Analysis on the spectra and level of wide-compatibility conferred by three neutral alleles in inter-subspecific hybrids of rice (Oryza sativa L.) using near isogenic lines

Wide‐compatibility varieties are a special class of rice germplasm that is able to produce fertile hybrids when crossed to either indica or japonica subspecies. Previous studies determined the f5 allele from ‘Dular’ (f5‐Du), f6 allele from ‘Dular’ (f6‐Du) and S5 allele from ‘02428’ (S5‐08) as neutral alleles conferring wide‐compatibility. However, the possible extent of the effect of the three neutral alleles has not been fully characterized because of the narrow range of the tester varieties used and the highly complex differentiation in Asian cultivated rice. In this study, we further developed the five near‐isogenic lines with higher recovery rates of the recurrent parent genome, and testcrossed to 14 japonica varieties, which have been widely used in rice breeding programmes in China. The results clearly revealed that all three neutral alleles exhibited substantial effects on spikelet fertility in most of the indica–japonica testcrosses, which indicated that these hybrid sterility loci have been extensively differentiated between indica and japonica varieties. The magnitudes of effects on spikelet fertility averaged over various crosses seem to be similar among the three neutral alleles, with f5‐Du, f6‐Du and S5‐08 alleles increasing spikelet fertility by 15.09%, 13.99% and 14.25%, respectively. The testcrosses involving f5‐Du allele generally showed much smaller variation in pollen fertility than others. The pyramiding lines with two neutral alleles showed a wider spectrum and a higher level of wide compatibility than others, whereas most of the increases in hybrid fertility couldn’t be simply explained by additive effects, suggesting the very complexity of wide compatibility and hybrid sterility. The indica–japonica hybrids involving restorer lines as one of their parents showed much higher pollen fertility (almost normal) and also higher spikelet fertility. The implications of the findings in rice breeding programmes are also discussed.     

Path and correlation analysis of perennial ryegrass (Lolium perenne L.) seed yield components

Maximum perennial ryegrass seed production potential is substantially greater than harvested yields with harvested yields representing only 20% of calculated potential. Similar to wheat, maize and other agriculturally important crops, seed yield is highly dependent on a number of interacting seed yield components. This research was performed to apply and describe path analysis of perennial ryegrass seed yield components in relation to harvested seed yields. Utilising extensive yield components which included subdividing reproductive inflorescences into five size categories, path analysis was undertaken assuming a unidirectional causal‐admissible relationship between seed yield components and harvested seed yield in six commercial seed production fields. Both spikelets per inflorescence and florets per spikelet had a significant (p < .05) direct effect on the overall seed yield; however, total path correlation coefficients showed that inflorescence size had the largest positive influence on seed yield via its indirect effects on other seed yield components. Our results will allow seed producers, seed production researchers and plant breeders alike to more efficiently increase harvested seed yield.     

Reproductive growth inLolium 1. Evaluation of genetic differences within an established variety by means of a diallel cross

Summary A genetic evaluation of S23 perennial ryegrass has been made by means of a 6 × 6 diallel cross. The characters studied were those associated with seminal reproduction and included seed yield and several of its primary components.The pattern of variation between the parents indicated a mainly additive system, itema of the Hayman analysis being significant for yield and all the other characters. Maternal effects were not a constant feature, but were evident for seed yield, plant weight and spikelet numbers/inflorescence. Dominance was a prevalent feature and invariably acted in the direction of the greater expression of the character.While a high level of incompatibility was evident from the paucity of seed setting on selfing, the variation between progeny yields following open pollination gave a range from the highly acceptable levels of commercial production to very low uneconomic levels. The results indicate that improvement in seed yield is feasible by breeding even though in this variety selection pressure has tended towards a restriction of variability for essentially agronomic features.     

Effects of Increasing Temperatures on Spikelet Fertility in Different Rice Cultivars based on Temperature Gradient Chamber Experiments

Spikelet sterility in rice (Oryza sativa L.) induced by high temperatures is a major concern given global warming predictions. We studied differences among eight rice cultivars in spikelet fertility at five different temperature levels in temperature gradient chamber (TGC) experiments. Six japonica and two indica cultivars were exposed to high‐temperature gradients in TGCs during the 2005 flowering season. Spikelet sterility increased with temperature in TGCs and differed among cultivars because of both variations in temperature tolerance and timing of heading. The correlation between spikelet fertility of individual panicles and both air temperature and panicle temperature during flowering was analyzed to compare tolerances among cultivars. The temperature (T₇₅) at which spikelet fertility was 75 % of maximum ranged from 34 to 39 °C air temperature and differed significantly among cultivars. Indica varieties had higher T₇₅ values than japonica varieties. The T₇₅ values based on panicle temperature also differed among cultivars, but the difference between indica and japonica varieties were less significant. We concluded that the higher temperature tolerances of indica cultivars in our experiments could be attributed to lower spikelet temperatures, and cultivars with similar spikelet temperatures still had different heat tolerances due to differences in pollination ability.     

Improvement of somatic embryogenesis and plant regeneration from durum wheat (Triticum turgidum var. durum Desf.) scutellum and inflorescence cultures

Scutellum and inflorescence explants of four genotypes of durum wheat(Triticum turgidum var. durum Desf.) were used to define culture conditions to obtain high frequencies of embryogenesis and plant regeneration in vitro. Under all conditions tested, scutellum cultures gave higher frequencies of embryogenesis and plant regeneration than inflorescence cultures. Two different auxins, 2,4-D(2,4-dichlorophenoxyacetic acid) and picloram(4-amino-3,5,6-trichloropicolinic acid), were compared for their effect on scutellum and inflorescence explant response in vitro. Picloram was found to significantly increase the frequency of plant regeneration from both explants. When cultures were grown on regeneration medium containing zeatin for two three-week passages, the frequency of plant regeneration increased by between 20–30% compared with cultures exposed to hormones for a single three-week passage. Finally, the addition of 1 mg/l 6-BAP (6-benzyl aminopurine) to the plantlet growth medium was found to enhance tiller production in regenerants. The optimized culture conditions were applicable to the four genotypes tested and frequencies of plant regeneration varied between 97% to 100% for scutellum cultures (2 mg/l picloram in induction medium) and between45% and 80% for inflorescence cultures (4 mg/l picloram in induction medium). The number of plants regenerated per explant was improved over previous procedures, with means of 34 plants per scutellum, and 16 plants per inflorescence explant. This revised version was published online in July 2006 with corrections to the Cover Date.     

Responses to photoperiod before and after jointing in wheat substitution lines

It has been hypothesised that wheat yields may be increased by lengthening the duration of the stemelongation phase. This paper reports studies on the effects of chromosomes carrying major photoperiod genes (Ppd-A1, Ppd-B1, Ppd-D1) in different genetic backgrounds, on responses to photoperiod before and after jointing, when the onset of stem elongation occurs, and on number of grains per spike. A field experiment considered the effects of two photoperiods on Chinese Spring and 12 substitution lines, in which chromosomes 2A, 2B or 2D had been substituted by those from four contrasting cultivars. The phase from seedling emergence to jointing (EM-JO) was more responsive than that from jointing to anthesis (JO-ANT), but no relationship was found between the duration of these phases. EM-JO length affected leaf and spikelet number and consequently grains per spike, but this component was further influenced by JO-ANT duration. Our results confirmed that the phases are independent in sensitivity, supporting the hypothesis that genetic manipulation of phase duration could enhance yield, but no evidence was found of any particular Ppd allele being responsible for major responses to photoperiod during stem elongation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chromosomal locations and genetic relationships of tiller and spike characters in wheat

Number of tillers per plant, plant growth habit in seedling and adult stages, and spike and spikelet characters are agronomically important features of the gross morphology of wheat. To localize to wheat chromosomes the genes for these traits, we scored them in a set of wheat recombinant-inbred mapping lines already well genotyped with molecular markers. Quantitative-trait analysis revealed a region near Gli-A2 (Xpsr10) on the short arm of chromosome 6A strongly affecting tiller number and the correlated trait of seedling growth habit. Genes with opposing effects on adult plant type were localized on the short arms of chromosomes2A and 3A, while genes affecting spike development were assigned to several A- and B-genome chromosomes. None of these genes showed synteny with counterpart QTLs reported to affect the same traits in rice. In the chromosome 2D region containing the photoperiod-insensitivity gene Ppd-D1, the major determinant of heading date in these autumn-sown lines, earliness alleles reduced tiller and spikelet numbers and increased erect seedling growth habit, but showed no influence on adult plant type or spike length. Though several of these morphological traits are generally considered to be associated with winter hardiness and their phenotypic intercorrelations were consistent with the genetic mapping evidence, no association was found between newly identified loci and known vernalization-response or frost-resistance loci. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of fertility between the F1, F2 and anther derived lines in the crosses of indica/japonica and japonica/indica in rice (Oryza sativa L.)

Summary Response of anthers in in vitro culture was examined in the indica-japonica hybrids of rice (Oryza sativa L.). Significant genotypic differences were observed for callus induction and regeneration among the different interracial hybrids of indica-japonica races. Induction frequency of haploids ranged from 57.7 to 72.9 per cent and doubled haploid androgenic lines ranged from 27.1 to 42.3 per cent in the anther culture of the different hybrids. The indica-japonica hybrids recorded partial pollen grain and spikelet fertility in F1 (29.9 to 41.5% and 19.4 to 48.7% respectively) as well as in F2 (42.7 to 50.6% and 37.1 to 54.4% respectively). In contrast, the androgenic doubled haploid lines recorded significant increase and the pollen grain and spikelet fertility was 76.3 and 78.6 per cent respecitively. The results suggested that the sterility barriers for realising genetic recombinants and fixation of fertile homozygous lines in indica-japonica hybridization programme could be overcome through F1 anther culture technique.Abbreviations BAP Benzyl Amino Purine - 2,4-D 2,4-Dichlorophenoxy acetic acid - MS Murashige & Skoog medium - IAA Indole Acetic Acid     

Mapping and candidate gene analysis for a new top spikelet abortion mutant in rice

Grain numbers is one of the determinations for rice yield and directly associated with spikelet numbers per panicle and its normal development. Lots of genes responsible for spikelet numbers and spikelet early development have been identified, but the molecular information about the spikelet development at later development is still limited. Here, we isolated a rice spikelet abnormal development mutant, which shows degenerated spikelet at the top panicle and named aborted top spikelet mutant 1(Ats1). The spikelets derived from the middle and bottom branches per panicle of Ats1 show normal development with those of wild type. However, a large number of branches and spikelets with arrested development were often observed only on apex panicle. The abnormality did not appear until the stage In8 when rachises elongate rapidly and reproductive organs get mature, based on observations through SEM analysis. The aborted spikelet could develop the complete floral organs with a pair of rudimentary glume, a pair of empty glume, two lodicule, six stamens and one carpel. But all these floral organs did not develop maturity. Genetic analysis on two F₂ populations indicated that the Ats1 was controlled by a single dominant gene. By using bulked segregant analysis of F₂ population developed from Ats1 crossing with Songjing6, ATS1 was mapped on chromosome 8 between RM3819 and RM5556. Then, the fine mapping was performed with 1078 F₂ population developed from Ats1 and IR36. The ATS1 locus was finely mapped in an 85.7 kb region between RM22448 and STS8‐2 with 8 genes according to the rice annotation project database. Sequence analysis of the candidate genes within the delimited region of the Ats1 and Akihikari showed two‐nucleotide changes, including single‐nucleotide substitutions corresponding to an amino acid substitution from asparagine to lysine acid in exons 3 and a 1‐bp deletion resulting in a premature stop codon in exon 22 at the candidate gene, LOC_Os08g06480. A cleaved amplified polymorphic sequence (CAPS) marker, CAPS‐ats1, was developed from the 1‐bp deletion site. The complete cosegregation of the CAPS genotypes with the matching phenotypes were observed in the F₂ populations. This suggested that Os08g06480 is most likely the ATS1 gene. These results will provide more information for better understanding of the molecular mechanism governing top spikelet abortion within a short developmental period.     

Enhanced grain yield in rice hybrids through complementation of fertility restoration by Rf3 and Rf4 genes as revealed by multilocation evaluation of tropical japonica derived rice (Oryza sativa) hybrids

Thirty-one tropical japonica derived Rf gene carrying rice hybrids were classified into three classes as Rf3, Rf4 and Rf3 + Rf4 hybrids. These hybrids were tested under three different mega-environments. Between Rf3 class and both the classes of hybrids possessing Rf4 genes, significant variation for spikelet and pollen fertility and grain yield was found. The pollen fertility was five times higher among Rf4 hybrids than that of hybrids carrying Rf3 alone. Likewise, spikelet fertility among Rf4 hybrids was two times higher than that of Rf3 hybrids. Parallel grain yield increase in Rf4 hybrids was 2.4 times than in Rf3 hybrids. However, Rf3 gene was found complementing Rf4 to truncate the range of pollen and spikelet fertility. Hybrids possessing Rf3 alone exhibited partial pollen and spikelet sterility, and significant negative standard heterosis for grain yield. The high yielding hybrid, “HYB36” carrying both the Rf genes was found to be widely adapted. The present study established that Rf4 gene is essential either alone or in combination with Rf3 for fertility restoration to achieve enhanced grain yield in WA-CMS based hybrids.     

Increasing biodiversity of irrigated rice in Africa by interspecific crossing of Oryza glaberrima (Steud.) × O. sativa indica (L.)

The African rice Oryza glaberrima, traditionally cultivated since more than 3.500 years, is of poor agronomic performance but resistant/tolerant to various stresses and diseases. The introduction of these characters into O. sativa cultivars is difficult since crossing barriers cause spikelet sterility in F1. Backcrossing can restore fertility and recently facilitated the development of fertile O. glaberrima × O. sativa ssp. japonica hybrid progenies for rain fed systems. With the objective to gain access to African rice germplasm for improvement of irrigated rice, crosses were performed with eighteen O. glaberrima and twenty O. sativa ssp. indica accessions. In total about one hundred F1-hybrid grains were obtained. The F1 plants were all completely sterile and backcrossing (BC) to O. sativa was performed in order to restore spikelet fertility. Monitoring of Tog5681 × IR64 hybrid progenies under field conditions revealed a broad genetic diversity within the BC₁ and BC₂ populations. Some BC₁ and BC₂ progeny plants outperformed the O. sativa parent, indicating that the heterozygocity level and complementary gene action after two backcrosses are still sufficient to positively influence plant vigor. Spikelet fertility of progenies was highly variable, but almost complete fertility was already observed within the BC₁F₂ population. High spikelet fertility was preserved in one out of two analyzed BC₁F₃ families and inmost of the BC₂F₃ families. The ability to restore spikelet fertility within few generations and the potential of the genetic diversity present in interspecific progenies facilitates the development of plant types specifically designed for the African irrigated and lowland environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of donor plant age and inflorescence age on microspore culture of Brassica napus L.

Summary Effect of age of donor plants and age of inflorescence on embryogenesis in microspore culture of B. napus was examined. Microspores isolated from buds of older plants had a higher embryo yield than those of younger ones. The effect of the age of inflorescence showed a different pattern. In older plants, a higher embryogenesis response was observed in microspores isolated from buds of new inflorescences, while in young plants, microspores isolated from buds of old inflorescences showed high embryo yield. These different responses were considered to be attributable to a difference in the developmental stage of pollen at the time of microspore isolation. Our results indicated that microspores collected from older inflorescences and older plants have sufficient embryogenic potential when the optimum developmental stage of pollen was used. Frequency of embryo to plant conversion was influenced by the size of embryos subcultured, but not by donor plant age or the age of the inflorescence.     

Growth, Development and Yield of Rice in Response to Cold Temperature

The effects of duration of cold, varying from 1 to 15 days, at 15/10 °C day/night temperature, were examined at floral initiation and booting in three cultivars of rice which differed in sensitivity to cold. Cold durations of greater than 1 day reduced spikelet number (up to 41 %), spikelet fertility (up to 90 %), inflorescence branch number (up to 43 %) and total length of branches (up to 34 %) relative to controls. Cold reduced spikelet number more than fertility at floral initiation, whereas cold reduced fertility more at booting. Some processes of growth — inflorescence branching, spikelet development, and dry matter accumulation — were directly and irreversibly damaged by cold. On the other hand, apex elongation showed thermal retardation (defined as a reduction in growth rate during the period of cold, only to the extent predicted from reduced thermal time) but recovered after cold was relieved. The sensitivity of a particular physiological process to cold may be related to the ability of the rate of that process to recover from cold stress and the opportunity (after cold stress is removed) for the completion of growth before the end of a development phase.     

A synthetic hexaploid (2n = 42) oat from the cross of Avena strigosa (2n = 14) and domesticated A. magna (2n = 28)

A synthetic hexaploid oat was produced by chromosome doubling of a sterile triploid hybrid between cultivated Avena strigosa (2n = 14) cv. Saia and a domesticated form of A. magna (2n = 28). The synthetic hexaploid was intermediate between its parents in panicle shape and lemma color, similar to the tetraploid parent in spikelet structure, and to the diploid parent in having a single, albeit partially shriveled seed per spikelet, and low protein content. By the third generation, plants with yellowish lemmas, mostly two seeds per spikelet and better filled grains had been selected. Rust resistance of the diploid parent was retained in the synthetic hexaploid, but not tolerance to barley yellow dwarf virus disease (BYDV). Chromosome associations at meiosis in the triploid hybrid was low, with over 60% of them being univalents. Bivalent association was the rule in the synthetic hexaploid with an occasional one or two quadrivalents. Regular meiosis with 21 bivalents was observed in further generations. The preferential pairing of homologous chromosomes in the synthetic hexaploid was probably contributed by the A. strigosa genome which exhibits this tendency in artificial allopolyploid situations. Selection of yellow lemma color and two seeds per spikelet suggests that the genes controlling these traits are located on the chromosomes involved in quadrivalents in the synthetic hexaploid. The potential and limitations of utilizing the synthetic hexaploid in oat research and breeding are briefly discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation in developmental patterns of wild barley (Hordeum spontaneum L.) and cultivated barley (H. vulgare L.)

Summary Six wild barley (Hordeum spontaneum) accessions, from a diverse range of habitats, and two spring-cultivated barleys, were examined for variation in durations of development phases. The durations of the leaf initiation and spikelet initiation phases were longer and spikelet growth phases shorter, in wild than in cultivated barley. Across all wild and cultivated barleys the rate and duration of spikelet initiation were negatively correlated, but neither was related to the number of spikelets per spike. The number of spikelets was positively correlated with the number of leaves and the ratio of the number of spikelets to the number of leaves declined with increasing time to anthesis, indicating that each successive leaf was associated with a diminishing increase in the number of spikelets. The duration of culm elongation and final culm length were shorter in accessions of cultivated barley compared with wild barley. This paper also discusses the feasibility for increasing the number of spikelets per spike through breeding for genetic changes in lengths of pre-anthesis phases of development.Abbreviations ANOVA Analysis of variance - HV Hordeum vulgare - CE Culm elongation - DR Double ridge - HS Hordeum spontaneu - ° Cd Degree days     

Microspore culture is successful in most crop types of Brassica oleracea L.

Summary Microspore culture was shown to be applicable to a broad range of accessions belonging to six horticulturally important crop types of Brassica oleracea: broccoli, white cabbage, cauliflower, savoy cabbage, Brussels sprouts and curly kale. Of 64 accessions tested 86% were responsive. Large genotypic differences were found in number of embryos produced per flower bud, and in frequency and mode of regeneration of plants from embryos. B. oleracea was characterized by a strong asynchrony of microspore development within single buds. Microspore populations optimal for culture contained a large proportion (10–40%) of binucleate pollen. An initial high temperature treatment was essential for microspore embryogenesis. Growth conditions of the donor plants during inflorescence formation were less critical.     

Complex segregation analysis of day-neutrality in domestic strawberry (<Emphasis Type="Italic">Fragaria ×ananassa</Emphasis> Duch.)

Thirty bi-parental progenies were generated using 45 genotypes from the University of California (UC) strawberry breeding population as parents. Both parent genotypes and their offspring were classified for photoperiod insensitivity, or day-neutrality, for flowering using a late-season flowering score and the number of inflorescences per plant recorded during late summer. Complex Segregation Analysis of these traits indicated that their distributions were best explained by a genetic model that postulates a single major locus with partial dominance for day-neutrality in combination with a background of polygenic and environmental variation. The frequency of the allele conferring day-neutrality was estimated as p = 0.59–0.62 in this experimental population. Genotypic values for the inferred major locus were estimated as a = 1.12 and d = −0.81 for the flowering score, and a = 4.93 and d = 2.41 for inflorescence number. Further resolution of inheritance patterns were obtained by comparing the phenotypic variance for each trait with estimates obtained by insertion of these genotypic class values and allele frequencies into standard quantitative genetic models, and by the comparing variance components estimated using a mixed model analysis with and without inferred genotypic classes as a fixed effect. These comparisons suggest that the major gene determines 80.5% and 73.9% of the additive genetic variance for flowering score and inflorescence number respectively. One complicating feature of the results obtained here is that a non-Mendelian model of segregation fit statistically better than a fixed Mendelian model. The genetic parameters estimated using this non-Mendelian model were essentially identical to those obtained with fixed segregation; hybrid and octoploid ancestry, selection affecting flowering response, and limited number of generations in the analysis are discussed as possible explanations of this result.     

Inheritance of genes controlling supernumerary spikelet in wheat line 51885

The supernumerary spikelet character of bread wheat (Triticum aestivum L.) is an abnormal spike with extra spikelet per spike. Supernumerary spikelet line 51885 was derived from the cross between octoploid Triticale and common wheat Fei 5056. The number of spikelet is over 30, and supernumerary spikelet is genetically stable. The inheritance of supernumerary spikelets in the line 51885 was genetically analyzed by crossed with seven commercial cultivars of normal head type. The results indicated that supernumerary spikelet in the line 51885 was controlled by two dominant genes which shows complementary function. Transgressive segregation for increasing the number of spikelet were observed in the four crosses, suggesting the possibility of the presence of minor effect gene(s) or modification gene(s) affecting the expression of the supernumerary spikelet in addition to the two major dominant genes. The relatively simple inheritance, feature of dominance plus the stability of expression of supernumerary spikelet in the line 51885 indicate that line 51885 could be easily incorporated in high yield breeding program in common wheat.     

The Amphiploid Hordeum chilense× Triticum aestivum ssp. sphacrocoecum. Variability in Octoploid Tritordeum

In order to increase variability in the octopioid tritordeum a new octoploid tritordeum has been synthesized after colchicine doubling of the chromosomes of the Hordeum chilense × Triticum aestivum ssp. sphaerococcum hybrid. The amphiploid showed full pairing in 30 % of the pollen mother cells, The fertility was higher than in previous tritordeums (1.65 grains per spikelet and 43.00 grains per spike). Hybrids between the octoploid tritorceums were sterile. However, hexaploid × octopiold trhordeum hybrids were fertile and secondary tritordeums were extracted with increased fertility.