Crossability relationships of andean potato varieties of three ploidy levels

Summary The formation of triploids in 4x×2x crosses was studied in primitive potato varieties from the Andes of Peru and Bolivia. Triploid frequencies were as high as 80–100% in some crosses, although the actual number of seeds was low. The triploid block varied according to the tetraploid parent, and was never entirely absent. The likelihood of triploid formation was studied in 4x×2x crosses where pollen from both tetraploids and diploids of known genotype was used. Even though the diploids were chosen for their non-production of 2n gametes, as detected in pollen grain size, 66% of the hybrids were tetraploid. The amount and probability of gene-flow between the tetraploid and diploid gene-pools, either directly through 2n gametes or through triploid bridges was examined. Most of the 20 cultivars of S. x chaucha produced seed in crosses with tetraploids and diploids, although the fertility of the latter crosses was four times higher. The female fertility of some of the triploids thus allows them to exchange genes with the tetraploids and diploids, although gene-flow occurs more rapidly through direct transfer from diploids to tetraploids via 2n gametes.     

Somatic hybrids between navel orange (Citrus sinensis) and grapefruit (C. paradisi) for seedless triploid breeding

Protoplasts from cell suspension cultures of ‘Bonnaza’ navel orange (Citrus sinensis (L.) Osbeck) were electrically fused with mesophyll protoplasts isolated from seedless ‘Red Blush’ grapefruit (Citrusparadisi). After 6 months of culture, a total of 20 plants were regenerated. Root tip chromosome counting revealed that 4 of them were tetraploids (2n = 4x = 36)and the rest were diploids (2n = 2x = 18) morphologically resembling the mesophyll parent. After 6 months of transplantation into the greenhouse, 4 of the diploidmesophyll regenerants unexpectedly flowered, but this phenomenon disappeared in the next year. This is the first report of precocious flowering in citrus via protoplast fusion. RAPD analysis further confirmed that the tetraploid regenerants were somatic hybrids while the diploid regenerants were mesophyll parent type. This somatic hybrid will be utilized as a possible pollen parent for improving the seedy pummelo cultivars in China by producing triploid seedless pummelo hybrid. The mechanism of early flowering was also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cross compatibility between diploid and tetraploid perennial ryegrass (Lolium perenne L.)

Summary A number of crosses were made between diploid and tetraploid Lolium perenne plants in order to determine the degree of cross compatibility between the two different ploidy levels. The range of hybridization undertaken involved diploidxdiploid, and tetraploid x tetraploid control crossings, diploid x tetraploid crosses, tetraploid x diploid crosses and mixed pollinations. The seed set, the germination capacity of resultant hybrid seeds, and the chromosome constitution of derived seedlings was determined. In addition attempts were made to culture 12 day-old hybrid embryos from the diploid x tetraploid and reciprocal crosses.The crossing results obtained indicated no barrier to fertilization between diploids and tetraploids, but the irregular and poor development of the seed accompanied by much reduced germination, indicated unsatisfactory endosperm establishment of large numbers of triploids, as confirmed by the result of culturing embryos dissected from 12-day old seeds.In preliminary studies undertaken to determine the extent to which diploid and tetraploid ryegrass varieties intercross when grown in close proximity for seed production, Griffiths and Pegler (1966) observed a very low incidence of triploids amongst the progenies of diploid plants exposed to the pollen of tetraploid plants and also amongst the progenies of tetraploid material exposed to the pollen of diploid plants.In these and subsequent studies it was noted that only a very small proportion of the so-called fully formed seeds derived from diploid x tetraploid crosses did actually germinate. When, in 1964, propagules of clones of S.24 perennial ryegrass, well separated from one another, were placed in drills of the tetraploid perennial ryegrass variety Reveille, the proportion of florets which formed caryopses, and of caryopses which germinated was as follows (Table 1).The data (Table 1) clearly indicate that a considerable proportion of the S.24 florets had been fertilized by 2n pollen and had formed caryopses, but most of these had proved to be defective. As noted in the previous studies, the frequency of triploid seedlings was low, thus suggesting incompatibility between the n and 2n gametes for the production of viable zygotes.Of the total progenies classed as tetraploids in the early seedling stage, 80% proved to be ryegrass x tall fescue F₁ hybrids. These arose as a result of free crossing with     

Reproductive diversity and strategies for breeding in St. John's wort (Hypericum perforatum L.)

St. John's wort (Hypericum perforatumL.) is a medicinal crop plant that reproduces by apomixis. To study the reproductive diversity within the initial breeding material, 656 plants from 92 accessions of the species were analysed by flow cytometric seed screening (FCSS) and compared to 66 plants of cv. ‘Topaz’ as a control. Among plants from the 92 accessions, 16 were obligate sexuals, nine were obligate apomicts and all remaining ones were facultative apomicts. Among controls, there were six obligate and 60 facultative apomicts. Tests of reproductive pathways indicated that, in the investigated collection, all apomictic plants were tetraploid and all sexual plants were diploid. A strategy for a cross breeding program in St. John's wort is illustrated by crossing one diploid obligately sexual mother plant with six tetraploid facultative apomicts. All 91 F₁ progeny obtained were triploid with 58% being obligate sexuals and 42% facultative apomicts. To quantify the different routes of seed formation within facultatively apomictic individuals, the reproductive pathways were reconstructed from about 30 single seeds from each of five tetraploid and two triploid plants using FCSS. The frequency of seed formed by apomixis ranged from 54% to 67% among four tetraploid genotypes. One triploid and two tetraploids formed 23% to 73% of the so-called B_III hybrids (fertilization of unreduced eggs) and one triploid was obligately sexual. The results show that while the mode of reproduction in H. perforatum is highly variable, tetraploid obligate apomicts as well as diploid and triploid obligate sexuals can be selected as starting material for breeding purposes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selection from Solanum tuberosum group phureja of genotypes combining high-frequency haploid induction with homozygosity for embryo-spot

Summary In this article the term pollinators refers to diploid genotypes of Solanum tuberosum Group Phureja which are used to pollinate autotetraploid potatoes for the induction of autonomous growth of unfertilized egg cells. Plants obtained from such egg cells contain chromosomes of the female only and are called dihaploids. The successful selection of ideal pollinators from Group Phureja is described. A pollinator is considered to be ideal, if it has good male fertility, homozygosity for the dominant seed marker embryo-spot and the ability to induce in autotetraploid cultivars a high number of dihaploids both per berry and per 100 seeds. Embryo-spot is a deep purple or reddish coloration at the base of the cotyledons of the embryo, visible on both sides of the flat seeds. It is one manifestation of a set of complementary genes with pleiotropic action, causing a concentration of anthocyanins at the base of all plant organs that are homologous to leaves. The complementary genes are: a basic gene P for anthocyanin production (this is the seedling marker coloured hypocotyl) and allele B ^d of locus B. Thirty-three homozygous B ^d B ^d PP plants and 41 B ^d bPP plants were identified and a number from both classes tested as pollinator onto varieties.Among the homozygous group a number of ideal pollinators could be detected. Unexpectedly a few polyploids grew from spotless seeds when using homozygous pollinators on varieties; 1.4% showed purple hypocotyl and 2.6% were matroclinous. The latter group of polyploids probably may have arisen from unreduced egg cells of the cultivars used.In the progenies from crosses between recessive cultivars (bbbbpppp) and heterozygous pollinators (B ^d bPP) the ratio of spotted: spotless hybrid seeds appeared to deviate greatly from 1:1. This is ascribed to unreduced gametes, which originate during meiosis, presumably through lack of formation of the reductional (= first division) cell wall. On this assumption a map distance of 30 and 24 cross-over units is estimated for locus B on the basis of the proportion of nulliplex tetraploid hybrids in two different crosses.High- and low-seed-set pollinators were found at a ratio of 8:21 suggesting monogenic dominance of low seed-set. Seeds per berry and haploids per berry are significantly correlated using low-seed-set pollinators on cultivars, but they are not when pollinators are used that produce high numbers of seeds per berry. A hypothesis is put forward to explain this phenomenon.     

Flow cytometric identification of sexually derived polyploids in hop (Humulus lupulus L.) and their use in hop breeding

The New Zealand hop breeding programme is based solely on the development of seedless triploid cultivars. This relies on the use of tetraploid parents. While a sexually derived tetraploid parents have been used successfully, sexually derived tetraploids offer a useful alternative. They may have a higher level of heterozygosity and are easier to obtain. Methods for the identification of tetraploids from seedling populations by flow cytometry are described. Two studies were conducted; one on field-grown plants, the other on plants grown in a glasshouse. Approximately 15%of seedlings in the two studies were identified as tetraploids (2n = 4x = 40). Between 70 and 80% were identified as either diploid (2n =2x = 20) or triploid (2n = 3x = 30). Remaining plants were haploid, pentaploid or probably aneuploid. Within triploid and tetraploid populations from 40to 50% of seedlings were female, 30 to40% were monoecious, with the remainder being males and non-flowering plants. Sexually derived tetraploid parents have been used successfully in the breeding programme, from which several promising triploid selections have subsequently been made. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aneuploids of perennial ryegrass (Lolium perenne)

Summary Aneuploid plants of perennial ryegrass (Lolium perenne L.) with 2n=15 to 30 chromosomes were obtained by crossing a near-triploid (2n=3x+1=22) with a diploid or on open-pollination with diploids and tetraploids. Aneuploids occurred with a frequency of 83% in near triploid × diploid progeny and 92% on open-pollination with diploid and tetraploid plants. Aneuploid plants with 15 to 18 chromosomes resembled diploids in morphology and those with 19 to 30 chromosomes were akin to tetraploids. Meiotic studies suggested that most aneuploid plants resulted from transmission of aneuploid egg cells (n=8 to 23). Aneuploid plants with 2n=27 to 30 chromosomes in the progeny of 22×14 cross originated from unreduced egg cells. Plants with 19 to 21 chromosomes were recovered only by immature seed culture. Aneuploid plants with 26 to 30 chromosomes and triploids (2n=21) had higher pollen fertility and bigger seeds than plants with 15 to 22 chromosomes.     

The production and behaviour of hexaploid hybrids between Hordeum vulgare and H. Bulbosum

Summary To produce hexaploid (or other polyploid) hybrids, diploid or tetraploid Hordeum vulgare was crossed with hexaploid or octoploid H. bulbosum, and perennial triploid hybrids between the two species were treated with colchicine. The crosses did not yield viable plants: seedset was low, the seed aborted and embryo culture was unsuccessful. The colchicine treatments geve rise to plants in which hexaploid chromosome numbers were observed. At the hexaploid level chromosomal instability occurred, resulting in chromosome elimination.The colchicine-treated triploid hybrids showed in the first years after the treatment better fertility after open flowering than untreated plants, but the level of fertility remained very low. The offspring consisted of haploid, diploid and approximately triploid plants like H. vulgare, tetraploid and approximately tetraploid plants like H. bulbosum, and plants with hybrid morphology and unstable chromosome number, which were highly sterile. Thus the crossing barrier between H. vulgare and H. bulbosum could not be broken down at higher ploidy level.     

Polyploidy and sexual polyploidization in the genus Vaccinium

Genus Vaccinium,consisting of blueberries, cranberries, lingonberries and many related wild species, includes diploid, tetraploid, and hexaploid species. Most evidence indicates that the tetraploid species are autotetraploids, with non-preferential bivalent chromosome pairing. Although homoploid interspecific crosses usually produce numerous fertile hybrids if the parents are from the same section of the genus, inter sectional crosses at the diploid level normally produce no seedlings, weak seedlings, or seedlings that are very low in fertility. There is a strong but not complete triploid block within Vaccinium. Even with insections, tetraploid × diploid (and the reciprocal) crosses normally give only a few tetraploid and a few triploid hybrids. Hexaploid × diploid crosses within sections are very hard to make, and the few hybrids that have been obtained are pentaploid. The frequency of 2n gametes varies,both among genotypes within species and among species. Vaccinium pollen is normally shed in tetrads, and the frequency of large pollen grains shed in dyads can be used to estimate 2n gamete frequency. Cultivated blueberries occur at both the tetraploid and the hexaploid levels, and there are important genetic resources in the diploids. Unreduced gamete production has permitted transfer of genetic material from the diploid to the tetraploid level and from the diploids and tetraploids to the hexaploid level via triploid hybrids. Intersectional crosses can occasionally produce tetraploid Vaccinium hybrids that appear to behave as amphidiploids and have medium to high fertility. CommercialVaccinium cultivars are normally propagated by cuttings. Intersectional hybridization, chromosome doubling, and asexual propagation could permit the production of novel hybrid combinations with value as ornamentals or in fruit production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bilateral sexual polyploidization in potatoes

Summary Bilateral sexual tetraploidization was achieved by intercrossing diploid Phureja × haploid Tuberosum hybrids. Both parents contribute gametes with the somatic number of chromosomes (diplogynoids and diplandroids). More than 100 tetraploid progeny appeared in nine 2x–2x families. The tetraploids are generally more vigorous and significantly outyield their diploid full-sibs. The superiority of the tetraploids is interpreted on the basis of the mode of diplandroid (2n pollen) formation by first meiotic division restitution (FDR). Heterozygosis, epistasis and genetic diversity, if present in the parental diploid genotype, are largely maintained in the 2n gametes formed by FDR. and syngamy brings about more opportunities for heterotic responses in conjunction with polyploidization. Differences in tuber yield among tetraploid clones within families were found to be statistically significant (P <0.05). Thus, unlike somatic chromosome doubling, which can merely increase chromosome numbers, or somatic hybridization, which could in addition bring about heterosis, sexual polyploidization is also associated with genetic variability. The degree of inbreeding and the genotypic structure in the population of newly arisen tetraploids is discussed. The relationship among asexual reproduction, preservation of favorable nonadditive genetic effects by FDR. restoration of sexuality, and polyploidization is explored. The possible evolutionary significance of these factors is emphasized.     

Polyploidy in the glasshouse cucumber (Cucumis sativus L.)

Summary An autotetraploid form of the parthenocarpic variety Butchers Disease Resisting was produced by colchicine treatment. Immersing the seeds in water before treatment in 0.2% colchicine for 24 hours gave the highest percentage of vigorous tetraploids. The yield of fruit from tetraploids was better than the diploids at first but it declined later in the cropping period owing to slow sideshoot production.Self-fertility of autotetraploids was lower than diploids. When crossed with diploids, triploid plants were obtained. These were self-sterile but they could be crossed with diploids to give trisomic plants. Aneuploids with one additional chromosome were found amongst tetraploids and triploids.     

Effect of ploidy on stomatal and other quantitative traits in plantain and banana hybrids

Summary Ploidy polymorphism occurs in the hybrid offspring derived from interspecific crosses between triploid plantains (Musa spp. AAB group) and diploid bananas (M. acuminata). Therefore,Musa breeders are interested in the determination of ploidy and its effects on phenotypic expression of quantitative traits. The aim of this research was to examine the reliability of stomatal and other phenotypic traits to determine ploidy in segregating plantainbanana hybrid families. Stomatal density and size were significantly correlated (P<0.01) with ploidy, although the correlation coefficients were not high (r=–0.49 and r=0.47, for stomatal density and size, respectively). High density of small stomata was correlated with low ploidy level, and vice versa. However, stomatal size and density were also influenced by a significant genotype effect (P<0.001) within the same ploidy level. Ploidy had an important effect on fruit traits and plant height in the hybrids of Obino l'Ewai×Calcutta 4, but this was not so clear in Bobby Tannap×Calcutta 4 hybrids. Obino l'Ewai derived tetraploids have medium to tall plants with large bunches and big fruits. Most of the tetraploids derived from Bobby Tannap have short stature due to the gene action of the dwarf,dw, allele. Also, a few selected diploids derived from Bobby Tannap outyielded their non-selected tetraploid full-sibs. In conclusion, chromosoem counting remains the only accurate proof of ploidy levels inMusa germplasm.     

The breeding of the cultivated potato species solanum x juzepczukii and S. x curtilobum II. The resynthesis of S. x juzepczukii and S. x curtilobum

Summary The possibility of combining anew the genomes of wild and cultivated progenitors of triploid S. x juzepczukii and pentaploid S. x curtilobum by following the known evolutionary pathway of these species was investigated.The resynthesis of S. x juzepczukii was easy, and a wide range of synthetic forms was bred. Among these were forms with higher frost resistance (-5°C) than has been found in natural S. x juzepczukii. The total tuber glycoalkaloid content of several synthetic hybrids was lower than or as low as that of natural clones. Most synthetic hybrids were more vigorous than natural S. x juzepczukii and produced about the same types of tubers as are found in the natural range of variation. The best diploid parents were found in the species S. goniocalyx.The attempt to resynthesize pentaploid S. x curtilobum has not been successful but tetraploid plants were obtained in the process. An explanation for the occurrence of tetraploids resulting from triploid x tetraploid and/or diploid crosses is offered.The newly bred tetraploids contain at least one genome from S. acaule (possibly two) and hybridize easily with ssp. andigena. They thus provide a means for the transfer of S. acaule germ plasm into the tetraploid cultivated gene pool which would profit from the frost resistance of S. acaule.     

Production of intraspecific aneuploids in the genus Solanum. 1. Doubling the chromosome number of seven diploid species

Summary The variation which persisted in interspecific aneuploids even after one or two generations of backcrossing prevented the morphological distinction of the aneuploids from their diploid sibs. An attempt was made to unmask the effects of aneuploidy by producing aneuploids within the more uniform genetic background of a single species.The procedure to produce trisomics and aneuploids of diploid species was to vegetatively double clones or seed of the species with colchicine, cross the induced tetraploids to diploids to produce triploids, and then cross the triploids to the original diploids. An attempt was made to use the same diploid clone and/or introduction of a species in all successive stages of the procedure in order to obtain as much uniformity as possible. This was largely unsuccessful because self-incompatibility and inbreeding reduced seed set. Seventy-three different tetraploids were obtained from seven species. Tuber treatments were noticeably unsuccessful in producing doubled plants. The two vegetative treatments were most successful and produced 41 different tetraploid clones while 30 doubled plants were obtained from four seed treatments.Morphologically the tetraploids resembled the diploid and there was no noticeable increase in size of leaflets and floral parts in the tetraploid. The average number of chloroplasts in ten pairs of guard cells was a distinctive feature which could efficiently be used in distinguishing the induced tetraploid plants from diploids. The tetraploids were generally less fertile than their diploid counterparts and the majority of them exhibited self-compatibility in comparison to self incompatibility of the diploids.Cooperative investigations of the Agricultural Research Service, U.S. Department of Agriculture and the Wisconsin Agricultural Experiment Station. Supported in part by grants from the Research Committee of the Graduate School, University of Wisconsin.     

Chromosome numbers of hybrid tuberous begonias

Summary The somatic chromosome numbers of 67 hybrid tuberous begonia cultivars (Begonia x tuberhybrida Voss) have been determined. The material concerned was collected in Belgium and England and represents several distinct groups, comprising both cultivars originated at the turn of this century and those introduced recently.The old Pendula group is characterized by the diploid chromosome number 28. Among the Multiflora group diploid, triploid and tetraploid cultivars are represented almost equally, while the investigated Multiflora maxima and large-flowered begonias proved to be tetraploid except for one triploid.Mitotic instability, to the extent reported by some authors, was not observed. The variation of the diploid number between 27 and 28, the triploid number between 41 and 42 and of the tetraploid number between 52 and 56 can be explained as a consequence of the fact that in the original hybridization wild tuberous species with 2n=26 and 2n=28 were involved.Publication 351, Laboratorium voor Tuinbouwplantenteelt, Landbouwhogeschool, Wageningen, the NetherlandsPublication 23, Rijksstation voor Sierplantenteelt, Melle, Belgium     

Somatic hybrids between Gossypium hirsutum L. (4×) and G. davidsonii Kellog (2×) produced by protoplast fusion

Somatic hybrids were obtained from electrofused protoplasts derived from embryogenic suspension cultures of tetraploid cotton (G. hirsutum L. cv. Coker 201) and embryogenic callus of diploid wild cotton G. davidsonii. The regenerants were initially identified as hybrids by RAPD (random amplified polymorphic DNA) analysis. Subsequently, observation on chromosome counting, morphology and SSR (simple sequence repeat) confirmed the hybrid status. Cytological investigation of the metaphase root-tip cells of the regenerated plants revealed there were 74 to 84 chromosomes in the plants, close to the expected 78 chromosomes. SSR analysis revealed the regenerated plants contained specific genomic fragments from both fusion partners, further confirmed their hybridity. The morphology of the plants was intermediate between the two fusion partners. The regenerants were difficult to develop into mature plants because their roots browned and they wilted from the stem apex before forming 3 to 5 true leaves. The hybrid plants were transferred to soil by grafting in vitro onto rootstocks.     

Bilateral polyploidization inDactylis glomerata L. subsp.lusitanica: occurrence,morphological and genetic characteristics of first polyploids

Summary InDactylis glomerata L. subsp.lusitanica, triploid and tetraploid plants were obtained by bilateral sexual polyploidization in crosses between diploid parents known to produce 2n gametes. The polyploid and diploid progeny were compared for allozyme diversity (allele number and heterozygosity), phenological (pollen fertility, inflorescence emergence date), cellular (stomatic cell size) and morphological characters (vegetative biomass, seed weight, total seed number per plant, panicle number and seed number per panicle length) in four contrasting growing conditions. Stomatal cell size and the total number of alleles were significantly lower in diploids than in polyploids which sometimes recovered more than two distinct alleles at a locus via 2n gametes possessing heterozygous loci. Pollen fertility was similar in diploids and tetraploids, but significantly lower in the triploids. Seed weight was significantly higher in polyploids, although the average number of seeds per panicle length and the total number of seeds was higher in the diploid sibs. No phenological differences occurred among ploidy levels whereas such differences were observed inDactylis in all the natural areas of sympatry between diploids and their related polyploids. Our results provide evidence for an instantaneous effect of polyploidization on certain traits and suggest that natural selection would act subsequently on several other traits to allow the establishment of new polyploids.     

Development of triploid hybrids in asparagus breeding employing a tetraploid landrace

The development of triploid cultivars from crosses between the tetraploid landrace ‘Morado de Huetor’ and diploid commercial cultivars was studied in this work. Fertility of both 4x × 2x and 2x × 4x crosses, as measured by fruit set and number of seeds per fruit, and germination rate of triploid seeds, were studied in order to assess the viability of producing commercial seeds. Seven triploid hybrid progenies were included in a progeny test together with thirteen experimental tetraploid hybrids and four cultivars (‘Grande’, ‘UC157-F2’, ‘Purple Passion’ and ‘Morado de Huetor’). Marketable spear production and mean diameter of spears were evaluated in both 2007 and 2008. Crosses 4x × 2x were more successful than 2x × 4x, and a wide variation from high to very low fertility within tetraploid females was found. Germination rate of triploid seeds was similar to diploid ones. After 2 years of evaluation experimental hybrid triploids were more productive than tetraploids with values similar to the highest yielding diploid cultivar (‘Grande’). Spear diameter of triploid hybrids was in general thicker than the tetraploid hybrids and significantly thinner than the standard cultivars ‘Grande’ and ‘Purple Passion’ (tertraploid). These results suggest that the development of new triploid cultivars (4x × 2x) employing a tetraploid landrace from a different genetic background of current diplods could be interesting in asparagus breeding, broadening the spectrum of cultivated asparagus cultivars.     

Polygon of crossability between eleven species of Ipomoea: section Batatas (Convolvulaceae)

Summary Crossability within Ipomoea section Batatas is complex because of genetic, cytogenetic and physiological interactions. This complexity is responsible for the fact that knowledge about phylogenetic relationships within this section remains preliminary. Between 1988 and 1991, studies of planting, pollination and evaluation of progenies were carried out at the facilities of CIP in La Molina and San Ramon, Peru. Self-compatibility was found in nine diploid species. Self-incompatibility was found in diploid I. trifida, tetraploid I. tiliacea and both tetraploid and hexaploid I. batatas. Sexual compatibility is though to be related to a multiallelic sporophytic incompatibility system which is expressed in the stigmatic papillae. Altogether, 4,162 cross pollinations were made between 11 species and 76 interspecific combinations of 110 possible combinations in a diallel 11×11 design. From these 76 interspecific combinations only 38 survived, and in these crossability (= proportion between number of fruits harvested/pollinations made) ranged from 0.01 to 1.00 at La Molina. At San Ramon, 11 out of 17 cross combinations were successful, and in these crossability ranged from 0.01 to 0.71. Crossability appeared to be influenced by latitude which is related to flowering synchrony. The early death of seedlings is hypothesized to be related to a genomic imbalance between embryo and endosperm. Maternal effects were found in crosses between diploids and tetraploids which produced triploid and diploid offspring, and in crosses between hexaploids and diploids which gave rise to hexaploid offspring. This study shows that I. trifida and I. x leucantha may act as bridge species for gene flow from wild Ipomoea species to the genepool of the sweetpotato.     

Controlled pollination transfer of a nuclear male-sterile gene from a diploid to a tetraploid watermelon line

Summary A recessive male-sterility (ms) gene was transferred from a diploid to a tetraploid watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) line through a series of controlled pollinations. Progeny testing and morphological characteristics confirmed the development of tetraploid lines containing the ms gene. The possible use of this gene transfer system in the improvement of tetraploid watermelon cultivars and the use of male-sterile tetraploids in the production of seedless triploid watermelons is discussed.