Production of 27-, 28-, and 56-chromosome apomictic hybrid derivatives between pearl millet (2n=14) and Pennisetum squamulatum (2n=54)

Summary An interspecific hybridization program designed to transfer gene(s) controlling apomixis from Pennisetum squamulatum Fresen. (2n=6x=54) to induced tetraploid (2n=4x=28) cultivated pearl millet, Pennisetum americanum (L.) Leeke resulted in four offtype plants, two with 27 chromosomes and two with 28 chromosomes. These plants were found among 217 spaced plants established from open-pollinated seed of an apomictic 21-chromosome polyhaploid (2n=21) plant derived from an apomictic interspecific hybrid (2n=41) between tetraploid pearl millet and Pennisetum squamulatum. It appeared that a 21- (or 20-) chromosome unreduced egg from the apomictic polyhaploid united with a 7-chromosome pearl millet (2n=2x=14) gamete to produce a 28- (or 27-) chromosome offspring. Meiotic chromosome behavior was irregular averaging from 3.60 to 4.05 bivalents per microsporocyte in the 27- and 28-chromosome hybrids. The 27- or 28-chromosome hybrids, like the 21-chromosome female parent, shed no pollen, but set from 1.8 to 28 seed per panicle when allowed to outcross with pearl millet. Progeny of the 28-chromosome hybrids were uniform and identical to their respective female parents, indicating that apomixis had been effectively transferred through the egg. In addition, a 56-chromosome plant resulting from chromosome doubling of a 28-chromosome hybrid was identified. Pollen was 68 per cent stainable and the plant averaged 2.3 selfed seeds per panicle. Chromosomes of the 56-chromosome plant paired as bivalents (x=10.67) or associated in multivalents. Three to nine chromosomes remained unpaired at metaphase I. Multiple four-nucleate embryo sacs indicated the 56-chromosome hybrid was an obligate apomict. The production of 27-, 28-, and 56-chromosome hybrid derivatives were the results of interspecific hybridization, haploidization, fertilization of unreduced apomictic eggs, and spontaneous chromosome doubling. These mechanisms resulted in new unique genome combinations between x=7 and x=9 Pennisetum species.     

Stabilization of new types of diploids (2n=22, 24) through selfing of aneuploids (2n=21, 22) derived from crossing of sesquidiploids (2n=29, AAC) and Brassica campestris (2n=20 AA)

Summary Aneuploids with 2n=21 and 2n=22 derived from crossing of sesquidiploids (2n=29, AAC) and Brassica campestris (2n=20, AA) were selfed successively in order to follow the changes in chromosome number of the progenies for three consecutive generations. Progenies with 2n=22, 23 and 24 obtained after selfing of S₀ generation and the succeeding S₁, S₂ and S₃ generations were analyzed in terms of pollen stainability, % seed set as well as cytogenetically based on meiotic behaviour with the aim of determining the possibility of addition of one or more alien chromosomes into n=10 species which may lead to differentiation of single or plural disomic addition lines. The generation of aneuploids with 2n=21 progressed in such a way that most plants seem to revert to the 2n=20 chromosome number of B. campestris after selfing. From 2n=22 aneuploids, however, the succeeding progenies showed high frequency of plants with two additional chromosomes which accounted for 50.6% and 52.9% of total S₃ progenies via 2n=22 and 2n=24 S₂ generations, respectively. The meiotic behaviour of these progenies indicated evidence for a rule governing the frequency distribution of chromosome number among these addition lines and high possibility to breed such disomic plants with 2n=22. A method of selecting stable aneuploids was suggested in addition to the possible role of pollination biology at various processes of such breeding program.     

Monohaploids (n=x=12) from autotetraploid Solanum tuberosum (2n=4x=48) through two successive cycles of female parthenogenesis

Summary Viable monohaploids with 12 chromosomes in their somatic cells were obtained from an autotetraploid cultivar of Solanum tuberosum (2n=4x=48) by inducing female parthenogenesis successively in the tetraploid cultivar and in dihaploids from that cultivar. Both dihaploids and monohaploids were induced using the diploid S. phureja clones IvP 35 and IvP 48 as pollinators. The average frequencies of dihaploids and monohaploids in 1973 were 2.0 and 0.14 per berry respectively.Non-homologous chromosome associations (bivalents, trivalents and even quadrivalents) were observed at metaphase I of meiosis in pollen mother cells of the two monohaploids studied.The occurrence of non-homologous associations of chromosomes during meiosis is discussed. In addition the potential significance of monohaploids for basic research andfor breeding of potatoes is considered.     

Male fertility and cytology of triploid hybrids between tetraploidSolanum commersonii (2n=4x=48, 2EBN) and Phureja-Tuberosum haploid hybrids (2n=2x=24, 2EBN)

Summary Solanum commersonii Dun. is a diploid (2n=2x=24, 1EBN) wild species of potential value for potato breeding. It is a reproductively isolated species and cannot be crossed with Tuberosum haploids (2n=2x=24, 2EBN) or other diploid 2EBNSolanum species. In order to overcome the EBN barriers, triploid hybrids were produced between Phureja-Tuberosum haploid hybrids, which form 2n pollen grains by parallel spindles, and tetraploidS. commersonii. Microsporogenesis analysis of the triploids indicated a trend towards low values of chromosome distribution at Anaphase I; lagging chromosomes were often observed as well. Despite these abnormalities, the percentage of stainable pollen was very high, ranging from 5.0% to 74.3%. A high variation in pollen grain diameter was also evident. Parallel and tripolar orientation of spindles at Metaphase II of microsporogenesis was a common feature of all the triploids analyzed, but dyads and triads were observed at a very low frequency. Therefore, also the frequency of 2n pollen was very low; the different size of stainable pollen appears to represent the ploidy levels which are possible according to the distribution of chromosomes in Anaphase I. The results obtained also suggest thatS. commersonii could have minor genes acting at the end of meiosis in such a way that, despite the presence of parallel/tripolar spindles, dyads/triads are not formed.Contribution no. 124 from the Research Center for Vegetable Breeding.     

Effect of selection and temperature stress on the production of 2n gametes in Lotus tenuis

Five Lotus tenuis genotypes showing 2n pollen production greater than 5% and producing 4x progenies in 4x(male sterile Lotus corniculatus) × 2x crosses were found in different natural populations. Meiotic analyses of these plants showed some second metaphases with disoriented spindles, i.e. parallel and tripolar spindles which resulted in diads and triads, respectively. Because useful genes for winter growth and resistance to powdery mildew from L. tenuis can be introgressed into L. corniculatus via 2n gametes, a programme of selection was started to increase the occurrence of 2n gamete production in L. tenuis. Controlled crosses were performed between genotypes producing 2n gametes and the progenies were checked for 2n pollen production and meiotic aberration, following the same methodologies as used for the parental plants. Although in some progeny plants the frequency of 2n gamete production increased remarkably, the overall progeny level of 2n gamete production remained unchanged. This suggested that different genes for meiotic aberration exist in natural populations and/or differential seed-setting ability exists between genotypes. Diallel crosses between selected genotypes confinned this hypothesis. Evaluation of 2n pollen production of clones grown under different temperature regimes (warm vs. temperate) showed that, in L. tenuis, the effects of genotype, temperature and genotype × temperature interaction are significant. In particular, high temperature increases the level of 2n gamete production in L. tenuis.     

Genetic strategies to determine the mode of 2n egg formation in diploid potatoes

Summary In this study, nineteen diploid potato clones (Solanum spp. 2n=2x=24) were identified as 2n egg producers on the basis of fruit set in 2x–4x crosses. The segregation of three genes mapped close to the centromere, Got-1 (1.1 cM), Pgm-2 (2.0 cM), and Sdh-1 (8.3 cM), were analyzed in the tetraploid offspring in these 2x–4x crosses to discriminate between First Division Restitution (FDR) and Second Division Restitution (SDR) modes of 2n egg formation. The co-dominant nature of these markers lead to more precise estimates of the recombinational frequencies as a result of completely classifying the segregating progenies. 2x–4x data revealed a predominance of SDR mechanisms occurring in 20 of the 21 families analyzed. With a SDR mode established, half-tetrad analysis (HTA) of four distal loci, 6-Pgdh-3, Mdh-1, Pgi-1, and Aps-1, revealed two SDR segregation patterns in some of the families. One pattern fit the expectations for the distal arm position. The gene-centromere map distances based upon SDR modes in the families following this pattern, were generally close to 4x–2x (FDR) estimates suggesting similar recombination rates between micro- and mega-sporogenesis. Heterozygosity transmission, on average, was 39.1%. In the other segregation pattern, in which the diploid parents were derived from S. chacoense PI 230580, higher than expected homozygosity levels were found in the female 2n gametophyte populations. A post-meiotic doubling of the reduced megaspore, which generates homozygous 2n eggs, is suggested to operate in three families. The common genetic background of the diploid clones suggested a heritable nature of this mechanism. Pooled data from these three deviant families calculated that 1.8% of the heterozygosity was transmitted to the tetraploid progeny.It is concluded that utilization of seven enzyme-coding loci, with previously established gene-centromere map distances, in 2x–4x crosses improved half-tetrad analysis (HTA) as a means to determine the mode of 2n gamete formation in megasporogenesis and megagametogenesis of diploid Solanum species.     

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.     

Transmission of bacterial wilt resistance by First Division Restitution (FDR) 2n pollen via 4x×2x crosses in potatoes

Summary Resistance to bacterial wilt (Pseudomonas solanacearum) found in tuber-bearing Solanum species was transferred into a diploid potato breeding population. Simultaneous selections were made for agronomic characters, production of first division restitution (FDR) 2n pollen, and resistance. Diploid resistant genotypes were identified via inoculation with a virulent isolate (CIP-204) of race 3 of Pseudomonas solanacearum. These resistant diploid genotypes were crossed to susceptible tetraploid potatoes. An investigation was made to assess whether resistant diploid genotypes transmit resistance to bacterial wilt, which is a quantitatively inherited trait, to tetraploid potatoes via FDR 2n pollen. Tetraploid seedlings from 4x×2x crosses were inoculated with the same isolate CIP-204, and the percentage of surviving seedlings was scored. Some 4x×2x families from resistant diploid genotypes demonstrated a high level of survival rate. The transmission of bacterial wilt resistance was achieved by the use of FDR 2n pollen. It was speculated that a female x male interaction effect on the survival rate of the evaluated progeny may exist. Selecting proper 4x and 2x parents would be important for obtaining a higher frequency of transmission of resistance to bacterial wilt in the progeny.     

Ploidy races in Actinidia chinensis

Summary Ploidy levels were examined in 26 accessions of Actinidia chinensis: 20 accessions were diploid (2n=2x=58) and 6 accessions were tetraploid (2n=4x=116). There was no evidence of variation in ploidy level within an accession. Our results are consistent with tetraploid A. chinensis coming from a restricted part of China.Interploid crosses within A. chinensis produced only low numbers of seedlings which were mainly triploid. Crossing hexaploid A. deliciosa with pollen of tetraploid A. chinensis produced a large family of plants and those checked were pentaploid.Counts on 83 genotypes of different ploidy levels (2x, 3x, 4x, and 5x) confirmed that the basic chromosome number in Actinidia is 29.     

Numerically unreduced (2n) gametes and sexual polyploidization in Actinidia

This is the first report on the production of numerically unreduced (2n) gametes in Actinidia species. Their occurrence was apparent from direct observations of microporogenesis in 2x, 3x, 4x and 6x plants and was inferred from ploidy analysis of progenies from 4x-2x and 2x-4x crosses. Both male and female plants were found to produce 2n gametes. 2n gamete producers were detected only in interploidy crosses. The observed frequency of 2n gametes was either similar to or towards the lower end of the ranges reported for other genera. The role of sexual polyploidization in the evolution of the genus is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pollen donor impact on progenies of pseudogamous blackberries (Rubus subgen. Rubus)

A series of intra- and interspecific crosses in Rubus was evaluated with DNA markers to assess the proportion of apomictically vs. sexually derived progeny plants. The species were well separated from each other with these markers and the interspecific hybrids were easy to ascertain. Ten plants each were derived by selfing of R. hartmanni, R. polyanthemus and R. vestitus, and evaluated with RFLP. Of these plants, only one R. hartmanni plant appeared to be sexually derived, whereas the remainder appeared to have arisen by apomixis. Between 10 and 19 progeny plants, derived by open pollination of R. Hartmanni, R. infestus, R. laciniatus and R. vestitus, respectively (total of 63plants), were evaluated with RFLP. Of these, 6 plants (approx. 10%) showed as lightly deviating band pattern, which may indicate sexual seed set but could also result from experimental artefacts. The three species combinations 1) R. hartmanni and R. sprengelii, 2) R. infestus and R. vestitus, and 3)R. laciniatus and R. polyanthemus were cross-pollinated reciprocally and the resulting progenies evaluated with RAPD (combinations 1 and 2)and leaf shape (combination 3). Combination1 produced 20% (3 out of 15 progenies)sexually derived progenies, combination 2produced 100% (26 progenies) and combination 3 produced 64% (16 out of 26progenies), respectively. The hybrid progenies appeared to have been derived through pollination of unreduced as well as reduced egg cells. The ability to produce interspecific hybrid progenies is most probably dependent upon the species combination. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of neurotoxin (ODAP) content, flower and seed coat colour in grass pea (Lathyrus sativus L.)

Summary A strong epidemiological association is known to exist between the consumption of grass pea and lathyrism. A neurotoxin, -N-Oxalyl-L-, -diaminopropanoic acid (ODAP) has been identified as the causative principle. This study was undertaken to investigate the mode of inheritance of the neurotoxin ODAP, flower and seed coat colour in grass pea. Five grass pea lines with low to high ODAP concentration were inter-crossed in all possible combinations to study the inheritance of the neurotoxin. Parents, F₁ and F₂ progenies were evaluated under field condition and ODAP analyzed by an ortho-phthalaldehyde spectrophotometric method. Many of the progenies of low x low ODAP crosses were found to be low in ODAP concentration indicating the low ODAP lines shared some genes in common for seed ODAP content. The F₁ progenies of the low ODAP x high ODAP crosses were intermediate in ODAP concentration and the F₂ progenies segregated covering the entire parental range. This continuous variation, together with very close to normal distribution of the F₂ population both of low x low and low x high ODAP crosses indicated that ODAP content was quantitatively inherited. Reciprocal crosses, in some cases, produced different results indicating a maternal effect on ODAP concentration. Blue and white flower coloured lines of grass pea were inter-crossed to study the inheritance of flower colour. Blue flower colour was dominant over the white. The F₂ progenies segregated in a 13:3 ratio indicating involvement of two genes with inhibiting gene interactions. The gene symbol LB for blue flower colour and LW for white flower colour is proposed.     

Manova and discriminant analyses of phenotypic data as a guide for parent selection in kiwifruit (Actinidia deliciosa) breeding

Evaluation of seedling populations assists in identifying those plants most desirable as parents for a breeding programme. A multivariate analysis of variance (Manova) showed that the seedling populations from 6 kiwifruit (Actinidia deliciosa) crosses differed in their overall merit. Discriminant functions were then used to identify the characters most powerful in distinguishing between populations. The populations were placed in order of their function scores which represent overall merit, and the superior populations were determined. The results showed that `Bruno' was a superior female parent for the production of seedlings with high discriminant function scores for floriferousness (male offspring)and productivity and high vitamin C content (female offspring). One male parent D-1-20 was consistently superior to D-1-6 the other male parent. This revised version was published online in July 2006 with corrections to the Cover Date.     

High regeneration potential in vitro of sunflower (Helianthus annuus L.) lines derived from interspecific hybridization

Successful selection of interspecific hybrid progenies with superior ability to regenerate shoots from apical meristems was performed in sunflower which now allows for the development of lines for improved biotechnological applications. Early generations of interspecific hybrids originating from crosses between the two H. annuus CMS lines ‘HA89’ and ‘Baso’, and 9 wild species were screened for their ability to regenerate in vitro. Evaluation of 36 progenies allowed to identify seven progenies from crosses involving H. mollis, H. giganteus, H. strumosus, and H. decapetalus which showed a significantly higher regeneration potential than the commercial hybrid ‘Albena’ regarding the number of shoots per explant. Among these progenies, 47.2 to 62.4% of explants produced shoots with an average of 2.3 to 3.5 shoots per cultured explant. Regeneration in vitro was significantly determined by the genotype. More than half of the investigated interspecific hybrids performed better than the inbred ‘HA89’ demonstrating that the high regeneration potential available in the wild species can be efficiently transferred to cultivated sunflower. The seven progenies with high regeneration potential in vitro were characterised by agronomic performance in the field. Two of the interspecific hybrids derived from H. strumosus and H. decapetalus not only showed a superior regeneration potential but also proved to be competitive to commercial hybrids with regard to important agronomic traits, e.g. fat content and TGW. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fertile somatic hybrids of Solanum etuberosum (+) dihaploid Solanum tuberosum and their backcrossing progenies: relationships of genome dosage with tuber development and resistance to potato virus Y

The wild non-tuberous species Solanumetuberosum is resistant to biotic andabiotic stresses, but is very difficult tocross with cultivated potato. Therefore,interspecific somatic hybrids between adihaploid clone of potato S.tuberosum (2n=2x=24, AA genome) and thediploid species S. etuberosum(2n=2x=24, EE genome) were produced byprotoplast fusion. Among the 7 fertilefusion hybrids analysed by genomic insitu hybridisation (GISH), three groups ofplants were found with the genomicconstitution of AAEE, AAEEEE and AAAAEE.Four fusion hybrids had exactly theexpected chromosome composition, while eachof the three aneuploid hybrids had lost twochromosomes of S. etuberosum. Twobackcross progenies were developed, andGISH analysis was applied to analysetransmission of the parental chromosomesinto the sexual generations. BC₁hybrids derived from the crosses of thehexaploid somatic hybrids with tetraploidpotato were pentaploid with thetheoretically expected genomic compositionor with slight deviation from thisexpectation. In the three BC₂ hybridsanalysed by GISH seven to 12 chromosomes ofS. etuberosum were detected in thepredominant S. tuberosum background.No recombinant chromosomes in the hybridswere detected. Genome dosage affects tuberformation in hybrids and their progenies,but has less effect on resistance to potatovirus Y (PVY) in fusion hybrids. Severalgenotypes of the fusion hybrids andBC₁ progeny did not show viralinfection even in the graftingexperiments.     

Evaluation of globe artichoke and related germplasm for resistance to artichoke latent virus

Summary A total of 280 globe artichoke (Cynara scolymus L.) clones were assessed for reaction to naturally occurring artichoke latent virus (ALV) in field trials. Also, the reaction of 17 seed progenies of crosses which produced ALV-resistant clones in the field, 20 self progenies of parental genotypes common to these crosses, self progenies of 17 artichoke cultivars, and 7 accessions ofC. cardunculus andC. syriaca were evaluated in the glasshouse following mechanical inoculation with ALV. The local and systemic presence of ALV, and the relative virus content in infected plants were determined by ELISA.All cultivars and most of the otherC. scolymus material tested (clones and seed progenies) proved to be highly susceptible to ALV. Ten progenies were less susceptible to mechanical inoculation as shown by the moment the virus could be detected, and a slow rate of accumulation of viral antigen. Some plants in two seed progenies possessed apparently a high resistance as could be concluded from the failure of ALV to spread or to multiply in plant.Accessions ofC. cardunculus andC. syriaca were also highly susceptible to ALV. However, an interspecific hybridC. cardunculus x C. syriaca presented the highest level of resistance, the plants remained free of local and systemic infection. Ratios of resistant to susceptible plants in self progenies of resistant plants suggested that this resistance was dominant.     

Interspecific hybridization between rice bean (Vigna umbellata) and its wild relative (V. minima): Fertility-sterility relationships

Summary The pre-and post-fertilization barriers in the interspecific crosses between Vigna umbellata and V. minima were investigated. In the reciprocal crosses (V. minima as the parent) the entry of pollen tubes into the ovary was delayed by about 4 h, and no seed set was observed. However, no pre-fertilization barriers were encountered in crosses involving V. minima as the parent and V. umbellata as the parent (normal cross). The delay/absence of divisions in the endosperm and the failure of embryo to divide were the post-fertilization barriers responsible for somatoplastic sterility in normal crosses which yielded a few hybrid seeds. The hybrid seeds showed poor germinability. The F₁ hybrids were intermediate between the parents in most morphological characters, and are completely sterile for pollen. Backcrossing of F₁ hybrid with both the parents did not restore fertility in the progenies. V. minima is considered as the tertiary gene pool of the rice bean.     

The transfer of the synaptic gene (sy-2) from 1EBN Solanum commersonii Dun. to 2EBN germplasm

Summary Crosses of a synaptic mutant (sy-2) of 2x(1EBN) S. commersonii Dun. with 2x(2EBN) S. chacoense Bitt. resulted in the production of 3x(2EBN) hybrids. Low levels of 2n egg production in these hybrids allowed subsequent crosses to be made with 4x(4EBN) cultivars, resulting in 5x(4EBN) S. commersonii-S. chacoense-Group Tuberosum hybrids. Seed set in these crosses averaged 0.22 seeds per fruit. Fifty-four unspotted seeds were produced in 1225 crosses of the 5x(4EBN) hybrids with 2x(2EBN) Group Phureja haploid extraction clones carrying dominant seed spot markers. These seeds yielded a total of 15 confirmed haploids having a diploid (2n=2x=24) or near diploid chromosome number. One haploid clone recovered displayed a synaptically abnormal phenotype similar to the original S. commersonii synaptic mutant. Preliminary test crosses of these haploids indicated that all haploids produced were 2EBN.This method for the transfer of alleles across EBN levels holds promise for the recovery of both qualitative and quantitative traits from 1EBN species. In these recovered, cross-compatible forms, clones with desired characteristics may be selected and easily incorporated into breeding programs.     

An evaluation of intergeneric hybridization of Fragaria spp. x Potentilla spp. as a means of haploid production

Summary Unidirectional intergeneric crosses using octoploid and hexaploid Fragaria species as a female parent and diploid and tetraploid Potentilla species as a male parent were performed, producing in toto 1345 fruits with an estimated 46089 seeds. The germinating seeds obtained by using P. anserina mostly perished at the cotyledon stage, a few developing into matroclinous octoploid seedlings. The seeds obtained by using P. fruiticosa produced two types of seedlings: matroclinous octoploids and genuine intergeneric hybrids. It is considered that the matroclinous octoploids resulted from contamination during the emasculation process. A great majority of the hybrid seedlings were slow in growth and development and perished before reaching the flowering stage. No haploids have been recovered. Possible reasons for absence of haploid genotypes in the progenies of these intergeneric crosses are considered. Among the seedlings of F. x ananassa cv. Tioga x P. fruiticosa, a seedling of an unusual phenotype was noticed, which upon chromosome determination, proved to be 2n=9x=63. This genotype displayed complete male and female sterility. The implication of the sterility of this seedling for bridging of the two genera is discussed.This research was performed as part of a cooperative project on genetic improvement of cultivated strawberry between the Fruit Laboratory, USDA, Beltsville and NJAES. Paper of the Journal Series, D-12102-16-83. New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick, NJ 08903.     

The occurrence of 2 n eggs in the dihaploids derived from Rosa hybrida L.

Dihaploid derivatives of 4x Rosa hybrida L. obtained through parthenogenesis induced by irradiated pollen significantly produce 2n eggs,often accompanied by gametes with higher ploidy levels (3x and 4x). The gamete ploidy level distribution depends on the originating genotype. The ploidy level of the male parent affects the maternal /paternal ratio in the endosperm, which can be involved in the counter-selection of triploid genotypes resulting from the crosses made between dihaploid and 4x Rosa hybrida.The observed segregation ratios of flower doubleness in triploid progenies of dihaploids × R. wichuraïanasuggest that the relevant locus could be positioned far from the centromere.