2x Crosses Research Articles

Interploid crosses in heather (Calluna vulgaris)

Heather (Calluna vulgaris), an important bedding plant in Northern Europe, is a monotypic genus with rather limited phenotypic variability. Hence, it is aimed at starting a breeding program of polyploids that might constitute a new phenotypic class of varieties with attractive strong habitus and larger flowers. As a basis, data on fertility of genotypes of different ploidy levels, fertility of interploid crosses, and ploidy levels of the offspring were generated by setting up a complete interploid crossing experiment in C. vulgaris including diploid (2x), triploid (3x), and tetraploid (4x) genotypes.Determination of pollen viability by staining, in vitro germination and evaluation of pollen tube growth in vivo generated contradicting results and none of the methods proved to be suitable to explain fertilization success as determined by seed set. Seed yield was significantly reduced in any cross direction compared to 2x×2x crosses. However, the germination rate did not correlate with seed set, hence for determining crossing success, the number of germinating seedlings has to be considered. The rate of germinating seedlings per pollinated flower was 2.95 in the 2x×2x crosses and on average only 0.025 in the interploid crosses. 2x×4x and 4x×2x crosses differed significantly with regard to seed yield, germination rate and ploidy of the offspring. 3x genotypes were generated only by 4x×2x crosses. Aneuploids resulted from 2x×3x, 3x×4x, 4x×2x and 4x×3x crosses.

2n megagametophyte formed via SDR contributes to tetraploidization in polyembryonic 'Nadorcott' tangor crossed by citrus allotetraploids.

2 n megagametophyte formation plays an important role in polyploidization in polyembryonic citrus and is valuable for plant improvement. Tetraploid plants are frequently observed in the seedlings of diploid polyembryonic citrus genotypes. However, the mechanisms underlying the formation of tetraploids are still indistinct when apomictic citrus genotypes are used as female parent to cross with tetraploids. Herein, 54 tetraploid progenies, which were unexpectedly obtained previously from four 2x × 4x crosses using polyembryonic 'Nadorcott' tangor as seed parent, were analyzed by 22 simple sequence repeat (SSR) markers, aiming to reveal their genetic origin and the mechanism underlying 2n megagametophyte formation. The results showed that 13 tetraploids from all these four crosses were doubled diploids as indicated by their identical SSR allelic profile with their female parent; while the remaining 41 tetraploids apparently exhibited paternally derived alleles, which confirmed their zygotic origin. Furthermore, the genotyping of all hybrids indicated that all of them arose from 2n megagametophytes. Based on the genotypes of 2n megagametophytes, the analysis of maternal heterozygosity restitution (HR) for each marker showed that it varied from 0.00 to 87.80 % with a mean value of 40.89 %. In addition, it was observed that 13 markers displayed a lower rate than 50 %. On the basis of the above results, it can be speculated that the second division restitution (SDR) is the mechanism underlying the 2n megagametophyte formation in 'Nadorcott' tangor. The elucidation of the mechanism of 2n megagametophyte formation will be of great help to optimize further sexual hybridization for polyploids in citrus.

Interploid and intraploid hybridizations to produce polyploid Haskap (Lonicera caerulea var. emphyllocalyx) plants

We produced polyploid Haskap (Lonicera caerulea var. emphyllocalyx) plants by performing interploid and intraploid crosses of wild accessions. Embryo rescue in a tetraploid (4x) × diploid (2x) cross produced triploid plants; reciprocal 2x × 4x cross failed to produce viable seeds. Intraploidy crosses of 4x × 4x produced mostly tetraploids but also several hexaploid (6x) and octoploid (8x) plants. Using hexaploids obtained from this cross, we examined reciprocal 4x–6x crosses and found that both produced pentaploid plants. An octoploid was produced by applying colchicine to a tetraploid; a 4x × 8x cross using this plant and aided by embryo rescue culture produced three hexaploid plants, with an aneuploid number of chromosomes. Several plants obtained in this study flowered and set fruits. We discuss the overall efficiency of producing polyploid plants in interploid and intraploid crosses.

Ploidy manipulation and introgression breeding in Darwin hybrid tulips

Meiotic polyploidisation via crossing with 2n gamete producing genotypes and interploidy crosses are two of the main methods currently used to obtain polyploid tulips. In our study diploid 2n gamete producing F-1 hybrids of Darwin hybrids (Tulipa gesneriana x Tulipa fosteriana) and triploid hybrid resulting from 'Rhodos' x 'Princeps' cross were used as pollen donor and crossed with cultivars of T. gesneriana in the following combination: 2x x 2x, 3x x 2x, 2x x 3x, and 3x x 3x. The progenies resulting from crosses at diploid level were mostly diploid, whereas a few seedlings were triploid. In 3x x 2x crosses aneuploids with chromosome constitution in between triploid and tetraploid (43-45 chromosomes) were predominant, but also one tetraploid (2n = 4x = 48) and four pentaploids (2n = 5x = 60) were obtained. In 2x x 3x crosses most progenies were triploid with the exception of a few aneuploids (3x + 1 and 3x - 1), whereas in 3x x 3x cross diploid and aneuploid genotypes were recorded with chromosome number varied from 27 to 34. These results indicate that triploid parents produced aneuploid as well as euploid (x, 2x, 3x) gametes and that success in ploidy manipulation in tulip depends to a large degree on the ploidy level of the parental genotypes used for hybridization. Genome constitution of selected population of F1 and BC1 hybrids was analyzed through genomic in situ hybridization (GISH). GISH analysis of the BC1 showed a considerable amount of intergenomic recombination which is desirable for introgression breeding.

Microsatellite and flow cytometry analysis to help understand the origin of Dioscorea alata polyploids.

Background and Aims Dioscorea alatais a polyploid species with a ploidy level ranging from diploid (2n = 2x = 40) to tetraploid (2n = 4x = 80). Ploidy increase is correlated with better agronomic performance. The lack of knowledge about the origin of D. alata spontaneous polyploids (triploids and tetraploids) limits the efficiency of polyploid breeding. The objective of the present study was to use flow cytometry and microsatellite markers to understand the origin of D. alata polyploids.MethodsDifferent progeny generated by intracytotype crosses (2x × 2x) and intercytotype crosses (2x × 4x and 3x × 2x) were analysed in order to understand endosperm incompatibility phenomena and gamete origins via the heterozygosity rate transmitted to progeny.ResultsThis work shows that in a 2x × 2x cross, triploids with viable seeds are obtained only via a phenomenon of diploid female non-gametic reduction. The study of the transmission of heterozygosity made it possible to exclude polyspermy and polyembryony as the mechanisms at the origin of triploids. The fact that no seedlings were obtained by a 3x × 2x cross made it possible to confirm the sterility of triploid females. Flow cytometry analyses carried out on the endosperm of seeds resulting from 2x × 4x crosses revealed endosperm incompatibility phenomena.ConclusionsThe major conclusion is that the polyploids of D. alata would have appeared through the formation of unreduced gametes. The triploid pool would have been built and diversified through the formation of 2n gametes in diploid females as the result of the non-viability of seeds resulting from the formation of 2n sperm and of the non-viability of intercytotype crosses. The tetraploids would have appeared through bilateral sexual polyploidization via the union of two unreduced gametes due to the sterility of triploids.

Diploid Potato Hybrids as Sources of Resistance to Blackspot Bruising

Resistance to bruising is an important quality trait of potato depending on multiple genetic and environmental factors. Thirty one interspecific diploid hybrids and two tetraploid standards were evaluated for resistance to bruising in 2 years. Diploids originated from crosses between wild and primitive Solanum species and dihaploids of S. tuberosum. Two-year mean values of bruising resistance of tested genotypes indicated significant variability of the trait. Analysis of variance showed significant effects of genotype, year and interaction between them on the trait. Broad-sense heritability of bruising resistance was estimated as moderately high, Hb = 0.73. Content of L-tyrosine, the main substrate of discoloration reaction, was not significantly correlated with brusing in tested diploid hybrids. The clones, that were resistant to blackspot bruise, also combined quality traits and resistances to many pathogens. All these traits could be transferred together into tetraploid level in 4x × 2x crosses via male 2n gametes produced by diploid hybrids.

Study on reproductive barriers in 4x–2x crosses in Dianthus caryophyllus L

Reproductive barriers often exist in 4x–2x crosses in Dianthus caryophyllus L. and seriously lead to low seed set and breeding efficiency. In this study, we inspected pre-fertilization and post-fertilization barriers in crosses between ‘butterfly’ (2n = 4x = 60) and the fertile breeding line ‘NH10’ (2n = 2x = 30) and ‘NH14’ (2n = 2x = 30) using fluorescence microscopy and paraffin section. The growth of the pollen tubes up to the ovary was observed in crosses of ‘butterfly’ × ‘NH10’ and ‘butterfly’ × ‘NH14’. However, abnormalities of pollen tube including coiled and spiky and callose deposition in incompatible tubes has also been detected. In the cross ‘butterfly’ and ‘NH10’, the number of pollen tubes at the base of the style reached the highest value, 67.2 ± 10.8 at 48 h after pollination, but the number of ovules with micropylar penetration was low, 19 ± 4.5. Similarly, in the cross between ‘butterfly’ and ‘NH14’, pollen tubes at the base of the style at 48 h after pollination was 51 ± 6.2 and the ovules with penetrated micropyles per ovary at 48 h after pollination was 11 ± 2.4. It indicated the pollen tubes difficultly penetrated into the ovules in 4x–2x crosses. In addition, it was observed that non-fused of egg and sperm or non-fused polar nucleus resulted in endosperm without embryos, and embryos without endosperm. Embryos aborted at different developmental stages (prembryo to cotyledon embryo, main globular embryo), which might be related to a deficient endosperm development in reciprocal crosses. The results suggested that pre-fertilization and post-fertilization barriers were detected in these combinations. Due to the post-fertilization barriers in 4x–2x crosses, the embryo rescue might be contributed to obtain hybrid F1 plants.

Fertilization fitness and offspring ploidy in 3x × 2x matings in potato

The main objective of the current research was to study the reproductive behaviour of artificial triploid potato hybrids between wild Solanum commersonii and the cultivated potato Solanum tuberosum. When used in 3x × 2x crosses, triploids gave aneuploid progenies with somatic chromosome number ranging from 29 to 36. Fertilization fitness data suggested that the survival rate of gametes produced by the triploid parents may be related to their chromosome number. In addition, consistent with molecular data, our results indicated that fitness of gametes and chromosome number of progenies are influenced by the genome dosage of interspecific triploids. Since a main route to polyploidy formation is via 2n gametes and triploids, our study may contribute to a better understanding of polyploid plant reproduction, evolution and breeding.

Effect of cross direction and cultivars on crossability of interspecific hybridization between Vaccinium corymbosum and Vaccinium virgatum

The purpose of this study was to evaluate the effect of cross direction and cultivars on interspecific hybridization between Vaccinium corymbosum (4x) and Vaccinium virgatum (6x). Totally 99 reciprocal interspecific crosses were made by using ten cultivars of V. corymbosum and five cultivars of V. virgatum. Percent fruit set, number of developed seeds per berry, germination rate and number of seedlings per flower were evaluated. The progeny seedlings were obtained from 65% of all cross combinations, and more than 99% of the seedlings were revealed to be pentaploid hybrids by flow cytometric analysis. In the comparison of crossability between interspecific reciprocal crossing, 4x×6x crosses showed 2.9 times higher rate of mean fruit set (67%) and 3.3 times higher rate of number of seeds per berry (6.2) than the opposite crosses (6x×4x). However, the average germination rate (38%) of the 6x×4x crosses was 1.7 times higher than that of 4x×6x crosses. Consequently, average number of seedlings per flower (0.96) in 4x×6x was 3.8 times higher than that in the opposite crosses. According to the number of seedlings per flower significant difference in the crossability was found among the cultivars in V. corymbosum in the reciprocal crosses with V. virgatum, whereas no such significant difference was observed in V. virgatum.

Crude Protein Content in Diploid Hybrid Potato Clones of Solanum phureja–S. stenotomum

Breeding potato (Solanum tuberosum L.) with high protein content is important for food value and for the processing sector to extract protein from potatoes. Due to the narrow genetic base of the common potato, a new germplasm has to be explored for breeding high-protein potatoes. The objectives of this research were to quantify the crude protein content of diploid hybrid Solanum phureja–Solanum stenotomum (PHU-STN) genotypes, determine the correlation of crude protein content with dry matter and other important agronomic traits, and estimate broad sense heritability for crude protein content. In 2008 and 2009, 70 diploid clones were grown in replicated field plots in Jiagedaqi, Heilongjiang. Also, two tetraploid potato cultivars, Kexin 2 (table potato) and Atlantic (high dry matter), were included in this experiment as controls. The crude protein content on a fresh weight basis (FW) over replications and years for PHU-STN clones ranged from 2.85% to 5.01% with a grand mean of 3.72%, and most of these clones were significantly or highly significantly higher in crude protein content than the two tetraploid controls. Correlations between crude protein content and agronomic traits were all negative, but the magnitudes were generally weak or moderate. Therefore it should be possible to select diploid clones with high crude protein content and acceptable agronomic traits. The estimate of broad sense heritability for crude protein content (FW) on a clonal mean basis was 0.66 with a 95% confidence interval of 0.46 to 0.79, whereas for crude protein content on a dry weight basis the estimate of broad sense heritability was 0.65 with a 95% confidence interval of 0.43 to 0.78. These results suggest that these diploid hybrid PHU-STN clones would be a good germplasm source for a high-protein-breeding effort, and identification of tetraploid progenies with high protein content and acceptable agronomic traits from 4x–2x cross should be possible.

Dihaploid induction ability of three clones of Solanum phureja (2n = 2x = 24) in interploidy cross with S. tuberosum (2n = 4x = 48)

Potato,Solanum tuberosumL. (2n = 4x = 48), is an autotetraploid species, the breeding of which at the tetraploid level is complicated by tetrasomic inheritance. Dihaploids (2n = 2x = 24) from the tetraploid cultivated potato have great potential for breeding and genetic studies. The common method deployed to obtain potato dihaploids is to make interspecific-interploidy (4x × 2x) crosses between a tetraploid seed parent and special clones from the diploidS. phurejaas pollinator. Pollinators carrying a marker gene have been used, but unfortunately, these clones were very weak, with rare flowering and low male fertility under the given conditions. To find a suitable pollinator, three clones were selected fromS. phurejabased on flowering, pollen shed and male fertility and were crossed with five cultivated tetraploid potatoes to evaluate their dihaploid induction ability. A total of 1529 interploidy crosses were made, resulting in 1116 berries and 1456 seeds. The progeny were divided into two groups based on stem, flower and tuber colour: hybrids and non-hybrids (putative dihaploids). Chromosome counting in non-hybrid genotypes detected 39 dihaploids. The clonephu 3and cv. Picasso, with 12.1 and 10.7 dihaploids per 100 berries, respectively, were the best dihaploid inducer and seed parent for dihaploid production.

PLOIDY LEVELS OF DEGENERATED EMBRYOS IN THE CROSSES BETWEEN DIPLOID AND TETRAPLOID CYCLAMEN

In reciprocal crosses between diploid and tetraploid cyclamen, almost all zygotic embryos were degenerated. Ploidy levels of the degenerated embryos were, however, obscure. Therefore, embryo rescue by the ovule culture in the crosses were examined. A lot of progenies were obtained in the 2x x 4x crosses, when the ovule culture used medium with coconut water was done 28 days after pollination. Although almost all fruits were dropped 28 days after pollination in the 4x × 2x crosses, some progenies were obtained by the ovule culture at 7-28 days after pollination. Almost all progenies obtained by the ovule culture after the reciprocal crosses between the diploids and tetraploids were triploids, suggesting that many triploid zygotes were formed in the crosses. However, some tetraploid, pentaploid and hexaploid progenies were also obtained. Origin of the pentaploids was suggested the fertilization between a reduced gamete from a diploid plant and an unreduced gamete from a tetraploid plant. These results suggest that many triploid zygotes are formed in the crosses between diploid and tetraploid cyclamen, whereas development of the triploid embryo without the embryo rescue is inhibited. It also shows the possibility that pentaploids and hexaploids as well as triploids and tetraploids can be obtained in the crosses using the ovule culture, depending on the cross combination.

‘Garbí’ Mandarin: A New Late-maturing Triploid Hybrid

Citrus is the most extensively produced fruit tree crop in the world (FAO, Food and Agriculture Organization, 2009). There are two clearly differentiated markets: fresh fruit and processed juice. In 2007, the main citrus fruit-producing countries were China (17.9%), Brazil (17.8%), the Mediterranean countries (17.1%), and the United States (8.5%) (FAO, Food and Agriculture Organization, 2009). These areas account for more of two-thirds of the total production of citrus fruits. In the Mediterranean area, citrus fruits are primarily produced for the fresh fruit market. Spain is the principal producer in the area with a total planted area of 330,000 ha and a production 6.3 million tons. Seedlessness is one of the most important characteristics for mandarin on the fresh fruit market, because consumers do not accept seeded fruits. Parthenocarpy is an essential trait for seedless fruit production, and this characteristic is present in citrus germplasm. Triploidy gives rise to seedless commercial cultivars. However, triploid plants have very low fertility, are generally sterile, and do not induce seeds in other cultivars by crosspollination (Frost and Soost, 1968). Several methods have been developed to obtain triploid citrus (Navarro et al., 2002; Ollitrault et al., 2008). One exploits natural events of polyploidization such as 2n gametes using embryo rescue and flow cytometry to select triploids in 2x · 2x crosses. Second meiotic division restitution has been proposed for diploid megagametophyte development in clementine (Luro et al., 2004), whereas Chen et al. (2008) proposed first meiotic division restitution in sweet oranges. In Spain, the structure of mandarin cultivars poses several problems. Mandarins include satsumas, clementines, and mandarin hybrids. Satsumas are harvested from the beginning of September until the beginning of November. They are cultivars with a high degree of parthenocarpy and have sterile pollen and ovules. Clementines are the most representative cultivars of mandarin in Spain. They are picked from mid-September until mid-February. They are self-incompatible, but pollen and ovules are viable, being able to pollinate and to be pollinated by other cultivars. Mandarin hybrids, like ‘Nova’ [C. clementina · (C. paradisi · C. tangerina)], ‘Fortune’ (C. clementina · C. tangerina), and ‘Ortanique’ tangor (natural hybrid between mandarin and C. sinensis), were introduced in the Spanish citriculture to cover the demand of late-season mandarins by international markets. They enable the harvesting period to be extended until May. These hybrids are also self-incompatible, but pollen and ovules are viable and cross-pollinate with clementines, producing fruits with seeds in both groups of cultivars. A triploid breeding program has been carried out in Spain since 1996 based on sexual hybridization, embryo rescue, and ploidy analysis by flow cytometry (Navarro et al., 2002). The objective is to produce new high–quality, late-season and seedless triploid mandarin hybrids. In this article, we describe ‘Garbi’ mandarin, a new triploid hybrid developed within this program characterized by its high-quality, late-season ripening and seedless fruits.

A new approach for obtaining rapid uniformity in rice (Oryza sativa L.) via a 3x x 2x cross

A triploid (2n = 3x = 36) rice plant was obtained by screening a twin seedling population in which each seed germinated to two or three sprouts that were then crossed with diploid plants. One diploid plant was chosen among the various F1 progenies and developed into an F 2 population via self-pollination. Compared with the control variety Shanyou 63, this F 2 population had a stable agronomical performance in field trials, as confirmed by the F-test. The stability of the F 2 population was further substantiated by molecular analysis with simple sequence repeat markers. Specifically, of 160 markers assayed, 37 (covering all 12 chromosomes) were polymorphic between the parental lines. Testing the F 1 hybrid individually with these markers showed that each PCR product had only a single band instead of two bands from each parent. The bands were identical to either maternal (23 markers) or paternal (eight markers) bands or distinct from both parents (six markers). The amplified bands of all 60 randomly selected F 2 plants were uniform and identical to those of the F 1 hybrid. These results suggest that the F 1 plant is a non-segregating hybrid and that a stable F 2 population was obtained. This novel system provides an efficient means for shortening the cycle of hybrid rice seed production.

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.

High frequency of spontaneous triploids in barley

HereditasVolume 64, Issue 1 p. 131-134 Open Access High frequency of spontaneous triploids in barley Jens Sandfaer, Jens SandfaerSearch for more papers by this author Jens Sandfaer, Jens SandfaerSearch for more papers by this author First published: February 1970 https://doi.org/10.1111/j.1601-5223.1970.tb02280.xCitations: 2AboutPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Literature cited Derenne, P. 1967. Triploïdie et trisomie chez l'orge cultivée, I. L'état triploïde. Bull. Rech. Agron. Gembloux N. S. 2: 242– 259. Jacobsen, P. 1954. Chromosome numbers in the genus Hedera L. Hereditas 40: 252– 254. Jacobsen, P. 1965. An improved squash-slide freezer. Stain Technol. 40: 63– 65. Kerber, E. R. 1954. Trisomics in barley. Science 120: 808– 809. Nilan, R. A. 1964. The cytology and genetics of barley 1951–1962.—Monogr. Suppl. No. 3, Res. Studies 32, No. 1. Wash. State Univ.. Sandfaer, J. 1968. Induced sterility as a factor in the competition between barley varieties. Nature 218: 241– 243. Sandfaer, J. 1970. Barley stripe mosaic virus as the cause of sterility interaction between barley varieties. Hereditas 64: 150– 152. Smith, L. 1951. Cytology and genetics of barley. Bot. Rev. 17: 1– 355. Tsuchiya, T. 1958. Studies on cross compatibility of diploid, triploid and tetraploid barleys I. Seed development in reciprocal crosses between diploids and induced autotetraploids. Bot. Mag. Tokyo 71: 48– 52. Tsuchiya, T. 1960. Studies on cross compatibility of diploid, triploid and tetraploid barley, III. Results of 3x × 3x crosses. Seiken Zihô 11: 29– 37. Citing Literature Volume64, Issue1February 1970Pages 131-134 ReferencesRelatedInformation

Selection for verticillium wilt resistance in potato breeding populations derived from four cross types

AbstractVerticillium wilt (VW) is one of the important yield‐limiting diseases for potato production. To develop resistant clones, the potential for early generation selection was studied using three basic selection methods, individual, family, and within family selection, for two clonal generations. A total of 152 clones were derived from four cross types (2x × 2x, 2x × 4x, 4x × 2x and 4x × 4x). Clones were evaluated for maturity, symptom expression, yield and stem colonization in replicated trials. Heritability and selection response for the traits were estimated for each selection method. Direct selection in the second clonal generation and individual selection showed more gain than that from other methods. Both 2x × 2x and 4x × 2x families were higher yielding and had lower stem colonization scores than 2x × 4x and 4x × 4x crosses. Therefore, 2x × 2x or 4x × 2x crosses between carefully chosen parents with high yield and VW resistance may produce offspring with superior performance.

Production and Identification of Colchicine-derived Tetraploid Vaccinium darrowii and Its Use in Breeding

In Vaccinium L., most tetraploid hybrids between tetraploid cultivated highbush blueberry (V. corymbosum L.) and diploid darrow's evergreen blueberry (V. darrowii Camp) have been produced by exploiting V. darrowii's tendency to produce 2n gametes and a strong triploid block, which greatly reduces the number of triploid hybrids produced. Colchicine-derived tetraploids offer an alternative method of producing V. darrowii plants that will easily cross with tetraploid Vaccinium species. V. darrowii (2n = 2x = 24) seeds were imbibed in 0.2% aqueous colchicine solution for 24 h. The seeds were germinated and seedlings whose morphology suggested colchicine effects were selected at various stages of development. No macromorphological changes were consistently associated with chromosome doubling. However, stomatal guard cells and pollen size increased substantially as a result of chromosome doubling. Several types of plants were identified after colchicine treatment: 1) plants with a doubled LI (epidermal tissues) and LII (internal tissues) plant layers; 2) periclinal chimeras with a doubled LI layer and a normal LII; and 3) periclinal chimeras with a doubled LII layer and a normal LI. Of ≈4000 seedlings that emerged from colchicine-treated seeds, 200 were selected for further examination based on leaf and stem morphology. Of the 200, five appeared to be tetraploid in LI, LII, or both layers based on stomatal guard cell size and pollen size. Crosses between colchicine-derived V. darrowii (4x) plants and southern highbush blueberry (V. corymbosum) cultivars (4x) were successful compared with 4x–2x and 2x–4x crosses using diploid V. darrowii and tetraploid southern highbush blueberry. Stomatal guard cells and pollen screening of the colchicine-treated plants were used as indicators of doubled V. darrowii plants and periclinal chimeras. The results from the crosses between colchicine-treated V. darrowii plants and tetraploid highbush blueberry cultivars confirmed the information obtained by stomata and pollen screening.

Field performance and resistance to tuber soft rot of potato clones derived from sexual polyploidization

Unilateral sexual polyploidization through 4x x 2x crosses is widely used to introgress genes from the diploid gene pool to the tetraploid cultivated potato Solanum tuberosum. Clones of eight 4x x 2x families, from crosses between four tetraploid cultivated potatoes and two diploid S. phureja - S. tuberosum hybrids, produc- ing 2n pollen, were evaluated for yield performances and resistance to tuber soft rot. Significant differences in terms of tuber yield and number were found between families and clones, with several clones outyielding their tetraploid parents. The source of variation due to females (4x females GCA) was significant for both tuber yield and number. A number of clones displayed inheritance of resistance to tuber soft rot. No relationship was found between agronomic traits and resistance to soft rot. Results suggest that the genotypes produced represent genet- ic material with good potentiality in the constitution of new genotypes with interesting trait combinations.

Diploid endosperm formation in Tulipa spp. and identification of a 1:1 maternal-to-paternal genome ratio in endosperms of T. gesneriana L.

Most Liliaceae plants have the tetrasporic Fritillaria-type embryo sac and normally form diploid embryos and pentaploid endosperms derived from a 4:1 maternal-to-paternal genome ratio (4m:1p) after double fertilization. Here we characterize embryo sac and endosperm formation in Tulipa spp. of Liliaceae. Chromosome analysis using seeds derived from 2x x 2x crosses of Tulipa gesneriana (2n = 2x = 24) identified diploid chromosome number in the endosperm. Similarly, flow cytometric analysis confirmed diploid endosperm formation in T. gesneriana, T. fosteriana (2n = 2x = 24) and T. greigii (2n = 2x = 24). To further study the possible mechanism of diploid endosperm formation, we made interploidy crosses of triploid (2n = 3x = 36) x diploid in which aneuploid seeds with various chromosome numbers (2n = 25-36) were produced. Again, flow cytometric analysis confirmed the same ploidy level in both embryos and endosperms at all aneuploidy levels, suggesting that only a single haploid polar nucleus contributes to endosperm formation at fertilization. Histological observation further confirmed the physical separation of two polar nuclei by a large vacuole in the Fritillaria-type embryo sac of T. gesneriana that appeared to prevent the fusion of the two polar nuclei that originated at the micropylar and chalazal ends before fertilization. Taken together, these results indicate that diploid endosperms (1m:1p) are normally formed in Tulipa spp. by fusion of the micropylar polar nucleus (n) and a spermatid (n) but not by normal triple fusion. We also show that tulip endosperm partially overcomes the triploid block mechanism that occurs in interploidy crosses. Based on these observations, the possible role of triple nuclear fusion in double fertilization is discussed.