Pollen viability and meiotic behaviour in intraspecific hybrids of Hydrangea aspera subsp. aspera Kawakami group x subsp. sargentiana

Pollen viability and male meiosis in intraspecific hybrids of Hydrangea aspera subsp. aspera Kawakami group (2n = 2x = 36) and subsp. sargentiana (2n = 2x = 34) were investigated. Although it had been assumed that they were sterile, pollen viability was observed; it varied from 2.5% to 12.1%. The production of gametes with different chromosome numbers was implied by the analysis of the dispersion of the diameter distribution of pollen grains. Analysis of male meiosis made it possible to identify the origins with two major categories of meiotic aberrations: abnormal chromosome distribution (early chromosome migration at metaphase, lagging chromosomes at anaphase, micronuclei at telophase), leading to the formation of unbalanced tetrads and/or ones with supernumerary microspores; and abnormal spindle orientation in metaphase II (tripolar, fused and parallel spindles), leading to the formation of dyads or triads. The mode of 2n pollen formation is of the First Division Restitution type. The high level of parental heterozygosity that is normally associated with them should facilitate the transfer of a polygenic trait in breeding programme.     

Production of haploids of potato (Solanum tuberosum subsp. tuberosum) and their identification with electrophoretic analysis

Summary The frequency of haploid production following the interspecific pollination of eight tetraploid potato cultivars (Solanum tuberosum subsp. tuberosum) with Solanum phureja clone 1.22 was investigated. A total of 185 haploids were produced with an overall haploid frequency of 3.9 haploids/100 fruits. The haploid frequency was affected by the genotype of the maternal parent. Atlantic, ND860-2, Superior, Saginaw Gold, Spartan Pearl, Nooksack and Onaway had frequencies of 6.2, 5.1, 4.7, 3.9, 2.3 and 0.7 haploids/100 fruits, respectively. There were 60 and 57 haploids produced from Atlantic and Saginaw Gold, respectively, and no haploids were extracted from fruits of Lemhi Russet. Isozyme analysis and visual examination were performed independently to compare the efficiency of discriminating hybrids from haploids. Approximately 80% of total hybrids could be identified by electrophoretic analysis, while 77% were distinguished through visual examination. Pgm-2 ¹, which is unique in the clone 1.22 and absent from all seed parents, was found to be the most useful locus in hybrid identification and 50% of total hybrids could be distinguished by this allele. With similar rationale, Mdh-1 ¹ allozyme, which was absent in six of the eight parents, identified 37% of total hybrids. A combination of both visual and electrophoretic methods made hybrid identification even more efficient, with an average identification efficiency of 91%. A scheme was proposed to develop a new haploid inducer which would be homozygous for both Pgm-2 ¹ and embryo spot.     

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.     

Unreduced pollen frequencies versus hybrid production in diploid-tetraploid Vaccinium crosses

Summary Reciprocal crosses were made between highbush Vaccinium corymbosum L. cultivars (4X) and high, medium and low frequency diplandroid pollen producing diploid clones of V. elliotti (Chapm.) and diploid pubescent forms of V. corymbosum. Pollination of 3,342 flowers produced 15 hybrids-11 tetraploids and 4 triploids. All V. elliottii hybrids were tetraploid while each successful diploid-tetraploid V. corymbosum cross produced at least one triploid. The ratio of tetraploid hybrids to 100 pollinations (H: 100P) was considered to be the best criterion for assessing crossability. Vaccinium elliottii produced a total of 8 4X hybrids while 2X V. corymbosum only produced 3. Vaccinium elliottii was more successful as a female parent and diploid V. corymbosum only produced hybrids as male parents. High frequency diplandroid clones showed a lower mean crossability as pollen parents (H: 100P=0.2) than medium frequency diplandroid clones (H: 100P=0.8). As predicted, 0% diplandroid clones wer unsuccessful as pollen parents. Unreduced pollen production was essential for tetraploid hybrid production in 4X×2X crosses. There appeared to be no consistent, inherent pattern between 2n pollen and 2n egg production in a clone nor was a tetraploid genotype effect on crossability consistently observed.Paper No. 10781 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC., 27695-7601     

Morphological variation of Medicago sativa subsp. falcata genotypes and their hybrid progeny

Semi-hybrid alfalfa cultivars offer the possibility of capturing non-additive genetic variation. Medicago sativa subsp. falcata and subsp.sativa have been shown to form a heterotic pattern for biomass yield. Objectives of this study were to examine morphological variation in a broad range of falcate germplasm and to determine how falcate morphological variation per se is related to the performance of falcate germplasm in hybrid crosses with subsp. sativa. Falcata genotypes from 40 populations spanning the subspecies native range were selected and biomass yield, plant width, plant height, growth angle, biomass density, plant maturity, and regrowth after cutting were measured on the genotypes and their hybrid progeny three times throughout the growing season. In addition weekly plant heights were measured and growth rates were determined with a Gompertz function. Falcata parental genotypes exhibited a full range of phenotypes for plant width, plant height, growth angle, density, and maturity. Heterosis was not only observed for biomass yield but also for plant width, plant height, and more erect growth habit. The top yielding sativa-falcata hybrids had increased plant width, plant height, and plant density. European germplasm was taller and had faster regrowth than Asian material. Sativa-falcata hybrids produced biomass yield superior to the mid-subspecies mean only after two to three weeks of growth prior to first and third harvests. Prior to second harvest, biomass production was inferior to the mid-subspecies mean for 30 days. Hybrids using falcata as one parent are not currently adapted to intensive harvest management due to their slower regrowth. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resistance to Rhopalosiphum padi (Homoptera: Aphididae) in Hordeum vulgare subsp. spontaneum and in hybrids with H. vulgare subsp. vulgare

Summary Accessions of Hordeum vulgare subsp. spontaneum, the progenitor of cultivated barley, were screened in field and glasshouse trials for resistance to the aphid Rhopalosiphum padi. A few selected lines were furthermore hybridized with a modern barley variety and the resulting populations evaluated. High levels of resistance were found among some of the spontaneum accessions resulting in lower aphid growth rates (maximum reduction 57%). Segregation patterns among siblings in F2 populations were continuous, indicating the presence of several genes with possibly additive effects. The usefulness of H. vulgare subsp. spontaneum for breeding aphid resistant barley is discussed.     

Cyto-morphological evidence for segmental allopolyploid origin of Teasle gourd (<Emphasis Type="Italic">Momordica subangulata</Emphasis> subsp. <Emphasis Type="Italic">renigera</Emphasis>)

Teasle gourd [Momordica subangulata Blume subsp. renigera (G. Don) de Wilde, 2n = 56] exhibits morphological characters found in both M. dioica (2n = 28) and M. cochinchinensis (2n = 28). Morphological analysis of M. subangulata subsp. renigera suggests an allopolyploid origin. We present evidence elucidating the genomic relationships between M. dioica, M. cochinchinensis and M. subangulata subsp. renigera. A triploid M. dioica × M. subangulata subsp. renigera hybrid had an average of 12.76 bivalents, 13.84 univalents and 0.88 trivalents at metaphase I, while the M. cochinchinensis × M. subangulata subsp. renigera hybrid had an average of 13.08 bivalents, 12.96 univalents and 0.96 trivalents. F₁ hybrids of the two diploid species (M. dioica × M. cochinchinensis) showed an average of 9.12 bivalents and 9.76 univalents, suggesting that the genomes of these species are only partially homologous. A higher number of bivalents in the triploid hybrids suggests that M. subangulata subsp. renigera is a segmental allopolyploid of M. dioica and M. cochinchinensis and that its genomes have diverged from the parental genomes.     

Induction of novel organelle DNA variation and transfer of resistance to frost and Verticillium wilt in Solanum tuberosum through somatic hybridization with 1EBN S. commersonii

Somatic hybridization can be used to induce genetic variability in plastidial and mitochondrial genomes, and transfer multiple uncloned genes across sexual barriers. Somatic hybrids were produced between a dihaploid clone of the common potato, S. tuberosum subsp. tuberosum, and the wild sexually incongruent diploid species S. commersonii. Fusion products were selected on the basis of callus growth and regeneration in vitro. Genome composition of putative somatic hybrids was determined by flow cytometric analysis of nuclear DNA content, RAPD analysis, and Southern analysis with probes specific to organellar DNA. All regenerated fusion products proved to be hybrids based on RAPD analysis. Seventy per cent of somatic hybrids were (near) tetraploids, 22% (near) hexaploids and 8%(near) octoploids. A high correlation was found between the nuclear DNA content determined by flow cytometry and the number of chloroplasts in stomata guard cell pairs. Somatic hybrids inherited the parental plastids in a random manner. On the contrary, they preferentially inherited the mitochondrial DNA fragments of S. tuberosum. The majority of them had a rearranged mitochondrial genome with fragments from both parents. Hybrids were highly vigorous and morphologically more similar to the cultivated than to the wild parent, produced tubers on long stolons under long photoperiod conditions, showed a high degree of flowering, but did not produce pollen. In addition, somatic hybrids were generally more resistant to frost and Verticillium wilt than the cultivated parent, indicating the introgression of relevant resistance genes from the wild species into the genetic background of S. tuberosum. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sterility in the rice hybrids and its significance

The degree of sterility resulting in F₁ hybrids has been used by several workers while considering the relative affinity between the different varieties and sub-species within a species and that between different species. The authors contend that sterility cannot be used as a criterion and present spikelet sterility data in F₁ hybrids of inter-varietal, inter-subspecific and interspecific crosses in the cultivated rices to support this contention. They conclude that the evolution of the three sub-species (viz., japonica, javanica and indica) may have taken place independent of each other from the various ecotypes of the putative species Oryza perennis through introgression although the possibility of the indica giving rise to japonica on the one hand and the javanica on the other cannot be altogether ruled out.The consequences of such a mode of evolution are discussed and the possibility of the existence of a slightly different set of linkage groups in indica rices compared with that of japonica rices is suggested.     

Comparisons of genetic and morphological distance with heterosis between Medicago sativa subsp. sativa and subsp. falcata

Biomass yield heterosis has been shown to exist between Medicago sativasubsp. sativa and Medica gosativa subsp. falcata. The objective of this study was to gain a better understanding of what morphological and genetic factors were most highly correlated with total biomass yield heterosis. We calculated genetic distances among nine sativa and five falcate genotypes based on amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) DNA markers. Genetic distance did not correlate with specific combining ability (SCA) or mid-parent heterosis. In contrast, a morphological distance matrix based on seventeen agronomic and forage quality traits was significantly correlated with heterosis; the agronomic traits of maturity, midseason regrowth, and autumn regrowth showed strong association with heterosis. Heterosis was also correlated with subspecies. We suggest that in many cases progeny heterosis can be accounted for by the interaction of genes controlling morphologically divergent traits between the parents. In other cases, progeny heterosis could also be due to divergence between the parents at particular genetic loci that do not control field-level phenotypic differences. Genetic distanceper se between parental genotypes, based on neutral molecular markers, however, does not reflect the potential of individual genotypes to produce heterosis in their progeny. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetic relationship of Mediterranean mandarins (Citrus deliciosa Tenore) using RAPD markers

Summary Random amplified polymorphic DNA (RAPD) analysis was carried out to evaluate polymorphism and genetic similarity between 39 Mediterranean mandarin genotypes. One hundred eleven amplification products were identified using 21 random primers. An average of 2.2 RAPD markers was obtained for each primer, corresponding to 42% of the amplification products. Genotype-specific RAPD markers were also found, mainly in known hybrids. UPGMA cluster analysis revealed the low level of genetic variation between accessions of Mediterranean mandarins, whereas their hybrids with other Citrus species showed greater genetic dissimilarity. Twenty accessions yielded very similar patterns, suggesting either that they could be a single clone, or that the technique was not able to detect genomic variation. However, for the other specimens genetic polymorphism can easily be detected by RAPD, although the genetic variation between accessions was quite low. The large number of hybrids and the low polymorphism between accessions support the hypothesis that Mediterranean mandarins are all true hybrid of Common mandarins (Citrus reticulata Blanco).     

Identification of somatic hybrids of dihaploid Solanum tuberosum lines and S. brevidens by species specific RAPD patterns and assessment of disease resistance of the hybrids

Summary Symmetric somatic hybrids were produced by electrofusion of protoplasts of two dihaploid tuber-bearing potato (Solanum tuberosum L.) lines and Solanum brevidens Phil., a diploid non-tuber-bearing wild potato species. A total of 985 plants was obtained. Verification of nuclear hybridity of putative hybrids was based on additive RAPD patterns, general morphological characteristics and chromosome counts. 53 (90%) calli regenerated into plants which were identified as somatic hybrids. Most of the hybrids were aneuploids at the tetraploid (4×) or hexaploid (6×) level. The 20 hybrids tested expressed a high level of resistance to potato virus Y (PVY ^N ) characteristic of the S. brevidens parent. Resistance to late blight (Phytophthora infestans (Mont.) de Bary) varied between hybrids, but was on average better than that of the fusion parents. Resistance of hybrids to bacterial stem rot (Erwinia carotovora subsp. atroseptica (van Hall) Dye) was not superior to that of commercial potato cultivars.     

The resistance of Hordeum bulbosum and its hybrids with H. vulgare to common fungal pathogens

Summary Two tetraploid and two diploid clones of Hordeum bulbosum were screened for resistance to five isolates of powdery mildew which are virulent on cultivated barley. All were resistant and this resistance was also expressed in hybrids with H. vulgare. The tetraploid genotypes were also resistant to isolates of yellow rust and brown rust. These results show that H. bulbosum contains useful genes for resistance to these diseases and that there is a potential to transfer these into cultivated barley.     

Tetrahaploids from colchicine-induced octaploid Solanum acaule subsp. Aemulans

Summary Crosses between colchicine-induced octaploid Solanum acaule ssp. aemulans (2n=8x =96) with diploid S. tuberosum, Group Phureja, carrying a homozygous embryo marker, produced a high percentage of S. acaule tetrahaploids besides pentaploid and hexaploid F₁ hybrids. These results are discussed in relation to the behaviour of autopolyploid and allopolyploid Solanum species, which basically differ in the occurrence of haploids and the ploidy level of F₁ hybrids after pollination of both types of poly-ploids with Phureja.     

Crosses between Hordeum vulgare L. and H. bulbosum L. I. Production,morphology and meiosis of hybrids,haploids and dihaploids

Summary From crosses between diploid and autotetraploid cytotypes of Hordeum vulgare L. (cultivated barley) and H. bulbosum L. (bulbous barley grass) diploid, triploid and tetraploid interspecific hybrids were produced. Both directly and after vegetative segregation crosses in either direction also gave rise to haploids and dihaploids resembling H. vulgare. The use of embryo culture was necessary. Plant morphology of the hybrids was much like that of H. bulbosum, although the hybrid plants were less vigorous. Meiosis in the hybrids was more or less disturbed, and this seemed to be the main cause of the high level of sterility.     

Natural triploid hybrids between Solanum acauleBitter and S. infundibuliformePhilippi in the province of Jujuy,Argentina

Summary The occurrence of natural triploid hybrids between tetraploid Solanum acauleBitter subsp. acaule and diploid S. infundibuliformePhilippi both tuber-bearing Solanums, in the Puna region of Jujuy, is reported. The natural hybrids, which are morphologically intermediate between their putative parents and completely male sterile, have a somatic chromosome number of 36. The artificial hybrids, obtained by crossing both parental species, closely resemble the natural ones thereby confirming their parentage. It is proposed to designate the natural hybrids Solanum x viirsooi hybr. nov.     

Evidence of a new gene for high resistance to Meloidogyne spp. in Myrobalan plum, Prunus cerasifera

The Myrobalan plum Prunus cerasifera clones ‘P,2175’ and ‘P,1079’ carry single major genes (Ma1 and Ma2. respectively) for resistance to the predominant root–knot nematode (RKN) species Meloidogyne arenaria (MA), Meloidogyne incognita (MI) and Meloidogyne jaranica(MJ). The Myrobalan plum clone ‘P.2980’ is another complete–spec–trum source bearing favourable agronomic features. The genetics of its resistance to MA, MI, MJ and to the population Meloidogyne sp. Florida has been investigated from G1 crosses with the host Myrobalan plum clone ‘P.16.5’ (recessive for both Ma genes), the resistant clone ‘P.2175’ (heterozygous for Mal) and the ‘Nemared’ peach (homozygous for resistance to MA, MI and MJ but a host for M. sp. Florida). The segregation of the Gl hybrids from the intraspecific crosses into two significantly separated resistant and host classes, independent of the RKN species, indicates that resistance to all four species in ‘P.2980’ is also controlled by a single major dominant (heterozygous) gene. This gene is designated Ma3 because it shares the same spectrum and efficiency as Ma1 and Ma2, and its relationship with these last two genes is discussed. The first hybrids between ‘P.2980’ and ‘Nemared’ tested only segregated for resistance to M. sp. Florida. These results illustrate the possibility of cumulating (pyramiding) Ma3 and the ‘Nemared’ peach resistance gene(s) into new interspecific rootstocks.     

Inheritance of incompatibility in hybrids between Solanum tuberosum haploids and diploid species

The inheritance of self-incompatibility in two South American cultivated diploid species, Solanum phureja and S. stenotomum subsp. stenotomum, conforms with the Nicotiana scheme of oppositional S alleles. Similar studies in hybrids between S. phureja and a haploid (US-W 253) derived from S. tuberosum subsp. andigena were explained on the assumption of S alleles at homologous loci. The cut-stem technique employed in haploid extraction was used to facilitate the analyses. Maintenance of a series of heterozygous S allele testers is accomplished via matings with a clone homozygous for the S allele common to the series. True seed obtained from such crosses consists of only the desired S genotype.Departments of Horticulture and Genetics, University of WisconsinPaper No. 951 from the Department of Genetics, College of Agriculture, University of Wisconsin. Supported in part by the National Science Foundation and the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation.     

Yield and fruit quality losses caused by ToMV in pepino (Solanum muricatum L.) and search for sources of resistance

Tomato mosaic virus (ToMV) is a limiting factor for the success of pepino (Solarium muricatum) as a new crop. The effects of ToMV infection on total and marketable yield, fruit weight, length/width ratio and soluble solids content (SSC) have been studied in two commercial clones (`Sweet Long' and `Sweet Round'). ToMV infection depressed total yield in infected plants of `Sweet Long' (43.1%), while it had no effect on this trait in `Sweet Round' . Marketable yield was dramatically reduced by ToMV infection in both clones, 94% in `Sweet Long' and 100% in `Sweet Round'. Infected plants of clone `Sweet Long' had a lower weight than healthy plants. Although no differences in fruit weight were detected in `Sweet Round' between ToMV infected and healthy plants, many fruits from infected plants showed deformities. Changes in fruit length/width ratio and SSC as a result of ToMV infection were not relevant, but fruit quality was lower in infected fruits, most of which had corky-like flesh. Forty-two clones from cultivated (S. muricatum), wild (S. caripense and S. tabanoense) and interspecific hybrids were tested for ToMV resistance. All but seven clones (four from S. muricatum and three from interspecific hybrids S. muricatum × S. caripense) were susceptible. Non-susceptible clones showed variable degrees of resistance and developed hypersensitive local lesions. Among these clones the most promising as sources of variation for resistance to ToMV are those belonging to the cultivated species. Although no immunity was found, plants from these clones remained asymptomatic and absorbance values resulting from the DAS-ELISA tests in these plants were always lower than those of the susceptible control (cv. `Sweet Round'). These sources of resistance may be of great utility in developing commercial clones resistant to this severe disease affecting pepino. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unilateral ‘eclipse sterility’ in reciprocal crosses between Solanum verrucosum Schlechtd. and diploid S. Tuberosum L.

Summary F₁ hybrids from Solanum verrucosum CPC 2247 x diploid S. tuberosum exhibit a type of male sterility which after staining with lactophenol acid fuchsin gives the pollen grains an eclipse-like appearance, but size and shape are fully normal. The reciprocal cross, which could be made using some exceptional diploid S. tuberosum clones that did not show unilateral incompatibility with S. verrucosum, gives rise to F₁ hybrids without this eclipse sterility (Fig. 1 and 2).Pollen quality of the reciprocal hybrids was compared applying several direct and indirect methods. A critical cytological study revealed that meiosis in pollen mother cells is completely normal. When eclipse sterility is involved, the nucleus in the young microspore does not divide any more after meiosis and some kind of plasmolytic process occurs in the microspore leading to the characteristic eclipselike appearance.