Showing papers on "Heterosis published in 2003"

In Search of the Molecular Basis of Heterosis

Abstract: Heterosis, or hybrid vigor, refers to the phenomenon that progeny of diverse inbred varieties exhibit greater biomass, speed of development, and fertility than the better of the two parents ([Figure 1][1]). This phenomenon has been exploited extensively in crop production and has been a powerful

Single-locus heterotic effects and dominance by dominance interactions can adequately explain the genetic basis of heterosis in an elite rice hybrid

Abstract: The genetic basis of heterosis of an elite rice hybrid was investigated by using an “immortalized F2” population produced by randomly permutated intermating of 240 recombinant inbred lines from a cross between the parents of Shanyou 63, the most widely cultivated hybrid in China. Measurements of heterosis for crosses in the immortalized F2 population were obtained from replicated field trials over 2 years by inter-planting the hybrids with the parental recombinant inbred lines. The analyses were conducted making use of a linkage map comprising 231 segregating molecular marker loci covering the entire rice genome. Heterotic effects were detected at 33 loci for the four traits with modified composite interval mapping. The heterotic loci showed little overlap with quantitative trait loci for trait performance, suggesting that heterosis and trait performance may be conditioned by different sets of loci. Large numbers of digenic interactions were resolved by using two-way ANOVA and confirmed by randomization tests. All kinds of genetic effects, including partial-, full-, and overdominance at single-locus level and all three forms of digenic interactions (additive by additive, additive by dominance, and dominance by dominance), contributed to heterosis in the immortalized F2 population, indicating that these genetic components were not mutually exclusive in the genetic basis of heterosis. Heterotic effects at the single-locus level, in combination with the marginal advantages of double heterozygotes caused by dominance by dominance interaction at the two-locus level could adequately explain the genetic basis of heterosis in Shanyou 63. These results may help reconcile the century-long debate concerning the genetic basis of heterosis.

Genetic Diversity, Specific Combining Ability, and Heterosis in Tropical Maize under Stress and Nonstress Environments

Abstract: Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non-stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and non-stress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F 1 performance (F 1 ), SCA, midparent heterosis (MPH) and high-parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F 1 , SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

Genetic Analysis of Inbred and Hybrid Grain Yield under Stress and Nonstress Environments in Tropical Maize

Abstract: Drought and low soil N cause significant yield reductions in maize (Zea mays L.) grown in the tropics. Understanding the genetic basis of hybrid performance under these stresses is crucial to designing appropriate breeding strategies. This study evaluates under optimal, drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the genetic control and modes of gene action for grain yield; and (iii) the relationship between line per se and hybrid performance. Seventeen lowland white-grained tropical maize inbred lines were used in a diallel study. Lines and their hybrids were evaluated separately in trials under drought stress, low N, and optimal conditions in a total of 12 environments, The differences in grain yield between hybrids and inbreds (i.e., heterosis) increased with the intensity of drought stress. Significant interactions were observed for combining abilities under low and high N. The type of gene action appeared to be different under drought than under low N, with additive effects more important under drought and dominance effects more important under low N. The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parental lines to achieve acceptable hybrid performance under severe drought. Inbreds derived from the population 'La Posta Sequin' exhibited the highest GCA effects, stability coefficients, and frequency of dominant alleles for grain yield. Good performance across stress levels can be achieved in tropical maize hybrids.

Gene actions of QTLs affecting several agronomic traits resolved in a recombinant inbred rice population and two testcross populations.

Abstract: To understand the types of gene action controlling seven quantitative traits in rice, we carried out quantitative trait locus (QTL) mapping in order to distinguish between the main-effect QTLs (M-QTLs) and digenic epistatic QTLs (E-QTLs) responsible for the trait performance of 254 recombinant inbred lines (RILs) from rice varieties Lemont/Teqing and two backcross hybrid (BCF1) populations derived from these RILs. We identified 44 M-QTL and 95 E-QTL pairs in the RI and BCF1 populations as having significant effects on the mean values and mid-parental heterosis of heading date, plant height, flag leaf length, flag leaf width, panicle length, spikelet number and spikelet fertility. The E-QTLs detected collectively explained a larger portion of the total phenotypic variation than the M-QTLs in both the RI and BCF1 populations. In both BCF1 populations, over-dominant (or under-dominant) loci were more important than additive and complete or partially dominant loci for M-QTLs and E-QTL pairs, thereby supporting prior findings that overdominance resulting from epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice.

Genetic distance based on simple sequence Repeats and Heterosis in Tropical Maize Populations

Abstract: Heterotic groups and patterns are of fundamental importance in hybrid breeding of maize (Zea mays L.). The major goal of this study was to investigate the relationship between heterosis and genetic distance determined with simple sequence repeat (SSR) markers. The objectives of our research were to (i) compare the genetic diversity within and between seven tropical maize populations, (ii) test alternative hypotheses on the relationship between panmictic midparent heterosis (PMPH) and genetic distances determined with SSR markers, and (iii) evaluate the use of SSR markers for grouping of germplasm and establishing heterotic patterns in hybrid breeding of tropical maize. Published data of a diallel of seven tropical maize populations evaluated for agronomic traits in seven environments were reanalyzed to calculate PMPH in population hybrids. In addition, 48 individuals from each population were sampled and assayed with 85 SSR markers covering the entire maize genome. A total of 532 alleles in the 7 × 48 genotypes assayed were detected. The analysis of molecular variance (AMOVA) revealed that 89.8% of the variation was found within populations and only 10.2% between populations. The correlation between PMPH and the squared modified Roger's distance (MRD) based on SSR markers was significantly positive (P < 0.05) only for grain yield (r = 0.63). With SSR analyses, it was possible to assign Population 29 (Pop29) to the established Heterotic Group A and propose new heterotic groups (Pop25, Pop43). We conclude that SSR markers provide a powerful tool for grouping of germplasm and are a valuable complementation to field trials for identifying groups with satisfactory heterotic response.

Genetic basis of heterosis explored by simple sequence repeat markers in a random-mated maize population

Abstract: The genetic basis of heterosis in crop plants has not been completely resolved. Our objective in this study was to determine the level of dominance for quantitative trait loci (QTLs) that underlie heterosis in maize (Zea mays L.). An F2 population of an elite maize single cross, LH200 x LH216, was random mated for three generations in an attempt to break up repulsion linkages that might lead to pseudo-overdominance. The population was analyzed with 160 simple-sequence repeat markers. Phenotypic data analyses indicated overdominance for grain yield and partial dominance for plant height, grain moisture and stalk lodging. A total of 28 QTLs were identified for grain yield, 16 for grain moisture, 8 for stalk lodging, and 11 for plant height. For grain yield, 24 QTLs (86%) showed overdominance. In contrast, most of the QTLs for plant height, grain moisture and stalk lodging showed partial to complete dominance. Little epistasis was detected among the QTLs for any of the traits. Our results can be interpreted in one of two ways, or a combination of both: (1) QTLs for grain yield in maize exhibit true overdominance, or (2) QTLs for grain yield show partial to complete dominance, but they are so tightly linked such that three generations of random mating failed to separate their individual effects.

Modeling Factors Affecting the Severity of Outbreeding Depression

Abstract: Hybridization between populations may cause either increased fitness ("hybrid vigor") or de- creased fitness ("outbreeding depression"). Translocation between populations may therefore in some cases be a successful means of combating genetic erosion and preserving evolutionary potential, whereas in other cases it may make the situation worse by inducing outbreeding depression. Because genetic distance alone is a poor predictor of the success or failure of hybridization, we developed a computer model (ELAB) to explore other factors affecting the consequences of hybridization. Our model simulates diploid, unisexual popula- tions following Mendelian rules, and in this study we used it to test the effect of a variety of parameters on both the magnitude and duration of outbreeding depression. We focused our simulations on the effects of (1) divergence between populations, (2) the genetic basis of outbreeding depression (disruption of local adap- tation vs. intrinsic coadaptation), (3) population parameters such as mutation rate and recombination rate, and (4) alternative management schemes (50:50 mixture vs. one migrant per generation). The magnitude of outbreeding depression increased linearly with genetic distance, whereas the duration of outbreeding de- pression showed a more complex curvilinear relationship. With genetic distance held constant, magnitude in- creased with larger population size, lower mutation rate, cross-fertilization, and higher recombination rate, whereas duration increased with larger population size and partial self-fertilization. Fitness problems caused by disruption of local adaptation were stronger but more transient than those caused by a disruption of in- trinsic coadaptation. Finally, simulations showed that, depending on the genetic basis of outcrossing prob- lems, recurrent transfer of only one migrant per generation into a population of 100 individuals could cause as much or more damage as a one-time 50:50 mixture.

Prediction of hybrid performance in grain sorghum using RFLP markers.

Abstract: Heterosis is an important component of hybrid yield performance. Identifying high yielding hybrids is expensive and involves testing large numbers of hybrid combinations in multi-environment trials. Molecular marker diversity has been proposed as a more efficient method of selecting superior combinations. The aim of this study was to investigate the value of molecular marker-based distance information to identify high yielding grain sorghum hybrids in Australia. Data from 48 trials were used to produce hybrid performance-estimates for four traits (yield, height, maturity and stay green) for 162 hybrid combinations derived from 70 inbred parent lines. Each line was screened with 113 mapped RFLP markers. The Rogers distances between the parents of each hybrid were calculated from the marker information on a genome basis and individually for each of the ten linkage groups of sorghum. Some of the inbred parents were related so the hybrids were classified into 75 groups with each group containing individual hybrids that showed similar patterns of Rogers distances across linkage groups. Correlations between hybrid-group performance and hybrid-group Rogers distances were calculated. A significant correlation was observed between whole genome-based Rogers distance and yield (r = 0.42). This association is too weak to be of value for identifying superior hybrid combinations. One reason for the generally poor association between parental genetic diversity and yield may be that important QTLs influencing heterosis are located in particular chromosome regions and not distributed evenly over the genome. Variation in the sign and magnitude of correlations between Rogers distance and hybrid-group performance for particular linkage groups observed in this study support this hypothesis. The concept of using diversity on individual linkage groups to predict performance was explored. Using data from just two linkage groups 38% of the variation in hybrid performance for grain yield could be explained. A model combining phenotypic trait data and parental diversity on particular linkage groups explained 71% of the variation in grain yield and has potential for use in the selection of heterotic hybrids.

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

Abstract: 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

Heterosis for biomass yield and related traits in five hybrids of Arabidopsis thaliana L. Heynh.

Abstract: Heterosis is of utmost economic importance in plant breeding. However, its underlying molecular causes are still unknown. Given the numerous advantages of Arabidopsis thaliana as a model species in plant genetics and genomics, we assessed the extent of heterosis in this species using five hybrids derived from five ecotypes. Parents, F(1) and F(2), generations in both reciprocal forms were grown in a greenhouse experiment with four replications. Mid-parent heterosis (MPH) and best-parent heterosis (BPH) averaged across hybrids were surprisingly high for biomass yield (MPH: 60.3%; BPH: 32.9%) and rosette diameter (MPH: 49.4%; BPH: 34.8%), but smaller for flowering date (MPH: 27.5%; BPH: 18.5%), seed yield (MPH: 18.9%; BPH: 1.7%), and yield components. Individual hybrids varied considerably in their MPH and BPH values for all traits, one cross displaying 140.1% MPH for biomass yield. MPH was not associated with parental genetic distance determined from molecular markers. Reciprocal effects were significant only in a few cases. With a proper choice of hybrids, our results encourage the use of Arabidopsis as a model species for investigating the molecular causes of heterosis.

Advances in hybrid rice research and development in the tropics

Abstract: During the past five years, hybrid rice technology in the tropics has entered the commercialization phase in India, Vietnam, the Philippines, Bangladesh, and Indonesia. Almost all the commercial rice hybrids are derived from the cytoplasmic male sterile (CMS) system. An IRRI CMS line, IR58025A, has been widely used in the tropics. Even in China, this line has been used in Jiangxi and Hunan provinces to improve the grain quality of local hybrids. IRRI's hybrid rice breeding program has focused on developing improved CMS lines possessing cytoplasmic and genetic diversity, improved grain quality, a higher outcrossing rate, and resistance to diseases and insects. Most widely used wild abortive cytoplasm has been analyzed for its effect on grain quality and resistance to/tolerance of biotic and abiotic stresses. There was no negative effect on the traits except for grain chalkiness, which needs to be studied further. As reported earlier, there was no lack of restorer lines among elite indica lines bred in the tropics. Marker-aided selection (MAS) using the sequence-tagged sire marker RG140 with Pvull digestion linked with the Rf3 gene on chromosome 1 was useful for increasing screening efficiency for restorers. Progress was also made in developing stable TGMS lines possessing a low critical sterility point and good outcrossing. Several public and private hybrids were released in national agricultural research and extension systems and new elite hybrids were identified. Average seed yields were further improved in the national programs by further fine-tuning of the seed production technology, training, the choice of appropriate locations/seasons, and, above all, the experience of seed growers. A significant increase occurred in private-sector participation in the hybrid rice seed industry during the past five years. About 40 private companies are actively involved in hybrid rice research and/or seed production and marketing in tropical Asia. During 2001, private companies were producing 90% of the hybrid rice seeds in India. Hybrid rice in the tropics covered about 770,000 ha in 2002. Major constraints identified during the commercialization phase of the technology in the tropics were inconsistent performance, inferior grain quality, an inadequate level of disease/insect resistance of hybrids, the inadequate supply and high cost of hybrid seeds, and inadequate policy support. Future opprotunities include the development of hybrids possessing stronger heterosis and more stable yield performance, improved grain quality, an adequate level of disease/insect resistance, and higher seed yields. Both conventional and molecular methods will be used in hybrid breeding and in assessing seed purity. Agronomic management of hybrid rice in the tropics will also be improved to improve hybrid yield and stability in performance. Hybrid rice, which made a significant impact in China in the 20th century, should contribute significantly to food security and environmental protection in the tropical rice-growing countries in the 21st century.

Genetic architecture of differences between populations of cowpea weevil (callosobruchus maculatus) evolved in the same environment

Abstract: We investigated the genetic architecture underlying differentiation in fitness-related traits between two pairs of populations of the seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae). These populations had geographically distant (> 2000 km) origins but evolved in a uniform laboratory environment for 120 generations. For each pair of populations (Nigeria x Yemen and Cameroon x Uganda) we estimated the means of five fitness-related characters and a measure of fitness (net reproductive rate R0) in each of the parental populations and 12 types of hybrids (two F1 and two F2 lines and eight backcrosses). Models containing up to nine composite genetic parameters were fitted to the means of the 14 lines. The patterns of line means for all traits in the Nigeria x Yemen cross and for four traits (larval survival, developmental rate, female body weight, and fecundity) in the Cameroon x Uganda cross were best explained by models including additive, dominance, and maternal effects, but excluding epistasis. We did not find any evidence for outbreeding depression for any trait. An epistatic component of divergence was detected for egg hatching success and R0 in the Cameroon x Uganda cross, but its sign was opposite to that expected under outbreeding depression, that is, additive x additive epistasis had a positive effect on the performance of F2 hybrids. All traits except fecundity showed a pattern of heterosis. A large difference of egg-hatching success between the two reciprocal F1 lines in that cross was best explained as fertilization incompatibility between Cameroon females and sperm carrying Uganda genes. The results suggest that these populations have not converged to the same life-history phenotype and genetic architecture, despite 120 generations of uniform natural selection. However, the absence of outbreeding depression implies that they did not evolve toward different adaptive peaks.

Genetics of quantitative traits in Arabidopsis thaliana

Abstract: The genetic control of 22 quantitative traits, including developmental rates and sizes, was examined in generations of Arabidopsis thaliana derived from the cross between the ecotypes, Columbia (Col) and Landsberg erecta (Ler). The data were obtained from three sets of families raised in the same trial: the 16 basic generations, that is, parents, F1's, F2's, backcrosses, recombinant inbred lines (RILs) and a triple test cross (TTC), the latter produced by crossing the RILs to Col, Ler and their F1. The data were analysed by two approaches. The first (approach A) involved traditional generation mean and variance component analysis and the second (B), based around the RILs and TTC families, involved marker-based QTL analysis. From (A), genetic differences between Col and Ler were detected for all traits with moderate heritabilities. Height at flowering was the only trait to show heterosis. Dominance was partial to complete for all height traits, and there was no overdominance but there was strong evidence for directional dominance. For most other traits, dominance was ambidirectional and incomplete, with average dominance ratios of around 80%. Epistasis, particularly of the duplicate type that opposes dominance, was a common feature of all traits. The presence of epistasis must imply multiple QTL for all traits. The QTL analysis located 38 significant effects in four regions of chromosomes I, II, IV and V, but not III. QTL affecting rosette size and leaf number were identified in all four regions, with days to maturity on chromosomes IV and V. The only QTL for height was located at the expected position of the erecta gene (chromosome II; 50 cM), but the additive and dominance effects of this single QTL did not adequately explain the generation means. The possible involvement of other interacting height QTL is discussed.

Genetics of spike length in durum wheat

Abstract: Gene effects were analyzed using mean spike length of 12 populations, viz., both parents, F1, F2, first back cross generation, BC1 and BC2, second backcross generations, BC11,BC12, BC21 and BC22 along with BC1 self and BC2 self derived by selfing BC1 and BC2populations of three crosses involving six diverse cultivars of Triticum durumto determine the nature of gene actions governing spike length through generation mean analysis under normal and late sown environments. The six-parameter model was adequate in most of the cases to explain genetic variation among the generation means under both the sowing environments. Additive (d) gene effect was significant in all the cases, whereas dominance (h) gene effect was not so frequently observed significant. Epistatic effects, particularly digenic types were predominant over additive and dominance effects in most of the cases under both normal and late sown environments except in the cross Cocorit 71 × A-9-30-1 (normal sown).Additive × dominance × dominance (y), trigenic interaction played significant role in controlling the inheritance of this trait in the cross HI 8062 × JNK-4W-128under late sown condition. Duplicate epistasis was observed in the cross HI 8062× JNK-4W-128 (normal sown). Non-fixable gene effects were of higher magnitude than fixable gene effects in almost all cases, confirmed the major role of non-additive gene effects to control the inheritance of spike length in durum wheat. Significant heterosis over better parent was not observed. Similarly, inbreeding depression was not commonly observed. Favourable and suitable environment must be considered before finalizing breeding programme for its simple inheritance to get desirable improvement for high grain yield. Hybridization systems, such as biparental mating and / or diallel selective mating, which exploit both additive and non-additive gene effects, simultaneously, could be useful in the improvement of spike length in durum wheat.

Heterosis in a Broad Range of Alfalfa Germplasm

Abstract: Crosses between Medicago sativa L. subsp. sativa and subsp. falcata (L.) Arcing. produce progeny exhibiting heterosis for biomass yield. The purpose of this study was to determine which subsp. falcata germplasm produced the best hybrids in testcrosses with elite subsp. sativa material to guide future breeding efforts. Over 100 falcata genotypes from 40 populations were test crossed to four elite sativa populations. Testcross progeny and parental clones were grown for 2 yr in two locations and harvested three times per year to determine biomass yield. A broad range of testcross performance was observed, with mean heterosis values approximately zero. The highest yielding sativa-falcata hybrids were derived from European falcata germplasm. North American semi-improved falcata germplasm performed well in hybrid testcrosses. Preselection of parental falcata genotypes for autumn growth was associated with higher yielding testcross progeny. Positive heterosis was seen during the first harvest, but negative heterosis was often observed during second and, to a smaller extent, third harvests. Superior sativa-falcata hybrids were observed that showed good biomass yield and heterosis during all three harvests. Parental yield was least predictive of hybrid progeny yield during first harvest (h 2 = 0.12). Heritability increased during second and third harvest to 031 and 033, respectively. Expected genetic gain per selection cycle is greater from progeny testing compared with simple recurrent phenotypic selection.

Genetic distance and its association with heterosis in cacao

Abstract: The efficiency of cacao breeding program can be increased by choosing superior crosses to be made between divergent clones. We assessed the genetic distance among five clones with RAPD data (genetic distance - GD) and with yield component data (Mahalanobis distance - MD). The clones were evaluated in a diallel, during five years, for five yield components. A total of 130 RAPD bands were scored. GD and MD were used to determine the correlation between genetic distances among clones and the performance of their hybrids. The correlation between GD and MD was 0.67 (P=0.03). Both distances were related to heterotic performance of hybrids for wet seed weight/plant and wet seed weight/fruit. The average hybrid performance for the same two yield components was correlated with only MD. Hence, genetic distances measured by RAPD and yield components can be used as a guide to the choice of the superior crosses.

Analysis of reproductive traits in crosses among maternal lines of rabbits

Abstract: A crossbreeding experiment, between three maternal lines of rabbits, was carried out to estimate the effects of the crosses on reproductive traits. The experiment was designed as a complete diallel cross involving the lines, A, V and H, selected for litter size. A total of 1190 does, distributed between the nine types originating from the diallel cross, and born, reared and bred on the same com- mercial farm, were controlled. The recorded traits were the total number of young born, the number of born alive and the number of stillborn per litter (3759 records), and the interval between kindlings (2505 records). A repeatability animal model was used first to obtain the REML estimates of the com- ponents of variance. This was followed by a generalised least squares analysis and the estimates of the type of doe effects were obtained. Finally the differences among the lines within direct and maternal effects, and the corresponding values of the individual heterosis were estimated. Important differ- ences in the direct effects were found between lines V and A. All the maternal effects were not signifi- cant. Favourable heterosis (4-10%) was found for prolificacy traits in the crosses of line A with lines V and H, being the heterosis, very dependent on the performance and history of the lines involved. reproduction / crosses / maternal / lines / rabbits

Exploiting Heterosis in Pearl Millet for Population Breeding in Arid Environments

Abstract: In the desert region of Rajasthan, India, farmers mainly grow pearl millet [Pennisetum glaucum (L.) R. Br.] landraces. The adoption of modern cultivars is generally low because of their poor adaptation to extreme drought stress. The objective of this study was to evaluate the performance of six elite breeding populations and three landraces and to determine the heterotic pattern among the 36 diallel crosses of those populations. Field experiments were conducted in eight environments in India. Mean grain yields (GYs) in the three environments with favorable growing conditions were double to threefold those in the three arid environments. The elite populations generally showed higher GY than the landraces; stover yield (SY) was similar in both population types. The landraces flowered earlier, had a higher tillering potential, and smaller seeds. Mean level of midparent heterosis was generally low, ranging from 0.85% for time to flowering (TF) to 6.57% for SY. For GY, expression of heterosis for individual population crosses was between -14 and +30% under drought stress, and between -9 and +17% in the favorable environments. For SY, mean heterosis was always positive and higher than for GY. The elite x landrace population crosses with high mean GY and high levels of heterosis under drought stress could be beneficial to widen the germplasm base and to combine the high yield potential of elite materials with the good adaptation of the landraces.

Heterosis and combining ability for grain yield and other agronomic traits in winter triticale

Abstract: Hybrid breeding is a widely discussed alternative for triticale. Heterosis as well as general (GCA) and specific combining ability (SCA) effects were estimated for eight agronomic traits. The experiment comprised 24 F1 hybrids, produced by a chemical hybridizing agent, together with their six female and four male parents, grown in drilled plots in two locations. In comparison with the mid-parent values, hybrids averaged a 6.4 dt/ha (10.1%) higher grain yield, 8.4% more kernels per spike, a 6.8% higher 1000-kernel weight, 9.7% lower falling number (FN) and 4.4% greater plant height. SCA effects for grain yield were significant and ranged from 4.5 to 6.9 dt/ha for grain yield. Together with GCA x location interactions, they explained most of the variation. For 1000-kernel weight, GCA effects were predominant. SCA and interactions with location accounted for most of the variation in FN, whereas interactions were negligible for plant height. Correlations between mid-parent and hybrid performance and between GCA and per se performance of parents were tight for all traits except grain yield, which allows for pre-selection of parental lines. Although the amount of heterosis in triticale at present is closer to wheat than to rye, by selecting parents for combining ability and identifying heterotic patterns, grain yield heterosis of up to 20% appears sufficiently encouraging to embark on hybrid breeding.

Inbreeding and interbreeding in Darwin's finches.

Abstract: Studies of inbreeding and interspecific hybridization are generally pursued separately with different metrics. There is a need to integrate them because they have the common goal of seeking an understanding of the genetic and ecological basis of fitness variation in populations. We use mean expected heterozygosity as an axis of variation on which to compare the fitness of inbreeding and hybridizing Darwin's finches (Geospiza scandens and G. fortis) relative to the fitness of matched outbred controls. We find that relative fitness of inbred finches is less than one in the 1991 cohorts of both species. Inbreeding depression is stronger in the species (G. scandens) with the lower genetically effective population size. Relative fitness of hybrids (backcrosses) in the same cohort of G. scandens is greater than one. Evidence of heterosis in G. fortis is mixed. Thus the two interbreeding species displayed somewhat different fitness patterns under the same set of environmental conditions. Hybridization may enhance fitness to different degrees by counteracting the effects of inbreeding depression, by other additive and nonadditive genetic effects, and by producing phenotypes well suited to exploit particular ecological conditions.

Relationship between line per se and cross performance under low nitrogen fertility in tropical maize (zea mays L.)

Abstract: Improved maize yields cinder low nitrogen (N) fertility is desired for both low- input production systems and also for judicious application of fertilizer nitrogen for a sustainable and environment friendly production systems. A key question in breeding for tolerance to low-N fertility is to what extent the performance of maize hybrids can lie predicted on the basis of performance of their inbred lines under low-N supply. Past studies showed varied decree of correlalions between line and hybrid performance under low-N fertility, ranging from weak to fairly strong relationship. We attempted to examine the impact of low-N supply on relationship between morpho-physiological traits and grain yield measured on inbred parents, all advance generation lines and their single cross progenies. Reactions of various morpho-physiological traits, except days to anthesis (male flowering), shimmer significantly at low- and optimal level of N-fertilily. Superiority of hybrid progenies over parental lines declined tinder low-N stress, which indicates that hybrids were comparatively more affected than 10 lines clue to low-N supply. Data on individual contributions of heterosis and performance of mid-parent showed that per se performance of lines was relatively more important factor in >0 determining the hybrid performance tinder both tile levels ol N-supply Correlations between various secondary traits and grain yield cinder both low and optimal-N ferrilily wele moderate to strong in case of both parental lines and hybrids. 1 lowerve, it was comparatively stronger in case of hybrid progenies under low-N stress Correlations between morpho-physiological trails of lines and hybrids, except for ear rots, were fairly strong and significant under both the level of N supply. The relationship between grain yield of mid-parent and hybrids was comparatively stronger tinder low-N fertility. Our findings suggest that performance of hybrid progenies under low-N fertility can be predicted to some extent on the basis of advance generation elite inbred parents with proven performance across N-supply.

Heterosis expression in crosses between maize populations: ear yield

Abstract: The phenomenon of heterosis has been exploited extensively in maize (Zea mays L.) breeding. The objective of this study was to evaluate the genetic potential of ten maize populations for ear yield following the diallel mating scheme. Six parental populations were obtained through phenotypic selection of open-pollinated ears in Rio Verde, GO, Brazil, (GO populations) and four parental populations were synthesized in Piracicaba, SP, Brazil (GN populations): GO-D (DENTADO), GO- F (FLINT), GO-A (AMARELO), GO-B (BRANCO), GO-L (LONGO), GO-G (GROSSO), GN-01, GN-02, GN-03 and GN-04. Experiments were carried out in three environments: Anhembi (SP) and Rio Verde (GO) in 1998/99 (normal season crop) and Piracicaba (SP) in 1999 (off-season crop). All experiments were in completely randomized blocks with six replications. Analysis of variance grouped over environments showed high significance for heterosis and its components, although mid-parent heterosis and average heterosis were of low expression. The interaction treatments x environments was not significant. Total mid-parent heterosis effects ranged from de -4.3% to 17.3% with an average heterosis of 3.37%. Population with the highest yield (7.4 t ha-1) and with the highest effect of population (vi = 0.746) was GN-03, while the highest yielding cross was GO-B x GN-03 with 7,567 t ha-1. The highest specific heterosis effect (sii' = 0.547) was observed in the cross GO-B x GN-03.

Heterosis for Yield and Yield Components in Okra

Abstract: Significant heterotic effects In the positive direction were expressed for all the eight yield and yield contributing characters. Most of the high heterotic combinations were between geographically diverse parents. The crosses, SOH-02 x P. K., and SOH-02 x G. F., exhibited desirable negative and significant heterotic effects for the characters, days to initiation of flowering, days to initiation of fruit and days to maturity for green fruit. The cross DVR-3 x G.G. had recorded significant heterobeltiosis for yield plant -1 , fruits plant -1 , nodes plant -1 , branches plant-1 and plant height. The crosses JNDO-5x P. K. (153.43%) and NOL - 101 x GG (147.79%) also showed higher magnitude of heterosis over better parent.

Exploitation of heterosis for carotenoid content in African marigold (Tagetes erecta L.) and its correlation with esterase polymorphism.

Abstract: African marigold (Tagetes erecta L.), a major source of carotenoids, is also grown as a cut flower and a garden flower in addition to being grown for its medicinal values. We studied gene action, combining ability and heterosis, aiming at genetic improvement of T. erecta for enhanced carotenoid content in petals, and report for the first time that heterosis can be exploited for total carotenoids and its commercially important fractions. Total content of carotenoids and lutein appears to be governed by dominance (or non-additive) gene action, while content of xanthophyll esters is governed by both additive and dominance (or non-additive) gene actions. Specific combining ability variance was predominant for all the three traits. General and specific combining abilities and heterosis were highly significant. Heterobeltiosis was also positive. General combining ability (GCA) variances were not significantly correlated to performance per se. There was also no correlation between performance per se of normal petalled pollen parents and the performance of crosses made between male-sterile (female) and male-fertile (pollen) parents. These findings suggest that carotenoid content should not be the only criterion considered in the selection of parental lines. Studies on esterase in seeds and peroxidase in seedlings revealed a relatively high level of polymorphism in esterase with a total of 14 isoforms, whereas peroxidase showed low polymorphism. Similarity indices between different parental combinations, calculated based on seed esterase polymorphism, showed a significant negative correlation (r = -0.479, P = 0.05) with heterosis for carotenoid content. This indicates that the selection of parents with wider variation in their esterase profiles may possibly be exploited for genetic enhancement of carotenoids in T. erecta.

Exploitation and characterization of thermosensitive genic male sterile tgms lines for heterosis breeding in rice oryza sativa l

Abstract: EXPLOITATION AND CHARACTERIZATION OF THERMOSENSITIVE GENIC MALE STERILE (TGMS) LINES FOR HETEROSIS BREEDING IN RICE (Oryza sativa L.)