized as Linnaean taxa to see whether, in combination with prior morphological, molecular, and crossing data, some of the ploidy variants can be recognized as distinct taxa. •

METHODS

We summarize 2780 chromosome counts of landrace cultivated potatoes, provide georeferences to 2048 of them, and analyze these data for 20 environmental variables at 10-min resolution using the randomForest algorithm to explore associations with taxa and ploidy variants. •

KEY RESULTS

Except for the S. tuberosum Chilotanum Group and extreme northern and southern range extensions of the Andigenum Group, it is impossible to find distinct habitats for the ploidy variants of the S. tuberosum Andigenum Group. •

CONCLUSIONS

Our distributional and ecological data, in combination with prior results from morphology, microsatellites, and crossing data, provide yet additional data to support a major reclassification of cultivated potato species. A rational, stable, and universally accepted taxonomy of this major crop plant will greatly aid all users of wild and cultivated potatoes from breeders to gene bank managers to ecologists and evolutionary biologists.