Testcross Progeny Research Articles

Unraveling Epistasis With Triple Testcross Progenies of Near-Isogenic Lines

Libraries of near-isogenic lines (NILs) are a powerful plant genetic resource to map quantitative trait loci (QTL). Nevertheless, QTL mapping with NILs is mostly restricted to genetic main effects. Here we propose a two-step procedure to map additive-by-additive digenic epistasis with NILs. In the first step, a generation means analysis of parents, their F(1) hybrid, and one-segment NILs and their triple testcross (TTC) progenies is used to identify in a one-dimensional scan loci exhibiting QTL-by-background interactions. In a second step, one-segment NILs with significant additive-by-additive background interactions are used to produce particular two-segment NILs to test for digenic epistatic interactions between these segments. We evaluated our approach by analyzing a random subset of a genomewide Arabidopsis thaliana NIL library for growth-related traits. The results of our experimental study illustrated the potential of the presented two-step procedure to map additive-by-additive digenic epistasis with NILs. Furthermore, our findings suggested that additive main effects as well as additive-by-additive digenic epistasis strongly influence the genetic architecture underlying growth-related traits of A. thaliana.

A lupus-susceptibility C57BL/6 locus on chromosome 3 (Sle18) contributes to autoantibody production in 129 mice

Epistatic interactions between the non-autoimmune strains 129 and C57BL/6 (B6), used for generating gene-targeted animals, can induce a lupus-like disease. Genome-wide scan analyses of testcross progeny between these two strains have identified several lupus susceptibility loci, with the strongest linkage to the production of autoantibodies (auto-Abs) displayed by an interval on chromosome 1 of 129 origin (Sle16). However, the contribution of B6 loci to the lupus phenotype remained unknown. We used a congenic approach to deduce the contribution to the autoimmune traits of the B6 genomic interval on chromosome 3 (Sle18), previously shown to be linked to antinuclear Ab production. This interval, when transferred on a 129 background (a strain termed 129.B6-Sle18), promoted auto-Ab production targeting a broad spectrum of autoantigens, expansion of activated CD4(+)T and B cells and mild glomerulonephritis. Surprisingly, these immunological and serological defects were accompanied by a significant increase in the percentage of regulatory T cells (Tregs; CD4(+) Foxp3(+)). However, these cells, that expressed lower levels of Foxp3, had no impaired regulatory function when tested in vitro. These findings illustrate further the efficacy of congenic dissection for functional characterisation of individual lupus susceptibility loci and highlight the contribution of loci derived from non-autoimmune strains to the disease pathogenesis.

Hybrid maize breeding with doubled haploids. IV. Number versus size of crosses and importance of parental selection in two-stage selection for testcross performance

Parental selection influences the gain from selection and the optimum allocation of test resources in breeding programs. We compared two hybrid maize (Zea mays L.) breeding schemes with evaluation of testcross progenies: (a) doubled haploid (DH) lines in both stages (DHTC) and (b) S(1) families in the first stage and DH lines within S(1) families in the second stage (S(1)TC-DHTC). Our objectives were to (1) determine the optimum allocation regarding the number of crosses, S(1) families, DH lines, and test locations, (2) investigate the impact of parental selection on the optimum allocation and selection gain (DeltaG), and (3) compare the maximum DeltaG achievable with each breeding scheme. Selection gain was calculated by numerical integration. Different assumptions were made regarding the budget, variance components, correlation between the mean phenotypic performance of the parents and the mean genotypic value of the testcross performance of their progenies (rho( P )), and the composition of the finally selected test candidates. In comparison with randomly chosen crosses, maximum DeltaG was largely increased with parental selection in both breeding schemes. With an increasing correlation rho( P ), this superiority increased strongly, while the optimum number of crosses decreased in favor of an increased number of test candidates within crosses. Thus, concentration on few crosses among the best parental lines might be a promising approach for short-term success in advanced cycle breeding. Breeding scheme S(1)TC-DHTC led to a larger DeltaG but had a longer cycle length than DHTC. However, with further improvements in the DH technique and the realization of more than two generations per year, early testing of S(1) families prior to production of DH lines would become very attractive in hybrid maize breeding.

Analyses of phenotypes of recombinants and revertants from testcross progenies involving genes at the L group, conferring resistance to rust in flax

When testcross progeny between lines carrying any two genes of theL group were analysed, rare susceptible and modified recombinants were recovered. Further, some susceptible recombinants yielded rare resistant plants (revertants) in their progeny. A total of 39 recombinants and 5 revertants were analysed for their reaction to a maximum of nine rust strains each avirulent on genes involved in their origin. The data showed that: a) Some of the rare recombinants represent new specificities. b) Reaction of a recombinant to a particular rust strains is usually stable. c) Modified recombinanls may be susceptible to certain rust strains. d) Revertants do not always possess the parental specificity.

Epistatic Interactions in Crosses of Illinois High Oil × Illinois Low Oil and of Illinois High Protein × Illinois Low Protein Corn Strains

Epistasis has been proposed as a possible explanation for the long continued progress from selection in the Illinois long‐term corn (Zea mays L.) selection strains. From the crosses of Illinois High Oil (IHO) × Illinois Low Oil (ILO) and of Illinois High Protein (IHP) × Illinois Low Protein (ILP), 500 S2 lines were developed. Each IHO×ILO line was genotyped for 479 single nucleotide polymorphism (SNP) markers, and each IHP×ILP line was genotyped for 499 SNP markers. Per se and testcross progenies of each S2 line were evaluated for oil, protein, and starch. A large number of QTL were found to control these traits. The objective of this paper is to report the results of analysis of these crosses for two‐way epistatic interactions. More epistatic interactions were significant than expected by chance. The proportion of significant interactions that involved a marker significant in a single marker analysis (SMA) was no greater than expected by chance. The number of markers associated only with significant epistatic effects ranged from 46.3 to 72.2% of the total number of markers significant for either an interaction effect or from SMA. The number of QTL controlling a trait is much greater than will be found by analyzing for significant QTL main effects. Thus, epistasis could contribute to the long continued response to selection in the Illinois long‐term selection strains and also may help explain the continued success of commercial corn breeding.

Inheritance of Blast Resistance and Associated Microsatellite Markers in Rice Cultivar ‘Laxmi’

AbstractLeaf blast, caused by Magnaporthe oryzae is a major disease of rice (Oryza sativa L.) in many rice‐growing environments. Rice breeders continuously seek new sources of resistance for improving resistance of commercial cultivars. This study was undertaken to determine the inheritance of blast resistance in the rice cultivar ‘Laxmi’. Both parents, and their F1, F2, backcross, F3 and F4 progeny were studied to determine the inheritance of blast resistance in a cross between resistant (R) ‘Laxmi’ and susceptible (S) ‘Mahsuri’ cultivars. Ten homozygous resistant and 10 homozygous susceptible F5 progeny based on their blast reaction in F3 from the above cross were subjected to microsatellite marker analysis to determine the linkage between resistance and marker. The F1 plants were blast‐resistant. The test cross progeny segregated in a ratio of 1R:1S. The F2 population segregated into 3R:1S ratio. The F3 and F4 progeny showed a segregation ratio of 1R:2Sg (segregating):1S. RM25 and RM310 markers showed association with resistant and susceptible progeny. We conclude that blast resistance in Laxmi is governed by a single dominant gene located on chromosome 8 of rice, which is linked to RM25 and RM310 microsatellite markers. This source of blast resistance could be useful in the Himalayan and other rice‐growing environments. This information could be used in marker‐assisted selection for blast resistance in rice crosses involving Laxmi.

Genetic Basis of Heterosis for Growth-Related Traits in Arabidopsis Investigated by Testcross Progenies of Near-Isogenic Lines Reveals a Significant Role of Epistasis

Epistasis seems to play a significant role in the manifestation of heterosis. However, the power of detecting epistatic interactions among quantitative trait loci (QTL) in segregating populations is low. We studied heterosis in Arabidopsis thaliana hybrid C24 x Col-0 by testing near-isogenic lines (NILs) and their triple testcross (TTC) progenies. Our objectives were to (i) provide the theoretical basis for estimating different types of genetic effects with this experimental design, (ii) determine the extent of heterosis for seven growth-related traits, (iii) map the underlying QTL, and (iv) determine their gene action. Two substitution libraries, each consisting of 28 NILs and covering approximately 61 and 39% of the Arabidopsis genome, were assayed by 110 single-nucleotide polymorphism (SNP) markers. With our novel generation means approach 38 QTL were detected, many of which confirmed heterotic QTL detected previously in the same cross with TTC progenies of recombinant inbred lines. Furthermore, many of the QTL were common for different traits and in common with the 58 QTL detected by a method that compares triplets consisting of a NIL, its recurrent parent, and their F(1) cross. While the latter approach revealed mostly (75%) overdominant QTL, the former approach allowed separation of dominance and epistasis by analyzing all materials simultaneously and yielded substantial positive additive x additive effects besides directional dominance. Positive epistatic effects reduced heterosis for growth-related traits in our materials.

Resistance to Puccinia polysora in Maize Accessions.

A number of potential sources of general and specific resistance to southern corn rust were identified from 1,890 plant introduction accessions that were screened for reactions to Puccinia polysora race 9. Resistance appeared to differ among four accessions on which uredinia were not observed in initial screenings. Resistance to P. polysora in PI 186215 (Argentine inbred 2-687) was a chlorotic fleck, hypersensitive reaction that was conditioned by a single, dominant gene that was allelic with or very closely linked to the Rpp9 gene based on tests of allelism. All but 3 of 2,357 testcross progeny, (inbred 2-687 × Rpp9) × PS were resistant. Resistance in Ames 19016 (Va59) was effective in F1 progeny and appeared to be dominant and simply inherited; however, this resistance appeared to be a slow-rusting or incomplete resistance that was effective in adult plants but not in young seedlings. Severity of southern rust was less than 10% on resistant progeny from crosses with Va59 compared with severity exceeding 70% on susceptible progeny. Resistance in plant introduction (PI) 186209 (Venezuelan flint) and NSL 75976 (IA DS61) were not effective in F1 hybrid combination and, thus, probably have limited value in commercial maize. Resistance in PI 186209 may be conditioned by a single recessive gene and resistance in NSL 75976 may be co-dominant.

Segregation Among Test-Cross Progeny Suggests That Two Complementary Dominant Genes Explain the Difference Between Ascites-Resistant and Ascites-Susceptible Broiler Lines

Ascites syndrome (AS) is a major cause of economic losses to the broiler industry. The tendency of broilers to develop AS was found to be heritable, suggesting that selective breeding could provide a solution to this problem. To further elucidate the genetic control of AS, AS-susceptible (AS-S) and AS-resistant (AS-R) lines were established by 3 cycles of divergent selection on pedigree data of AS mortality under AS-inducing conditions. The rapid divergence between the lines suggested the involvement of a major gene with dominance for AS resistance. It was hypothesized that the difference between the lines is controlled by a single dominant gene, denoted A, with AA (and some Aa) individuals in the AS-R line, and aa individuals in the AS-S line. The current study was designed to test this hypothesis by test-crossing heterozygous (Aa) sires from the AS-R line and F(1) from the AS-R x AS-S cross, with recessive homozygous (aa) dams from the AS-S line. A ratio of 1:1 was expected between progeny with AS vs. healthy progeny when reared under AS-inducing conditions. Test-cross progeny of 5 sires from the AS-R line segregated 1:1, indicating that these sires were heterozygous (Aa) in the suggested major gene and thus supporting the hypothesis of a single major gene with dominance of AS resistance. There was segregation among test-cross progeny of 8 F(1) sires, but with a 3:1 ratio of AS progeny to healthy progeny. The 3:1 ratio is expected if the F(1) sires are heterozygous (AaBb) with complementary interaction between the dominant alleles in 2 unlinked major genes. The segregation among test-cross progeny of the 9 heterozygous AS-R sires could also be explained by the same model. These results suggested that 2 major genes are responsible for the difference between the AS-R and AS-S lines. Resource populations derived from these lines will facilitate an efficient genomic search for these 2 genes. Once the alleles of these genes are identified and genotyping tests are developed, breeders will easily be able to select against AS susceptibility.

Hybrid maize breeding with doubled haploids: III. Efficiency of early testing prior to doubled haploid production in two-stage selection for testcross performance

Early testing prior to doubled haploid (DH) production is a promising approach in hybrid maize breeding. We (1) determined the optimum allocation of the number of S(1) families, DH lines, and test locations for two different breeding schemes, (2) compared the maximum selection gain achievable under both breeding schemes, and (3) investigated limitations in the current method of DH production. Selection gain was calculated by numerical integration in two-stage breeding schemes with evaluation of testcross progenies of (1) DH lines in both stages (DHTC), or (2) S(1) families in the first and DH lines within S(1) families in the second stage (S(1)TC-DHTC). Different assumptions were made regarding the budget, variance components, and time of DH production within S(1) families. Maximum selection gain in S(1)TC-DHTC was about 10% larger than in DHTC, indicating the large potential of early testing prior to DH production. The optimum allocation of test resources in S(1)TC-DHTC involved similar numbers of test locations and test candidates in both stages resulting in a large optimum number of S(1) families in the first stage and DH lines within the best two S(1) families in the second stage. The longer cycle length of S(1)TC-DHTC can be compensated by haploid induction of individual S(1) plants instead of S(1) families. However, this reduces selection gain largely due to the current limitations in the DH technique. Substantial increases in haploid induction and chromosome doubling rates as well as reduction in costs of DH production would allow early testing of S(1) lines and subsequent production and testing of DH lines in a breeding scheme that combines high selection gain with a short cycle length.

Estimating the Proportion of Nitrogen Remobilization and of Postsilking Nitrogen Uptake Allocated to Maize Kernels by Nitrogen‐15 Labeling

ABSTRACTEstimating the proportion of N remobilization and postsilking N uptake allocated to kernels may help to improve N‐use efficiency in maize (Zea mays L.). In this study, we show theoretically and experimentally, that 15N labeling at the beginning of stem elongation can be used in the field to estimate the proportion of N remobilized from the vegetative parts to the kernels of maize by measuring 15N distribution only at maturity. In the same way, 15N labeling at silking allows a determination of the proportion of postsilking N uptake allocated to the kernels by measuring 15N distribution only at maturity. Two 1‐yr experiments with three and four genotypes and one 2‐yr experiment with testcross progenies from 66 recombinant inbred lines were developed. Nitrogen‐15 labeling during vegetative growth provided an estimate of the proportion of N remobilized with a greater accuracy compared with the “balance” method, which computes the difference between the amount of N present in the stover at silking and the amount of N present in the stover at maturity. The validity of our 15N method is mainly based on the assumption that less than 15 to 20% of 15N is taken up after silking. The determination of the proportion of postsilking N uptake allocated to kernels requires assumptions that are more difficult to fulfill. Nitrogen‐15 labeling in the field at the beginning of rapid stem growth appears to be a useful tool for studying the genetic variability of N remobilization using a large number of genotypes.

Doubled-haploid versus single-seed descent and S1-family variation for testcross performance in a maize population

Progress made in the in situ gynogen- esis technique since 1990 now allows production of a high number of maize (Zea mays L.) doubled-haploid (DH) lines. The aim of the study was to compare DH lines versus selfing lines for testcross performance. DH and single-seed des- cent (SSD) lines were produced from random S1 progenies of a broad-base population. For grain yield, kernel moisture, plant height, ear height and leaf length, the three population means were similar. Except for kernel moisture, the genetic variance of DH lines was nearly twice as high as the genetic variance of S1 families, as expected. On the other hand, genetic variance among SSD lines was only 1.5 times higher than the genetic variance of S1 families. This lower variance could be due to a selection bias in the method of production of SSD lines. However, for all traits, heritability of SSD or DH lines was higher than heritability of S1 families. Epistasis effects in DH progenies were not significant. The consequence was a high correlation between S1 testcross progenies and DH or SSD testcross progenies, meaning that the S1 testcross value can be used to select the best families from which DH lines will be extracted. As a whole, the observed variation in DH lines appeared to be more in accordance with the observed variation among S1 families than with the observed variation among SSD lines.

Clonal performance of perennial Oryza sativa/ O. rufipogon selections and their combining ability with O. sativa cultivars for survival, stolon production and yield

In Southeast Asia, upland rice ( Oryza sativa) is an annual crop typically grown for subsistence on hilly lands that are at risk for soil erosion. If perennial cultivars of upland rice were developed, they would provide farmers with an inexpensive tool to produce a preferred food while conserving soil. O. rufipogon, the undomesticated progenitor of O. sativa, includes perennial and stoloniferous forms. To evaluate the feasibility of developing perennial cultivars by combining genes of O. sativa and O. rufipogon, four trials were conducted in an upland field at IRRI: O. sativa/ O. rufipogon F 1 clone and cutting height trial, F 2 family trial, and two O. sativa/stoloniferous-selection factorial mating design trials. Plants were established at the beginning of the rainy season, then subjected to a 6-month dry season and evaluated for survival after 1 year. Of the 2101 cultivar control plants, only three IR47686-1-4-B individuals survived and none produced stolons. The 18 F 1 clones, which were selected from previous trials with less drought stress, ranged in survival from 4.4 to 91.4%. Cutting height at harvest did not affect survival. Survival among the F 2 families ranged from 9.4 to 31.9%. Segregation for stolon presence did not differ from a 3:1 ratio for five of the six F 2 families, suggesting the effect of a single dominant gene. Average yields per plant for the F 2 families were 1/3 to 1/9 of yields for the cultivars. However, by crossing the F 1s to cultivars, yield potential was almost fully recovered. For the full-sib families of the factorial trials, survival ranged from 0.0 to 48.6%. Azucena and IR47686-1-4-B, both japonica cultivars, exhibited greater general combining ability for survival relative to the six other cultivar parents. Thus, the process of developing perennial cultivars of rice should include screening annual cultivars to identify those with the best combining ability for survival. To develop cultivars of perennial upland rice, drought avoidance and/or tolerance from annual upland cultivars must be combined with the capacity for perennial growth from wild perennial species. The frequency of stolonifereous testcross progeny was lower than expected and was affected by the O. sativa parent. Thus, additional genes likely affected stolon penetrance and expression. Analyses of covariance indicated that stolons improved the likelihood of survival for progenies of the factorial trials but the effect was small ( b < 0.1), and that yield (g/plant) had a small negative effect on percent survival ( b = −0.13 to −0.32). This study demonstrated that it was possible to introgress genes for perennial growth from wild O. rufipogon accessions into domesticated O. sativa. Additionally, strategies for developing perennial cultivars of upland rice were improved.

Molecular mapping of the fertility‐restoration gene Rf4 for WA‐cytoplasmic male sterility in rice

AbstractThe wild abortive cytoplasmic male sterility (CMS‐WA) system, an ideal type of sporophytic CMS in indica rice, is used for the large‐scale commercial production of hybrid rice. Searching for restorer genes is a good approach when phenotyping is very time‐consuming and requires the determination of spikelet sterility in testcross progeny. To establish more precisely the genetical and physical maps of the Rf4 gene, high‐resolution mapping of this locus was carried out using simple sequence repeat (SSR) markers and newly designed markers in a F2 population. The genetic linkage analysis indicated that five SSR markers (RM6737, RM304, RM171, RM5841 and RM228) on the long arm of chromosome 10 were linked with the Rf4 gene. Rf4 was flanked by two SSR markers RM171 and RM6737 at distances of 3.2 and 1.6 cM, respectively. Also, within the region between Rf4 gene and RM171, a newly designed primer pair, AB443, produced two sterile‐specific markers, AB443‐400 and AB443‐500, 0.5 and 1.03 cM from the gene. The flanking markers identified give promise for their application in molecular marker‐assisted selection (MAS) and they are also suitable for starting chromosome walking to clone Rf4 gene in the near future.

(252) Inheritance of Phytophthora Stem Blight, Root Rot, and Foliar Blight Resistance in Capsicum

The oomycete fungus Phytophthora capsici Leon. is known to be a limiting factor of chile pepper (Capsicum spp.) production around the world. The genetics of the resistance is becoming better understood due to the specific nature of the host–pathogen interaction; i.e., all plant organs are subject to infection. This study determined whether stem blight was the same disease syndrome as root rot or foliar blight. Stem cuttings of a segregating F2 population and testcross progeny facilitated the ability to screen for two disease syndromes concurrently. When the three disease syndromes were compared separately, the F2 populations fit a 3R:1S ratio and the testcross progeny fit a 1R:1S ratio. When comparative studies were performed (stem vs. foliar and stem vs. root), the F2 populations fit a 9R/R:3R/S:3S/R:1S/S ratio and the testcross fit a 1R/R:1R/S:1S/R:1S/S ratio. These ratios are consistent of a single gene controlling the resistance of each system. Therefore, Phytophthora stem blight, root rot, and foliar blight are three separate disease syndromes.

Asexual reproduction in a close relative of Arabidopsis: a genetic investigation of apomixis in Boechera (Brassicaceae)

Understanding apomixis (asexual reproduction through seeds) is of great interest to both plant breeders and evolutionary biologists. The genus Boechera is an excellent system for studying apomixis because of its close relationship to Arabidopsis, the occurrence of apomixis at the diploid level, and its potentially simple inheritance by transmission of a heterochromatic (Het) chromosome. Diploid sexual Boechera stricta and diploid apomictic Boechera divaricarpa (carrying a Het chromosome) were crossed. Flow cytometry, karyotype analysis, genomic in situ hybridization, pollen staining and seed-production measurements were used to analyse the parents and resulting F1, F2 and selected F3 and test-cross (TC) generations. The F1 plant was a low-fertility triploid that produced a swarm of aneuploid and polyploid F2 progeny. Two of the F2 plants were fertile near-tetraploids, and analysis of their F3 and TC progeny revealed that they were sexual and genomically stabilized. The apomictic phenotype was not transmitted by genetic crossing as a single dominant locus on the Het chromosome, suggesting a complex genetic control of apomixis that has implications for future genetic and evolutionary analyses in this group.

Genetic Analysis of Corn Kernel Chemical Composition in the Random Mated 10 Generation of the Cross of Generations 70 of IHO × ILO

To identify and characterize quantitative trait loci (QTL) affecting oil, protein and starch concentration in the corn (Zea mays L.) kernel, plants from Generation 70 of the Illinois High Oil (IHO) and Illinois Low Oil (ILO) populations, previously developed by divergent selection for kernel oil concentration, were crossed. The cross was random mated 10 generations and selfed two generations to develop 500 F1RM10S2 lines. The lines per se and their testcross progenies were evaluated at three locations with two replications for 2 yr. Genotypes were evaluated using 479 SNP markers on a bulk of kernels from each line. Since the parent plants used to make the original cross were not available for genotyping, a multivariable optimization procedure was developed to estimate parental population parameters required for QTL mapping. Simple interval mapping was performed with software specially developed to account for the complex mating structure and the fact that the initial cross was made between populations. Correlations and signs of QTL effects suggest development of high oil–high starch lines would be difficult but that it should be possible to develop high oil–high protein lines. The identification of a large number of QTL (at least 40 each for oil, protein, and starch) with small effects has implications for breeding for improved corn chemical composition.

Doubled haploid versus S1 family recurrent selection for testcross performance in a maize population

Theoretically, in a recurrent selection program, the use of doubled haploids (DH) can increase genetic advance per unit of time. To evaluate the efficiency expected from the use of DH for the improvement of grain yield in a maize (Zea mays L.) population, two recurrent selection programs for testcross performance were initiated using testcross progenies from DH lines and S1 families. In 4 years one selection cycle using DH and two selection cycles using S1 families were carried out with the same selection intensity for both methods. As expected, testcross genetic variance was twice as high among DH lines as among S1 families. The predicted genetic gain was 8.2% for the DH selection cycle, and 10.6% for the two S1 selection cycles, giving a per year advantage of 29% for the S1 family method over the DH method with a cycle of 4 years. With a 3-year cycle for the DH method, both methods were expected to be equivalent. Using a tester related to the one used for selection, the genetic gains obtained were equivalent for both methods: 6.6% for the DH cycle and 7.0% for the two S1 cycles. With a 3-year cycle for the DH method, the advantage would have been in favor of DH method. Furthermore, the DH method has the advantage of simultaneously producing lines that are directly usable as parents of a hybrid. Thus, if the genetic advance per unit of time is evaluated at the level of developed varieties even with the same or with a lower genetic advance in population improvement, the DH method appears to be the most efficient.

Factors associated with emergence of Shrunken-2 maize in Korea

Seventeen supersweet Shrunken-2 (sh-2) maize lines and progenies of their crosses to two sh-2 hybrid testers were investigated for factors responsible for differences in field emergence in Korea in studies carried out in the field, greenhouse and laboratory. The lines and their testcrosses showed significant genetic variation for emergence. Two lines showed significant positive general combining ability (GCA) for emergence while two other lines had significant negative GCA effects. Because none of the specific combining ability effects was significant, emergence was primarily influenced by additive gene action, and could, therefore, be improved by recurrent selection. Seed weight was significantly (P&lt;0·05) correlated with emergence for inbreds (r=0·63) and testcross progeny involving the tester with the lower mean seed weight (r=0·54). Two lines also showed significant positive GCA effects for seed weight. The lines with positive GCA effects for emergence were different from those with positive GCA for seed weight, indicating that factors other than seed weight influence field emergence. The severity of Fusarium infection of seeds of the lines was significantly correlated to germination (r=−0·61, P&lt;0·05) and emergence (r=−0·67, P&lt;0·01), suggesting that practices that prevent pathogenic field infection, such as timely harvest and treatment with fungicides, are crucial for high seed quality. The field emergence of sh-2 inbred parents did not give an indication of emergence in hybrid combinations.

Combining ability analysis for agronomic traits in sunflower (Helianthus annuus L)

The general and specific combining abilities among 20 cytoplasmic male sterile inbred lines and four testers were estimated in order to study the potential of these materials in a sunflower (Helianthus annuus L.) breeding program. Test cross progenies were evaluated in three environments. Plant height, days to 50% flowering, 1000-kernel weight, seed oil content and grain yield were evaluated. Seed oil content, plant height and 1000-kernel weight presented the largest proportional contribution of cms inbred lines and testers, indicating the predominant role of the additive component for these traits. On the other hand, line × tester interaction exhibited the greatest contribution to grain yield suggesting the presence of non-additive genetic effects. The testers showed capacity to discriminate within the set of inbred lines.