Genetic variability among elite red clover ( Trifolium pratense L.) parents used in Chile as revealed by RAPD markers

Red clover (Trifolium pratense L.) is one of the main forage species of temperate regions. Cultivars of red clover are heterogeneous which makes their genetic analysis difficult. We applied RAPDs (Random Amplifed Polymorphic DNA) in order to assess the genetic relationship and levels of genetic variability existing among a group of 16 elite red clover parents organised in four subsets of 4 parents each. Out of 55 primers 21 provided reproducible results. A total of 135 reliable and polymorphic RAPD bands were detected which were used to estimate genetic distances among pair-wise combinations of elite parents. Nei and Li's similarity values ranged from 0.60 to 0.77, with a mean of 0.66, which reflects a rather high genetic variability among the genotypes evaluated. Lower levels of genetic variability, as detected by polymorphic loci and mean heterogeneity values, were detected in a subset of parents selected for resistance to the stem nematode. Cluster analyses resolved the different sets of parents in a manner consistent with what is known from their breeding origins. An Analysis of Molecular Variance detected substantial levels of variation within subsets of parents. RAPDs represent a valuable source of genetic information for red clover breeding programmes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Estimating the genetic relationship of hybrid bromegrass to smooth bromegrass and meadow bromegrass using RAPD markers

The two most widely grown bromegrass species in North America are smooth bromegrass, a hay type grass, and meadow bromegrass, a pasture type grass. Hybrid populations between these two species have been developed through hybridization and recurrent selection. The objective of this study was to determine the genetic relatedness of the hybrid bromegrass population S‐9073M to its parental populations using random amplified polymorphic DNA (RAPD) markers and to determine genetic variation within and between populations. Individuals from each of the three populations were genotyped at 43 polymorphic RAPD loci. One of the RAPD fragments was meadow bromegrass‐specific. Cluster analysis showed three groups representing the two parental populations and the hybrid population. An analysis of molecular variance (AMOVA), showed that the hybrid population had the highest within‐population variation, followed by meadow bromegrass and smooth bromegrass. The interpopulation genetic distance (phi‐statistic =Φ_st) was highest between meadow and smooth bromegrass and lowest between smooth and hybrid bromegrass. The hybrid population was genetically intermediate to smooth bromegrass and meadow bromegrass, but closer to smooth bromegrass, possibly reflecting the selection criteria used in the development of this hybrid.     

Structure and variation of subterranean clover populations from Sicily,Italy

Summary A collection of subterranean clover lines singled out from populations of the species Trifolium brachycalycinum and T. subterraneum collected in Sicily, Italy, was examined at two localities for flowering time and at one locality for oestrogen content and seed yield. The structure and variation of the populations of the two species were compared. The relationship between flowering time of the populations and some environmental features of their collection sites was examined to assess whether the maturity requirements of the two species were similar and to frame selection models focused on developing varieties of appropriate maturity. Populations of T. subterraneum were, on average, more complex than those of T. brachycalycinum, being characterized by higher number of lines per population and greater intra-population variation for flowering time, oestrogen content and seed yield. Furthermore, populations of T. subterraneum were, on average, about 15 days earlier than sympatric populations of T. brachycalycinum. Both the greater variation and the relative earliness of T. subterraneum occurred irrespective of the environments of origin of the populations. Inferences are drawn on the adaptive advantages that such features confer to T. subterraneum. Mean flowering time of the populations increased on increasing annual rainfall and altitude of the collection sites. However, the changes in maturity appeared almost exclusively related to variations in rainfall in T. subterraneum, while in T. brachycalycinum the effect of altitude was greater and that of rainfall less marked than in the former species.     

Variation for dry matter yield,seed yield and other agronomic traits in Ladino white clover landraces and natural populations

Summary A set of Ladino white clover ecotypes including the last available landraces and some natural populations collected from old permanent meadows, together with some white clover varieties of various origins, were evaluated as monocultures for dry matter (DM) and seed yield assessed in separate field experiments. Other agronomic traits were also recorded. The best performing variety, Espanso, was significantly outyielded by some ecotypes for DM and seed production. Compared to landraces, natural populations yielded on average far less seed but as much DM and tended towards higher persistence indicating that domestication of the native Ladino at the farm level only improved seed yield of the crop. Natural populations showed, besides lower values of most seed yield components, a distinct canopy architecture characterized by smaller leaves, longer internodes and denser stolons. Seed weight proved the best character discriminating between the two ecotype groups. Significant variation for most traits was found both among landraces, in which it was mostly relatable to differences among provenances, and among natural populations. The relations between the main multivariate patterns of phenotypic variation and a set of environment collecting variables suggested that evolutive adaptation to different agronomic practices was a major determinant of the overall variation found in landraces. Evidence is provided that a relevant part of the variation observed among ecotypes for some important traits could be genetic.Abbreviations DM dry matter - DMY dry matter yield - NRH number of ripe heads - NFH number of florets per head - PL petiole length - LS leaf size - IL internode length - NS number of stolons - PRO protein content - SY seed yield - TSW thousand seed weight - NSF number of seeds per floret - PCA principal components analysis - PC principal component     

Pollen Characteristics and Genetics of Induced and Spontaneous Genetic Male-Sterile Mutants in Rice

Pollen characteristics, inheritance and allelism of 23 genetic male-sterile (ms) mutants of rice (Oryza sativa L.) were investigated. Ten of the mutants were induced by ethyl methane sulfonate, one induced by ethyleneimine, seven induced by gamma rays, one induced by streptomycin, one derived from tissue culture and three were spontaneous mutants found in the field. Four pollen abortion types were observed among these ms mutants: 3 no-pollen (NP) type, 6 complete pollen abortion (CPA) type, 13 partial pollen abortion (PPA) type, and 1 stainable pollen abortion (SPA) type. Progeny tests over two years indicated that each of the mutants was inherited as a monogenic recessive. A partial diallel cross among the 23 ms mutants indicated that mutant E2 was allelic to G4, E5 was allelic to E9, N2 was allelic to N3, and that three other mutants, E3, G6 and T1, shared the same ms locus. Gene symbols ms-46 (t) through ms-63 (t) were assigned to these mutants.     

A geographical approach to identify phosphorus-efficient genotypes among landraces and wild ancestors of common bean

Cultivars of common bean (Phaseolus vulgaris L.) capable of yielding well at low levels of native or added phosphorus (P) are highly desirable in many tropical production systems. The objective of the present study was to identify geographical regions which might be sources of such genotypes. A total of 364 landraces, cultivars and wild genotypes, drawn from a broad geographic range, were divided on the basis of growth habit into four field trials, each comprising two levels of P, stressed and unstressed, on an infertile Andosol in Popayan, Colombia. The regression relationship between grain yield per plant in the presence and in the absence of stress was determined, and each genotype's deviation from this relationship was used as a measure of P-efficiency. There was highly significant variation in efficiency among genotypes in all growth habits, and in climbing beans there were consistent regional differences, superior genotypes being identified with greater frequency among those from Bolivia, West Mexico and South Mexico-West Guatemala. The latter region was promising for prostrate bush genotypes also. Wild beans in general performed relatively poorly; it appears that P-efficiency traits in P. vulgaris have been acquired during or after domestication. These results confirm that genetic differences in P-efficiency exist among common bean genotypes and suggest that these are related to geographic origin. Furthermore, the use of a representative sample of germplasm can help to identify segments of the gene bank that are especially promising as sources of desirable traits. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heterosis and Genetic Variances and their Implications for Breeding Improved Varieties of Spring Beans (Vicia faba L.)*

Handcrosses were made in a factorial manner with 24 Vicia faba inbred lines m three sets, each between four male and four female parents. The resulting 48 hybrids, their parental inbreds and three commercial varieties were grown in performance trials during the years 1985—87 with three replications at one location. Yield and yield components were recorded on single plant basis. The average superiority of the hybrids above the control varieties was 26 % in plant yield. The best crosses outyielded the controls by more than 50 %. However, a few inbred lines also reached, the yield level of the controls. The average heterosis was significant for all characters except for maturity date. In plant yield the average heterosis was 75%, ranging between 34% and 148% in the various cross combinations. The highest average heterosis of 110 % was observed in yield at the lateral stems. In all characters the best inbreds were superior to the lower hybrids and in some characters even equal to the average of the hybrids. A strong positive relationship was found for all characters between hybrid performance and mid-parent value and between the per se performance of the inbreds and their general combining ability. Genotypic differences between the hybrids as well as between the inbreds were highly significant. Most of the variability between the hybrids in all characters were attributed to general combining, ability with only little evidence for specific combining ability. In the discussion the results are evaluated according to the choice of the breeding category of the partial allogamous faba bean species.     

Genotypic Variation for Cold Tolerance among Twelve Maize (Zea mays L.) Populations

The objectives of this study were (1) to ascertain the genetic differences for cold-tolerance traits in a series of diallel crosses among 12 maize populations grown in field experiments, and (2) to compare, in controlled environment rooms, the modification of various metabolic parameters of two sets of four F1 variety crosses which, according to the field experiments, differed largely in seedling early vigour (cold-tolerant [Ct] vs cold-sensitive [Cs] Fl sets). Under field conditions, cold-tolerance adaptability, as monitored by shoot dry matter accumulation, appeared genetically controlled; moreover, considerable differences existed among genotypes. Field data suggested that shoot dry weight was a good indicator of plant adaptability to periods of unfavorable cool conditions. The growth-chamber experiments showed that shoot dry weights of the two sets of hybrids (Ct vs Cs), were more uniform at the highest temperature regimes than under suboptimal temperatures. Moreover, the laboratory study indicated that the interval of time of attain a specific stage of development provided a meaningful and useful criterion for differentiating the capacity of maize plants to grow at low temperatures. Although a clear association between a particular metabolite and the capacity of plants to grow at low temperatures was not observed, it was of interest to note that the total N concentration in the shoot was significantly higher in the Ct plants compared to the Cs ones.     

Computer-Simulated Evaluation of the Hybrid Weakness Gene System as a Means of Preventing Genetic Contamination of Rice Cultivars

Effectivness of the hybrid-weakness caused by a complementary interaction of two dominant genes Hwc₁ and Hwc₂ to suppress the spread of the red-grain gene into ordinary white-grained rice cultivars was investigated using the Monte Carlo computer simulation method. It was shown that the weakness genes, as far as being capable of reducing the fitness of the carrier by 80 percent or more, work quite effectively to suppress the genetic contamination of white-grained cultivars due to inflow of pollen from red-grained cultivars due to inflow of pollen from red-grain gene once the gene has been incorporated in to the population in the form of red-grained progeny produced as a result of the imperfect lethality of the initial hybrid between red-and white-grained cultivars. The weakness genes have little or no effect cultivars. The weakness genes have little or no effect either, when the contamination is initiated by an inflow of seeds or seedlings from ed-grained cultivars in these situations, removal (selection)of red grained plants Is the only countermeasure to exterminate the red-grain gene. The removal of red grained plants need not be intensive if carried out continuously. The spread of the red-grain gene after initial contamination is suppressed if the red-grain gene and weakness genes are linked. The suppressing effect of the linkage, however, is not substantial when the outcrossing rate is around or smaller than 0.01 as can be assumed in most rice cultivars.     

A Dominant Inhibitor Gene Inhibits the Expression of Ht2 against Exserohilum turcicum Race 2 in Corn Inbred Lines Related to B14

The purpose of this study was to elucidate the genetic cause for the lack of express ion of the gene Ht2 against Exserohilum turcicum race 2 on certain genetic backgrounds related to the inbred line B14. Two such inbreds (A635Rp and B59), susceptible to this pathogen were crossed onto the inbred Oh43—-Ht2Ht2. The following generations: F₁, F₂, F₃, F₄, backcrosses to both parents, selfings of the backcrosses, in addition to the parent lines, were evaluated for Ht2 expression in this study. Plants of each generation were inoculated with E. turcicum race 2 and evaluated for the expression of the chlorotic-lesion resistance determined by the gene Ht2. In spite of the reported dominance of Ht2 on some genetic backgrounds, the F₁ generations studied here did not show Ht2 resistant lesions. The data presented herein suggest that B₁₄ and related inbred lines carry a dominant gene(I) that inhibits the expression of the Ht2 gene. Chi-square analyses of the reactions of 108 progeny families studied supported this hypothesis.