Inbreeding and the production of commercial F1 hybrid seed of brussels sprouts

Inbred lines, many of which have reached or passed the I₄ generation, have been produced by successive generations of self-pollination of material selected stringently for desirable characteristics. Of 99 parent plants originally selected from three different non-inbred varieties only eight were represented in the I₄ generation, by which time individual inbred lines were virtually true breeding for characters of commercial importance.An F₁ hybrid named Avoncross, produced by crossing unrelated selected inbred lines, has given consistently good yields of high quality sprouts in trials grown during several seasons. In most of these trials Avoncross gave the greatest marketable yield as compared with various control varieties and its superiority was significant at the 5% level of probability. A further F₁ hybrid, as yet un-named, has also given promising results in recent trials.Genetic male sterility was found in the material but did not provide an economic method for the production of F₁ hybrid seed. The method developed for the field production of F₁ hybrid seed relies on natural crossing between partially sib-compatible inbred lines. The partial sib-compatibility makes it possible to multiply the parent inbred lines by using natural sib-pollination in the field, but leads to the presence of sibs in the F₁ hybrid seed produced by natural crossing. The percentage of sibs in field crops of the F₁ hybrid established from such seed, however, was acceptably low because the majority of the sib-seedlings initially present could be rejected, on the basis of their smaller size, at the time of transplanting from the seedbed to the field.Experimental seed production, both of parent inbred lines and of the F₁ hybrid Avoncross, indicated that commercial production of F₁ hybrid seed in this manner was likely to be economically feasible.     

The use of seed acid phosphatases in the determination of the purity of F1 hybrid brussels sprout seed

Summary Starch gel electrophoresis of single seeds of five different F₁ hybrids of Brussels sprouts showed that they possessed three cathodal acid phosphatases. By comparison with cathodal acid phosphatases present in their inbred parents these have been interpreted as the two parental types plus a hybrid enzyme. All of the parental material could be classified into two groups depending upon whether or not their cathodal acid phosphatase was fast or slow moving. It was shown that these acid phosphatases are suitable for the determination of sibs in F₁ hybrid sprout seed provided that one of the parents possesses the slow moving cathodal acid phosphatase and the other the fast moving one. A survey of 35 different F₁ hybrids showed that 18 could be analysed for sibs using this method, those which could not were assumed to have had parents who possessed cathodal acid phosphatases of the same mobilities.     

Yield of F1, F2 and F3 hybrids of rice (Oryza sativa L.)

Summary Yield and yield components of F₁ hybrids were studied in three experiments at 30×30 cm spacings and in one experiment at 15×15 cm spacings. In the 30×30 cm experiments, 10 of the 41 hybrids tested significantly outyielded their high parents. However, only 2 hybrids significantly outyielded the best cultivar: one hybrid yielded 23 % and the other 16 % more than their respective check cultivars. The four hybrids in the 15×15 cm experiment yielded only 59 to 92% as much as their high parents.In areas where rice is transplanted at relatively wide spacings, the observed levels of F₁ heterosis in selected hybrids may be sufficient to warrant production of hybrid rice, if enough hybrid seed can be produced. For direct-seeding at the high rates normal in the USA, the relatively small levels of heterosis and the difficulties of hybrid seed production preclude use of F₁ hybrid rice cultivars at present.None of 19 bulk F₂ and F₃ hybrids in two experiments yielded significantly more than its high parent. Similarly, none of the 12 mixtures included in one experiment yielded significantly more than its high parent. On the basis of yield alone, using bulk F₂ or F₃ or simple mixture populations is not merited.Contribution from the Western Region, Agricultural Research Service, U.S. Department of Agriculture, and the Department of Agronomy and Range Science, University of California, Davis, California 95616.     

Chasmogamous mutant,a novel character enabling commercial hybrid seed production in mungbean

Mungbean (Vigna radiata (L.) Wilczek) is a self-pollinating crop that displays significant hybrid vigor in seed yield of F₁ hybrids. Thus there is the possibility to use hybrid varieties as a breakthrough to raise the yield plateau of mungbean. However, hybrid mungbean seeds can only be accomplished by hand-pollination and thus commercial production is not possible. To encourage hybrid seed set, the plant breeder needs to develop characters that promote higher outcrossing rate such as open flower (chasmogamy). In this experiment, new chasmogamous mutants were induced by gamma irradiation at the rate of 100 and 200 Gy. The mutants were identified at a low rate of 0.4–0.7% in the M₂ generation of accession V1197, and observed for their purity by growing in plant-to-row in the M₃ and M₄ generations. A uniform chasmogamous line was hybridized to normal flower lines to study the inheritance of this character. All F₁ plants had normal flowers, while the F₂ plants segregated well with 3 normal : 1 chasmogamous ratio. When the F₁ was backcrossed to the chasmogamous parent, the progeny gave a ratio of 1 normal to 1 chasmogamous. Thus, chasmogamy was controlled by a single recessive gene, cha.     

Comparing mating designs to restore seed production of interspecific hybrids between Trifolium repens (white clover) and Trifolium uniflorum

Interspecific hybrids between Trifolium uniflorum and cultivated white clover (Trifolium repens) have highly useful characteristics for temperate pastoral systems derived from both parent species. However, the early hybrids (F₁ and BC₁) also have unacceptably poor seed production for commerce. This study analysed the basis for the poor seed production and investigated breeding strategies for overcoming the problem. The BC₁F₁ generation produced lower‐than‐expected numbers of heads per plant and seeds per floret. Backcrossing of selected hybrids to white clover corrected these deficiencies and created new variation. Seed numbers were also returned to near target levels by recurrent selection within the BC₁ generation. Thus, it was possible to retain a theoretical average of 25% of T. uniflorum genome and still achieve high seed production per plant. The BC₁F₂ and BC₂F₁ generations produced high seed numbers per plant, along with reasonable variation. Both of these second‐generation hybrid forms have high reproductive potential and should be the focus for the selection of the desired combinations of agronomic and seed production traits.     

Progenies of an interspecific hybrid betweenArachis hypogaea and A.stenosperma — pest resistance and molecular homogeneity

Summary Interspecific triploid hybrids were obtained betweenArachis hypogaea L. andA. stenosperma Krapov. and W.C. Gregory by adopting hybridization coupled with rescue of the developing hybrid embryos. Two hexaploid hybrid populations were generated from triploids, somatically doubled (SD) and sexually polyploidized (SP) hexaploids. Microscopic screening for the occurrence of 2n gametes in triploid hybrids was useful to predict the production of spontaneous hexaploids. In order to facilitate maximum intergenomic recombination, the hexaploids were allowed to self for several generations (F₄) in the greenhouse. Prolific vegetative growth, pollen stainability, and seed set were observed to decline with each selfed generation. Individuals of the F₃ generation from the two hexaploid populations were evaluated for resistance to nematode (Meloidogyne arenaria Chitwood, race 1) and late leafspotCercosporidium personatum (Berk. & Curt.) Deighton under greenhouse conditions. Both SD and SP populations performed significantly better than their cultivated parent, Sunbelt Runner, and a susceptible cultivar, Florunner. At the DNA level, no significant differences were detected among hybrid individuals using cDNA clones and RAPD primers polymorphic for the two parents. Seven cDNA clones were used to probe DNA from 17 F₂ individuals and two parents and 45 RAPD primers were used to amplify DNA from 21 F₂ and F₃ individuals. No significant differences in banding patterns were observed among hybrid individuals which suggested that little or no detectable intergenomic recombination had occurred.     

Assessment of DNA markers for seed contamination testing and selection of disease resistance in cabbage

Cabbage (Brassica oleracea L. var. capitata) is an important vegetable worldwide. Most Japanese commercial cultivars of cabbage use an F₁ hybrid seed production system. The purity of F₁ hybrid seeds is important and the assessment of purity based on DNA markers can be highly accurate. In addition, selection of agronomically important traits such as disease resistance based on DNA markers is useful for breeding of cabbage. The aim of this study is to demonstrate the effectiveness of DNA marker-assisted selection in cabbage. In this study we distinguished the parental S haplotypes in 35 F₁ hybrid cultivars by combining several linked DNA markers. Thirty-one highly polymorphic simple sequence repeats (SSR) markers were screened from 175 reported SSR markers, which are useful for assessment of the purity of F₁ hybrid seeds. We examined the relationship between the DNA marker based genotype and the phenotype by an inoculation test of clubroot disease. A co-dominant PCR–RFLP marker was developed for selection of Fusarium yellows resistance and the genotypes using this marker were consistent with inoculation test in all tested samples.     

Combining ability of seed vigor and seed yield in soybean

Studies have shown no consensus in relationships between seed yield and vigor in soybean [Glycine max (L.) Merrill]. The lack of information regarding the inheritance of seed vigor prompted this study to determine the types of gene action and combining ability estimates for seed vigor and its related traits. Five high and six low seed vigor soybean genotypes were crossed in a diallel, and selfed to produce 55 F₂ progenies, which were examined, along with the parents, for seed vigor, yield, and seed weight. Significant genotype and environment effects were found for seed vigor and yield. General combining ability (GCA) effects for seed vigor and seed yield were significant (p≤ 0.01) and larger than specific combining ability (SCA) effects. Significant GCA and SCA effects were found for seed weight, indicating that both additive and non additive genetic effects were involved in conditioning seed weight. The ratios of mean square, 2GCA / (2GCA+SCA), were 0.96 for seed vigor and 0.93 for seed yield. These ratios indicated that additive gene effects were more important than non additive gene effects for seed vigor and seed yield in these crosses. Mean seed vigor(83.8%), as determined by accelerated aging germination, and mean seed yield (2,155 kg ha^-1)in high vigor × high vigor crosses were higher than the high vigor × low vigor and low vigor × low vigor crosses. Mean percent accelerated aging germination rates in F₂ populations from diallel crosses were significantly related to mid-parent seed vigor(r² = 0.52^**) and midparent seed size (r² = 0.31^**). These results indicated that levels of seed vigor can be improved through breeding, while maintaining high yields because of the predominance of GCA effects in both seed vigor and seed yield. This revised version was published online in July 2006 with corrections to the Cover Date.     

The use of genetic,manual and chemical methods to control pollination in vegetable hybrid seed production: a review

Production of hybrid varieties of vegetable crops is currently a desired breeding goal, due to their remarkable agronomic performance and to the possibility of intellectual property protection. However, efficient hybrid production requires a careful pollination control to guarantee the hybrid nature of F₁ seed. Several technologies ranging from manual emasculation to genetic transformation are used to inhibit pollen production in mother plants. In this review, we examine the principles underlying strategies like genetically determined systems (genic male sterility, cytoplasmic–genic male sterility, self‐incompatibility) and other methods (manual emasculation, chemical‐hybridizing agents) in different species, considering the benefits and drawbacks of their adoption. Finally, we present the current state of the art for vegetable hybrid seed production.     

Micropropagation of intra and interspecific Lens hybrids

An intraspecific (Tetir × ILL 323) and an interspecific (Alpo × L. odemensis) lentil hybrid were multiplied in vitro in three consecutive micropropagation cycles to increase the production of F₂ seeds. Cloning efficiencies were slightly higher for Tetir × ILL323 (83%) compared to Alpo × L. odemensis (67%). A total of 982 F₂ seeds were produced in the experiment with Alpo × L. odemensis, consisting of the 334 F₂ seeds of the original hybrid and 648 F₂ seeds produced by the 12 plants cloned; consequently, F₂ seed production was increased three-fold over the original hybrid (194%). A total of 6050 F₂ seeds were produced in the experiment with Tetir × ILL 323, made up of the 483 F₂ seeds of the original hybrid and 5567 F₂ seeds produced by the 15 plants cloned; therefore, F₂ seed production was increased by more than twelve-fold over the original hybrid (1153%). In both hybrids the F₂ seed production of cloned plants diminished in the three consecutive multiplication cycles, with the plants having experienced less vegetative development. In lentil, F₂ seed production of cloned plants is related for the first time to the length of their period of vegetative development. In conclusion, micropropagation of hybrids is an interesting tool to construct from a single individual large F₁ populations that enable to increase by a manifold the production of F₂ seeds useful for genetic studies and breeding.     

Performance Studies in F6 Lines of Autotetraploid Fenugreek

Hybridization followed by pedigree selection was carried out between two autotetraploid strains of fenugreek (Trigonella foenum-graecum L.). Morphological and cytological investigations were carried out in two autotetraploid parents, their F₁ hybrids and F₆ lines derived from F₁ after selfing. Cytomorphological data of F₆ lines were compared with those of F₁, hybrids and of their parents. The hybrid plants were intermediate with respect to different vegetative traits, however, they showed increased seed set as compared to their autotetraploid parents. The mean pod number per plant in F₆ was one and half times as high as one of the high yielding tetraploid parents and approximately twice as high as that of F₁ hybrid. Meiosis in F₆ was more regular than in F, and the parents. Thus, hybridization followed by pedigree selection is helpful not only in regularizing the meiotic behavior, but vigour and seed fertility have also improved in autotetraploid fenugreek.     

Fertility improvement in tetraploid pearl millet

Summary Autotetraploid (2n=4x=28) pearl millet inbred lines, Tift 23BE and Tift 239DB, have been developed for use in crosses with other polyploid Pennisetum species. Each line set less than 1% and 3% selfed and open-pollinated seed, respectively. Seed germination was usually less than 17%. First generation (F₁) hybrids between the two inbreds set up to 61% seed while up to 40% of the seed from hybrids germinated.Seed weight per inflorescence for two planting dates averaged 0.32 g and 3.06 g for the two inbreds and the second generation F₂ progeny, respectively. One hundred seed weight was also significantly higher (0.78 g vs 0.48 g) in the F₂ progeny, probably due better endosperm development. Chromosome behavior and pollen stainability were similar in the inbred parents and hybrids. However, the hybrids shed more pollen than the inbred parents.Heterosis was evident in the F₁ hybrids and F₂ progeny which showed significant increases in plant height, leaf length, leaf width, and inflorescence length in addition to fertility improvement.     

Effect of inbreeding on pseudo-self-compatibility in red clover (Trifolium pratense L.)

Summary The effect of inbreeding on pseudo-self-compatibility (PSC) was investigated in I₀, I₁, and I₂ clones of red clover, Trifolium pratense L., at 40°C. PSC was found to decrease with inbreeding. Significant differences were found among I₀ clones, among I₁ clones, and among I₂ clones for PSC. Significant differences in PSC within generations were found in two of eight I₁ clones and one of five I₂ clones.A new method of inbred line maintenance is proposed which combines the desirable features of vegetative and seed maintenance of inbred lines. Superior I₁ clones with different but homozygous S genotypes and PSC values of 8 to 10% would be vegetatively increased. Clones would be isolated under high temperature field conditions and selfed seed (PSC) would produce the I₂. Selfed seed from two different I₁ clonal lines would then be mixed and sown to produce single-cross seed. Singlecross seed obtained from different clonal sources would be mixed and sown for the production of double-cross hybrid red clover.Contribution from the Kentucky Agricultural Experiment Station. This paper (72-3-125) is published with approval of the Director, Kentucky Agricultural Experiment Station, Lexington.     

Breeding and evaluation of Digitaria eriantha for improved spring yield and seed production

Summary Digitaria eriantha is a subtropical grass which is widely sown as a pasture species in South Africa and cultivars have recently been released in Australia. The species is variable in seed production and in other agronomic attributes. This paper describes a breeding program designed to combine the seed production of cv. Premier with the high productivity in winter-spring of an almost sterile genotype.Hybrids between the two accessions were either triploid and sterile, or diploid with low fertility. The F₂ to F₅ generations were selected intensively for fertility and, at two contrasting sites, for spring productivity, with both maternal and paternal selection. The F₆ generation was grown as replicated swards and yield in spring, and fertility, were used for selection of elite F₅ plants to comprise the parents of a new cultivar (cv. Advance).In evaluation trials, spring dry matter production of Advance averaged 40% higher than that of Premier. On average, seed production of Advance was 11% lower than that of Premier and botanical fertility was consistently about half that of Premier. Experimental yields of cleaned seed of Advance were up to 138 kg/ha. In commerce, seed yields of Advance are likely to be lower than those of Premier unless higher inflorescence densities than Premier can be achieved.     

Inheritance of siliqua locule number and seed coat colour in Brassica juncea

The inheritance of siliqua locule number and seed coat colour in Brassica juncea was investigated, using three lines each of tetralocular brown seeded and bilocular yellow seeded. Three crosses of tetralocular brown seeded × bilocular yellow seeded lines were attempted and their F₁, F₂ and backcross generations were examined for segregation of these two traits. Brown seed colour and bilocular siliqua characters were found to be dominant over yellow seed and tetralocular siliqua, respectively. Chi‐square tests indicated that each trait is controlled by different sets of duplicate pairs of genes. Bilocular siliquae or brown seeds can result from the presence of either of two dominant alleles, whereas tetralocular siliquae or yellow seeds are produced when alleles at both loci are recessive. A joint segregation analysis of F₂ data indicated that the genes governing siliqua locule number and seed colour were inherited independently.     

Inheritance of petal colour and its independent segregation from seed colour in Brassica rapa

The inheritance of petal (flower) colour and seed colour in Brassica rapa was investigated using two creamy‐white flowered, yellow‐seeded yellow sarson (an ecotype from Indian subcontinent) lines, two yellow‐flowered, partially yellow‐seeded Canadian cultivars and one yellow‐flowered, brown‐seeded rapid cycling accession, and their F₁, F₂, F₃ and backcross populations. A joint segregation of these two characters was examined in the F₂ population. Petal colour was found to be under monogenic control, where the yellow petal colour gene is dominant over the creamy‐white petal colour gene. The seed colour was found to be under digenic control and the yellow seed colour (due to a transparent coat) genes of yellow sarson are recessive to the brown/partially yellow seed colour genes of the Canadian B. rapa cvs.‘Candle’ and ‘Tobin’. The genes governing the petal colour and seed colour are inherited independently. A distorted segregation for petal colour was found in the backcross populations of yellow sarson × F₁ crosses, but not in the reciprocal backcrosses, i.e. F₁× yellow sarson. The possible reason is discussed in the light of genetic diversity of the parental genotypes.     

A new gene that controls seed coat wrinkling in soybean

Seed coat wrinkling is a major factor affecting the germinability of soybean [Glycine max (L.) Merr.] seed produced in high-temperature environments, such as in the Early Soybean Production System of the midsouthern United States. Exposure of seed to high temperatures, coupled with alternating periods of wet and dry conditions, promotes seed coat wrinkling. This can predispose the seed to mechanical damage at harvest, further reducing germinability, and reducing the usability of the grain for seed beans. Previous studies identified a single recessive gene (shr) in a mutant line (T-311), located on chromosome 13 (linkage group F), which causes seed shriveling and seed coat wrinkling. The current study was undertaken to identify and genetically map new gene(s) that affect seed coat wrinkling. Crosses were made between a smooth-seeded accession (PI 567743) and a wrinkled-seeded accession (PI 87623). The parents, F₁, F₂, and BC₁ generations were phenotyped for seed coat wrinkling in a greenhouse in Stoneville, MS during the summer of 2006. Genetic analysis indicated that the wrinkled seed coat trait in PI 87623 was inherited as a single recessive gene. A test for allelism, conducted in the greenhouse with a segregating F₂ population derived from T-311?×?PI 87623, showed that the gene from PI 87623 is different from the shr gene in T-311. A field study of a larger population, derived from a reciprocal cross of the same parents, confirmed these results, but also suggested epistatic interactions between the genes. A linkage map was developed using 195 SSR and SNP markers on 168 F₂ individuals of the cross PI 567743?×?PI 87623. Linkage analysis identified only one significant locus which was located on chromosome 5 (linkage group A1), confirming identification of a new gene that controls seed coat wrinkling in soybean. This study demonstrates genetic control of seed coat wrinkling, which offers the potential for selecting cultivars with less seed coat wrinkling for heat-stressed production environments.     

Genetics of seed coat colour in lentil

Summary Four grey mottled seed coat colour lentil lines/cultivars were crossed to one brown seed coat colour cultivar. The F₁ hybrids were brown seeded in all the crosses. Segregation pattern for seed coat colour in F₂ and F₃ generations revealed that it is under control of a single dominant gene, which is present in the parent UPL 175 while a recessive gene is responsible for grey mottled seed coat colour in Pant L 406, Pant L 639, LG 120 and Rau 101.     

Identification and validation of an over‐dominant QTL controlling soybean seed weight using populations derived from Glycine max × Glycine soja

Heterosis, or hybrid vigour, has been used to improve seed yield in several important crops for decades and it has potential applications in soybean. The discovery of over‐dominant quantitative trait loci (QTL) underlying yield‐related traits, such as seed weight, will facilitate hybrid soybean breeding via marker‐assisted selection. In this study, F₂ and F_2 : 3 populations derived from the crosses of ‘Jidou 12’ (Glycine max) × ‘ZYD2738’ (Glycine soja) and ‘Jidou 9’ (G. max) × ‘ZYD2738’ were used to identify over‐dominant QTL associated with seed weight. A total of seven QTL were identified. Among them, qSWT_13_1, mapped on chromosome 13 and linked with Satt114, showed an over‐dominant effect in two populations for two successive generations. This over‐dominant effect was further examined by six subpopulations derived from ‘Jidou12’ × ‘ZYD2738’. The seed weight for heterozygous individuals was 1.1‐ to 1.6‐fold higher than that of homozygous individuals among the six validation populations examined in different locations and years. Therefore, qSWT_13_1 may be a useful locus to improve the yield of hybrid soybean and to understand the molecular mechanism of heterosis in soybean.     

Conversion of the genic male sterility (GMS) system of bell pepper (Capsicum annuum L.) to cytoplasmic male sterility (CMS)

Non‐pungent bell pepper (Capsicum annuum L.) lacks the cytoplasmic male sterility (CMS) nuclear restorer allele, Rf, and CMS cannot be employed in its F₁ hybrid seed production. To demonstrate that the genic male sterility (GMS) system in non‐pungent bell pepper can be converted to the CMS male sterility system, the conversion of GMS to CMS for non‐pungent bell pepper line GC3 was conducted by introgression of S‐type cytoplasm and the Rf allele from tropical pungent donors. After morphological traits were evaluated, two lines from BC₁F₁ containing S‐type cytoplasm and four lines from BC₂F₂ containing Rf allele, phenotypically similar to GC3, were obtained and could be employed as CMS male sterile lines and restorer lines for non‐pungent bell pepper. Four molecular markers potentially linked to traits of interest were also evaluated in BC₁F₁ and BC₁F₂ populations. This is the first time that GMS has been successfully converted to CMS in bell pepper, a significant contribution for bell pepper hybrid seed production.