Hybridization barriers between diploid Solanum tuberosum and wild Solanum raphanifolium

Wild potato germplasm represents a unique, diverse and accessible resource for disease and pest resistance, along with useful agronomic traits that may be introgressed into the cultivated potato (Solanum tuberosum L.). Hybridization of diploid wild Solanum species with haploids (2×) of cultivated potato (4×) is generally an effective technique for introducing genetic diversity and desirable traits into potato. However, in this study, hybridization barriers were found in crosses between S. tuberosum haploids and the wild species S. raphanifolium. Male sterility, likely due to nuclear-cytoplasmic interactions, was observed in some haploid tbr?×?S. raphanifolium hybrids. In addition, pollen-pistil incompatibilities were observed in backcross, F2, and reciprocal cross hybridization attempts that failed to produce seed. More crosses were successful when F1 clones were crossed as females to wild clones than to cultivated clones. When crosses were made in the other direction, with F1 hybrids used as male parents, seeds were almost never produced.     

Crossability relationships between the common potato, Solanum tuberosum spp. tuberosum, and its wild diploid relatives S. kurtzianum and S. ruiz-lealii

The common potato, Solanum tuberosum spp. tuberosum (tbr), is a tetraploid species with a narrow genetic base, but with a large number of related species that harbor great genetic diversity for numerous characters of agronomic interest. S. kurtzianum (ktz) and S. ruiz-lealii (rzl) are wild diploid relatives with potential resistance to adverse biotic and abiotic factors. To evaluate if this wild germplasm can be incorporated by conventional crosses into the tbr gene pool, pollen-pistil compatibility relations and seed set in interspecific crosses with tbr were investigated. In 4x tbr × 2x ktz crosses and the reciprocals, 89% and 52.2% of genotypic combinations (respectively) were compatible at the pollen-pistil level. Seeds were obtained from some genotypic combinations in both directions of the cross, suggesting that functional 2n gametes might be produced by particular ktz genotypes. In 2x rzl × 4x tbr crosses and their reciprocals, 35% and 11.7% (respectively) of the genotypic combinations were compatible at the pollen-pistil level, but no seeds were obtained. These results indicate that the reproductive isolation between 4x tbr and 2x ktz is incomplete, and that gene exchange between them is feasible. Moreover, further studies will have to be carried out with rzl to ascertain its crossability with 4x tbr.     

Cytological mechanisms of 2n pollen formation in the wild potato Solanum okadae and pollen-pistil relations with the cultivated potato, Solanum tuberosum

Solanum okadae Hawkes et Hjert is a wild diploid potato species endemic to Argentina and Bolivia, of potential breeding value. However, no genetic studies have been carried out with this species and its crossability relations with the common tetraploid potato, S. tuberosum L. ssp. tuberosum, are unknown. Furthermore, accessions from both countries differed in their morphological phenotypes. To ascertain the feasibility of incorporating this wild germplasm into cultivated potato, 2n pollen screening was carried out in 10 accessions and families derived from crosses between accessions; also, pollen-pistil compatibility relations were studied in reciprocal interspecific crosses. Plants of four of the 10 accessions produced 2n pollen (0.1 to 5.0%) and 4n pollen (0.0 to 3.0%). Parallel and tripolar spindles at Anaphase II were the cytological mechanisms involved in 2n pollen formation; lack of chromosome migration in both meiotic divisions originated the 4n pollen. Both full compatibility and incompatibility at various sites along the pistil were observed in the S. tuberosum × S. okadae combinations; most reciprocal combinations were incompatible. Compatible genotypes produced 2n pollen. However, only a few seeds were obtained and chromosome counts could not be carried out in the hybrid progeny because seedlings died at an early stage. The pollen-pistil barriers are incomplete and can be circumvented by the appropiate choice of parents. The identification of the post-zygotic barriers will be the focus of further studies.     

Patterns of solanidine glycoalkaloid variation in four gene pools of the cultivated potato

Patterns of variation for the solanidine-based glycoalkaloids have been determined for tuber material of genebank accessions of landraces of the cultivated potato and three closely-related wild species. Total levels were low in the cultivated taxa investigated,S. phureja, S. stenotomum, and the tetraploidS. tuberosum in its two formsS. tuberosum ssp.tuberosum andS. tuberosum ssp.andigena. The only solanidine-based glycolkaloids found in the tubers investigated in this study were-solanine and-chaconine, with one additional related compound, dehydrocommersonine, found in tubers of an accession ofS. canasense. The ratio of chaconine : solanine differed markedly from a mean of 0.98 inS. phureja to 2.28 in ChileanS. tuberosum ssp.tuberosum. The chaconine : solanine ratio was relatively constant within these two taxa, reflecting their relatively narrow genetic bases, but was much more variable within the taxa regarded as ancestral,S. stenotomum andS. tuberosum ssp.andigena. The wild speciesS. sparsipilum has been implicated in the origin of the tetraploid potato,S. tuberosum, but there was no support for this from the patterns of variation in chaconine : solanine ratio. The cline of chaconine : solanine ratios in cultivated potatoes along the spine of S. America does not appear to relate to the influence of local wild species through introgression, but may instead reflect differing selective pressures acting in different parts of the native range of the cultivated potato.     

Genetic diversity of Musa diploid and triploid accessions from the Brazilian banana breeding program estimated by microsatellite markers

Banana (Musa spp.) is one of the most consumed fruits worldwide. Production is based mainly on triploid cultivars, and most genetic improvement programs aim to generate tetraploid hybrids obtained from the crossing of established triploid cultivars with a diploid parent genotype, improved or wild, exhibiting the trait of interest, normally resistance to biotic factors. Microsatellites were used to investigate the genetic variability and relationships between 58 Musa genotypes, including 49 diploids and nine triploid cultivars maintained at the Musa germplasm collection of the Brazilian dessert banana breeding program. Thirty-three primer pairs developed for banana were tested, and nine amplified reproducible and discrete fragments, producing a total of 115 alleles. The average number of alleles amplified per primer was 12.8, ranging from 10 to 15. The diploid genotypes presented the largest genetic variability, demonstrated by the large number of alleles detected, and the low similarity between the clones. The phenetic analysis clustered the triploid cultivars in a separated group, with the exception of the Nanica and Gros Michel cultivars, which showed high similarity with the diploid cultivar Mambee Thu. It was not possible to separate the wild diploid genotypes from the cultivated ones, indicating a common origin of these genotypes. A high proportion of duplicated alleles and/or loci was observed for diploid and triploid genotypes. The information gathered about the similarity between diploid and triploid accessions will help to define potential crosses to maximize the recovery of the typical fruit qualities required in Brazil (AAB, Pome and Silk dessert banana).     

The introgression of 2× 1EBN <Emphasis Type="Italic">Solanum</Emphasis> species into the cultivated potato using <Emphasis Type="Italic">Solanum verrucosum</Emphasis> as a bridge

Wild diploid Solanum species with an endosperm balance number (EBN) of one are a rich source of disease resistance, pest resistance, and tuber quality traits. They are not sexually compatible with 4× 4EBN or 2× 2EBN forms of the cultivated potato. In this study, the wild 2× 2EBN Mexican species S. verrucosum (ver) was used as a bridge species to access 2× 1EBN germplasm. Hybrids (V1) were created between ver as a female parent and the 1EBN species S. cardiophyllum, S. chancayense, S. commersonii, and S. trifidum. Most of the V1 hybrids were successfully crossed as females to cultivated 2× 2EBN clones. However, due to stylar barriers, reciprocal crosses were unsuccessful. Additional studies are underway to determine the effectiveness of ver as a bridge to introgress specific traits from 2× 1EBN germplasm into the cultivated potato.     

High proportion of diploid hybrids produced by interspecific diploid × tetraploid <Emphasis Type="Italic">Sorghum</Emphasis> hybridization

A perennial version of grain sorghum [S. bicolor (L.) Moench] would create opportunities for greatly reducing tillage and preventing soil degradation. Efforts to select for perenniality and grain production among progeny of hybrids between S. bicolor (2n = 20) and the weedy tetraploid perennial S. halepense (L.) Pers. (2n = 40) are complicated in that F₁ hybrids produced by diploid × tetraploid sorghum crosses are usually tetraploid. In 2013, a set of random pollinations between 19 diploid cytoplasmic male-sterile inbred lines and 43 tetraploid perennial plants produced 165 F₁ hybrid plants, more than 75% of which had highly atypical plant, panicle, and seed phenotypes. Phenotypic segregation in F₂ populations derived from atypical hybrids was also anomalous. Examination of mitotic metaphase cells in F₁ or F₂ root tips revealed that 129 of the 165 hybrids were diploid. Parentage of the diploid progenies was confirmed using simple-sequence repeat analysis. The mechanism by which diploid hybrids arise from diploid × tetraploid crosses is unknown, but it may involve either production of monohaploid (n = 10) pollen by the tetraploid parent or chromosome elimination during early cell divisions following formation of the triploid zygote. The ability to produce diploid germplasm segregating for S. bicolor and S. halepense alleles could have great utility, both for the development of perennial sorghum and for the improvement of conventional grain sorghum.     

Characterization of intergeneric hybrids of <Emphasis Type="Italic">Erianthus rockii</Emphasis> and <Emphasis Type="Italic">Saccharum</Emphasis> using molecular markers

Erianthus rockii, a wild relative of sugarcane, is drought and cold tolerant, and both are potentially important agronomic traits to the sugarcane industry worldwide. As such it is of interest as a source of parental germplasm to sugarcane breeders and is currently being used in sugarcane introgression programs in both China and Australia. To date morphological characters have been used to verify the putative hybrids produced. Two crosses were generated between two different Saccharum species and two E. rockii accessions. Over 400 AFLP markers were used to identify the intergeneric hybrid progeny as well as determine hybrid diversity. Both crosses generated hybrids but efficiency levels were very different and are probably related to the different Saccharum parent used in each cross. Cross 1 was between a Saccharum officinarum and E. rockii and generated 100% hybrid progeny. Cross 2, however, was between a sugarcane hybrid (S. officinarum × Saccharum spontaneum) and E. rockii and only 10% of the progeny were intergeneric hybrids. Inheritance of markers in the progeny was analysed and for both crosses there were equal numbers of markers from both parents indicating n + n transmission of chromosomes. This is the first verification of E. rockii hybrids with molecular markers. It may now be possible to exploit genes of value from E. rockii in sugarcane breeding programs.     

Inheritance of morphological characters and glycoalkaloids in potatoes of somatic hybrids between dihaploid Solanum acauleand tetraploid Solanum tuberosum.

Steroidal glycoalkaloids occur in potatoes and are reported to impart resistance to phytopathogens including bacteria, fungi, and insects. Because glycoalkaloids can be passed to progenies during breeding programs designed to develop improved potatoes, it is of importance to determine the quality of desired characteristics and the composition of glycoalkaloids of new somatic hybrids. The objective of this study was to determine the appearance, size, and shape (morphological characters) as well as the glycoalkaloid content of potato tubers of somatic hybrids between tetraploid Solanum tuberosum cv. Dejima (2n = 4x = 48 chromosomes) and the dihaploid clone ATDH-1 (2n = 2x = 24 chromosomes) induced by anther culture from Solanum acuale-T (acl-T, 2n = 4x = 48 chromosomes). Tuber size and shape in somatic hybrids were in accord with those of cv. Dejima, whereas the tuber skin color resembled that of ATDH-1. Thin-layer chromatography, high-performance liquid chromatography, and gas-liquid chromatography/mass spectrometry studies showed that the two steroidal glycoalkaloids (alpha-chaconine and alpha-solanine) were present in the tubers of S. tuberosum, whereas acl-T and ATDH-1 tubers were found to contain alpha-tomatine and demissine. The concentrations of total glycoalkaloids in both acl-T and ATDH-1 was >100 mg/100 g of fresh weight tuber cortex, much higher than in S. tuberosum. All somatic hybrids, except one clone, contained four glycoalkaloids (alpha-chaconine, alpha-solanine, alpha-tomatine, and demissine) derived from the fusion parents. The lack of alpha-tomatine in the remaining clone may be due to somaclonal variation. The results show that character expression is influenced by ploidy level and that total glycoalkaloid levels in most somatic hybrids were intermediate between those of the fusion parents. The possible significance of these findings for plant breeding and food safety is discussed.     

A Potato Collecting Expedition in the Province of Jujuy,Argentina and Disease Indexing of Virus and Fungus Pathogens in Andean Cultivars

A potato collecting expedition was carried out in the province of Jujuy, Argentina in March 24 to April 15, 2001. The objective was to collect local cultivars of potatoes and wild potato species, covering high mountain valleys not previously collected or areas where germplasm was not available. A total of 247 accessions was collected, 188 cultivated accessions of S. tuberosum subsp. andigenum, four of Solanum tuberosum subsp. tuberosum, two of S. curtilobum and 53 accessions of wild species. The wild species collected were S. acaule subsp. acaule, S. chacoense, S. infundibuliforme, S. megistacrolobum subsp. megistacrolobum, S. microdontum, S. gourlayi, S. × viirsooi and S. oplocense. Herbarium voucher specimens were obtained when possible. For the collection of cultivated potatoes, tubers were gathered from farmer's fields and in a few cases from stores or markets. Seed samples were generally obtained for the wild species. Detailed collection site data were recorded at every site. After breaking dormancy, the accessions of the cultivated species were screened for the presence of Potato virus Y (PVY), Potato virus X (PVX), Potato leafroll virus (PLRV), Potato virus S (PVS), Potato virus M (PVM), Potato virus V (PVV), Andean potato latent virus (APLV), Andean potato mottle virus (APMoV), Potato rough dwarf virus (PRDV), Potato spindle tuber viroid (PSTVd) and Spongospora subterranea f. sp. subterranea. PSTVd, APLV and PRDV were not detected, but different levels of infection are reported for the other pathogens assayed.     

Genetic diversity and origin of cultivated potatoes based on plastid microsatellite polymorphism

The origin of cultivated potatoes has remaining questions. In this study, 237 accessions of all cultivated species and 155 accessions of wild species closely related to cultivated potatoes, including their putative ancestors, were analyzed using 15 plastid microsatellites (SSRs) to investigate genetic diversity and their relationships with the wild species. We here used polymorphic plastid SSRs we developed from potato plastid genome sequences as well as already known plastid SSR markers. All 15 loci were polymorphic and identified a total of 127 haplotypes. Dramatic decreases in levels of genetic diversity were revealed in landraces in comparison with wild ancestor species. The plastid SSR results showed a decrease in haplotype number and diversity from Peru to both north and south. Phylogenetic analysis revealed two distinct groups. One of them, group A, contained the majority of accessions of cultivated species of the Solanum tuberosum Andigenum group including all accessions of cultivated diploid and triploid cytotypes of this group (S. chaucha, S. phureja, and S. stenotomum by a former taxonomic system) and most of tetraploid accessions of the S. tuberosum Andigenum group (S. tuberosum subsp. andigenum), and the majority of accessions of wild ancestors from the northern members of the S. brevicaule complex. Another group B comprised most of the wild species accessions and almost exclusively hybrid cultivated species which have introgressed plastid genomes from the other wild gene pools. Lack of clustering of traditional cultivated species (as used above) support a revised group classification of S. tuberosum.     

Identification of the tetrad-sterility-causing Solanum stoloniferum Schltdl. & Bouché cytoplasm in interspecific hybrids with S. tuberosum L.

Tetrad sterility, in which only clumps of four premature pollen grains are released from anthers, has been observed in some modern potato cultivars. It is a form of cytoplasmic male sterility caused by the cytoplasm derived from the Mexican tetraploid species Solanum stoloniferum Schltdl. & Bouché., an important source of resistance to Potato virus Y in potato breeding. However, since S. stoloniferum is highly polymorphic, the source of tetrad-sterility-causing cytoplasm is unknown among diverse S. stoloniferum accessions. In this study, we directly crossed 24 S. stoloniferum accessions with pollen from 4x?S. tuberosum and obtained 39 hybrids from 12 accessions. Nineteen hybrids from six accessions showed tetrad sterility, with either D/γ- or W/γ-type cytoplasm, and were triploid, tetraploid, or hexaploid. The W/γ-type cytoplasm was not necessarily associated with tetrad sterility. Sequence comparisons of 17 mitochondrial genes and their intergenic regions revealed a length polymorphism in the intergenic region between rpl5 and rps10, in which an amplified band of 859 bp was associated with tetrad sterility. This specific cytoplasm causing tetrad sterility is named TSC_sto. The 859-bp band will be a useful diagnostic marker for identifying TSC_sto in potato breeding.

     

Quantitative Trait Loci for Seed Dormancy in Wild Barley (Hordeum spontaneum C. Koch)

A quantitative trait locus analysis was carried out to unravel the genetic basis of dormancy in wild barley (Hordeum spontaneum) from Israel. Two accessions, Ashkelon and Mehola, from divergent environments were crossed to produce a mapping population. A linkage map was produced from the F₂ population, and F₄ seeds were used for germination experiments. Five quantitative trait loci (QTL) were detected for dormancy across the different germination experiments. These QTL were found on chromosomes 1, 2, 5, 6 and 7. The variation explained by each QTL varied between 8 and 25%. Ashkelon alleles increased the germination except for the QTL on chromosome 5. Three out of these five QTL co-locate with QTL found earlier in cultivated barley crosses, although this does not necessarily imply that they would be the same loci. The level of dormancy is much higher in wild barley than in cultivated barley and wild barley may have alleles that have not yet been utilised in breeding for optimally dormant barley.     

The genome donors of the groundnut/peanut (Arachis hypogaea L.) revisited

Arachis hypogaea, the cultivated groundnut is a tetraploid with an AABB genomic constitution. The available literature on the origin of groundnut reveals that there is general agreement that the cultivated groundnut has evolved from the wild tetraploid species A. monticola, with which it crosses freely to produce fertile hybrids. However, the issue of actual diploid ancestors of A. monticola is still unresolved. Both cytogenetic and molecular evidences support A. duranensis being the most probable progenitor and donor of the A genome to A. hypogaea. For the B genome, the cytogenetic evidence suggests A. batizocoi to be the most probable progenitor, but the RFLP banding pattern indicates that A. batizocoi is more distantly related to A. hypogaea than other species of section Arachis. RFLP banding pattern indicates A. ipaensis to be one of the closest species to A. hypogaea and the possible donor of the B genome. The present article critically analyzes the available data, which suggests that until an amphidiploid is produced synthetically between A. duranensis × A. ipaensis and crossed successfully with A. hypogaea to produce a fertile hybrid, this issue would remain unresolved. A. batizocoi would remain the most probable donor of the B genome because of its directly demonstrable cytogenetic affinity.     

Brassica sect.Brassica (Brassicaceae)

Summary An extensive crossing program including 10 wild taxa and 23 accessions representing 6 major cultivated forms and landraces of theBrassica oleracea group was carried out. Data for crossability, germination, viability, fertility inF ₁ andF ₂ were studied as well as the meiotic chromosomal pairing inF ₁ hybrids in some combinations. The fertility of hybrids between all cultivated forms and wild populations ofB. oleracea was high and it was concluded that all, including the Chinese endemicalboglabra form, belong to the same biological species. Among the other species,B. macrocarpa, B. montana, andB. rupestris had the lowest andB. cretica the highest fertility values in crosses with cultivated forms. The crossing data do not give any further information as to the genetic differentiation ofB. oleracea during the course of domestication. Introgression between wild and cultivated forms has probably occurred frequently. All members of theB. oleracea cytodeme belong to the primary and secondary gene pools of the 18 chromosomic crops and are as such of great importance for breeding purposes.     

The extent of embryo and endosperm growth following interspecific hybridization between Cicer arietinum L. and related annual wild species

In vivo interspecific pollinations were performed and immature seed development investigated by histological methods in order to study crossability barrier(s) in Cicer L. species wide hybridization. Seven of the eight wild annual Cicer species, belonging to the secondary and tertiary gene pools, were used in reciprocal crosses with the cultivated chickpea. It was confirmed that the zygote was formed in all interspecific crosses. The embryos showed continued and retarded growth at different rate in various crosses, but eventually aborted at an early pro-embryo stage in all crosses except C. arietinum L. ×C. echinospermum Dav. Reciprocal cross differences were observed in early embryo growth rate and could have implications in obtaining hybrids. This study further emphasizes the necessity for developing appropriate and efficient in vitro procedures for rescuing immature globular hybrid pro-embryos, which will make the wild Cicer gene resources amenable to chickpea improvement.     

Chloroplast DNA Variation in the Most Primitive Cultivated Diploid Potato Species Solanum stenotomum Juz. et Buk. and its Putative Wild Ancestral Species using High-Resolution Markers

Solanum stenotomum Juz. et Buk. (2n = 2x = 24) is considered to be the most primitive diploid cultivated species from which all the other Andean cultivated potatoes were originated (Hawkes 1990). To disclose chloroplast DNA (ctDNA) variability and the maternal origin of S. stenotomum, 36 accessions of S. stenotomum and 86 accessions of putative wild ancestral species were determined for ctDNA types and analyzed by high-resolution markers (seven ctDNA microsatellites and an H3 marker). High-resolution markers discriminated 57 different ctDNAs (haplotypes), which were classified into the W-type ctDNA group and C-, S- and A-type ctDNA group, and within the latter group S- and A-type ctDNAs were distinct from each other among many different haplotypes mostly having C-type ctDNA. This ctDNA relationship supported our previous findings obtained for mostly Andean cultivated species (Sukhotu et al. 2004). Compared with other putative ancestral wild species, S. stenotomum showed somewhat limited ctDNA diversity, having two major haplotypes 1 and 2 also found in different wild species in different places. Therefore, the ctDNA in S. stenotomum was of at least dual origins either by successive domestication from different species or else by introgression after initial S. stenotomum arose.     

The wild ancestry of the cultivated artichoke

Summary The genetic affinities between the cultivated artichoke Cynara cardunculus L. var. scolymus (L.) Flori (= C. scolymus L.) and its wild relatives were tested by means of a crossing programme. The following wild taxa were involved: (i) wild cardoon C. carcundulus L. var. sylvestris (Lamk) Fiori, (ii) C. syriaca Boiss., (iii) C. cornigera Lindley, (iv) C. algarbiensis Cosson, (v) C. baetica (Spreng.) Pau (= C. alba Boiss.), and (vi) C. humilis L. Only the wild cardoon was found to be fully cross-compatible and fully infertile with the crop. In contrast, all other five wild Cynara species turned to be almost fully-or fully-cross-incompatible with the crop, and the few interspecific F₁ hybrids recovered were partly or almost fully sterile. These finds establish the wild cardoon as the wild ancestor of the cultivated vegetable.     

Adaptive values of wild × cultivated sorghum (<Emphasis Type="Italic">Sorghum bicolor</Emphasis> (L.) Moench) hybrids in generations F<Subscript>1</Subscript>, F<Subscript>2</Subscript>, and F<Subscript>3</Subscript>

Gene flow between cultivated and their wild relatives is one of the main ecological concerns associated with the introduction genetically modified (GM) cultivars. GM sorghum cultivar has been developed and its commercial production may be possible in the near future. The rate of gene flow depends on the fitness of wild × cultivated sorghum hybrids. The study aimed at estimating adaptive values of wild × cultivated sorghum hybrids in generations F₁, F₂, and F₃ compared to their parents. Artificial crosses of four wild sorghums, five cultivated sorghums, and two male sterile lines were made to produce the F₁ generation, which were advanced to F₂ and F₃. Each hybrid generation and their respective parents were evaluated for their adaptive value at two sites in a randomised complete block design with seven replicates. The resulting progenies did not show serious fitness penalties. Some hybrids were as fit as their respective wild parents and no consistent differences exist between the three generations studied. Thus, the resultant wild × cultivated hybrids may act as avenue for introgression.     

Genetic variation within and among species of genus Arachis, section Rhizomatosae

The genus Arachis is endemic to South America and comprises 80 species, 69 of which have already been described and eleven not yet published. The genus includes the cultivated peanut (A. hypogaea) and several forage species, the most important ones being A. glabrata and A. pintoi. Accessions of section Rhizomatosae, including three tetraploid species 2n = 4x = 40 (A. glabrata, A. pseudovillosa and A. nitida nom. nud.) and one diploid species 2n = 2x = 20 (A. burkartii), were evaluated using RAPD markers to assay genetic variability within and among species. The ten random primers used yielded a total of 113 polymorphic bands. The data were scored as the presence or absence of each band in each sample. A distance matrix and dendrogram were obtained using Link's coefficient and the neighbor-joining method. Most accessions analyzed grouped into two major clusters: the first comprised most accessions of A. glabrata and accessions of A. nitida, and the second cluster comprised accessions of A. burkartii. Arachis pseudovillosa and a few accessions of A. glabrata and A. nitida were placed between these major clusters. The diploid and tetraploid species were grouped quite separately, suggesting that the tetraploids did not originate from the diploid species analyzed.