QTL mapping of <Emphasis Type="Italic">Sclerotinia</Emphasis> midstalk-rot resistance in sunflower

In many sunflower-growing regions of the world, Sclerotinia sclerotiorum (Lib.) de Bary is the major disease of sunflower (Helianthus annuus L.). In this study, we mapped and characterized quantitative trait loci (QTL) involved in resistance to S. sclerotiorum midstalk rot and two morphological traits. A total of 351 F₃ families developed from a cross between a resistant inbred line from the germplasm pool NDBLOS and the susceptible line CM625 were assayed for their parental F₂ genotype at 117 codominant simple sequence repeat markers. Disease resistance of the F₃ families was screened under artificial infection in field experiments across two sowing times in 1999. For the three resistance traits (leaf lesion, stem lesion, and speed of fungal growth) and the two morphological traits, genotypic variances were highly significant. Heritabilities were moderate to high (h²=0.55–0.89). Genotypic correlations between resistance traits were highly significant (P<0.01) but moderate. QTL were detected for all three resistance traits, but estimated effects at most QTL were small. Simultaneously, they explained between 24.4% and 33.7% of the genotypic variance for resistance against S. sclerotiorum. Five of the 15 genomic regions carrying a QTL for either of the three resistance traits also carried a QTL for one of the two morphological traits. The prospects of marker-assisted selection (MAS) for resistance to S. sclerotiorum are limited due to the complex genetic architecture of the trait. MAS can be superior to classical phenotypic selection only with low marker costs and fast selection cycles.     

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

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.     

Development of RGA-CAPS markers and genetic mapping of candidate genes for sugarcane mosaic virus resistance in maize

Three previously published resistance gene analogues (RGAs), pic13, pic21 and pic19, were mapped in relation to sugarcane mosaic virus (SCMV) resistance genes ( Scmv1, Scmv2) in maize. We cloned these RGAs from six inbreds including three SCMV-resistant lines (D21, D32, FAP1360A) and three SCMV-susceptible lines (D145, D408, F7). Pairwise sequence alignments among the six inbreds revealed a frequency of one single nucleotide polymorphism (SNP) per 33 bp for the three RGAs, indicating a high degree of polymorphism and a high probability of success in converting RGAs into codominant cleaved amplified polymorphic sequence (CAPS) markers compared to other sequences. SNPs were used to develop CAPS markers for mapping of the three RGAs in relation to Scmv1 (chromosome 6) and Scmv2 (chromosome 3), and for pedigree analyses of resistant inbred lines. By genetic mapping pic21 was shown to be different from Scmv2, whereas pic19 and pic13 are still candidates for Scmv1 and Scmv2, respectively, due to genetic mapping and consistent restriction patterns of ancestral lines.     

Prediction of testcross means and variances among F3 progenies of F1 crosses from testcross means and genetic distances of their parents in maize

 Prediction of the means and genetic variances in segregating generations could help to assess the breeding potential of base populations. In this study, we investigated whether the testcross (TC) means and variances of F₃ progenies from F₁ crosses in European maize can be predicted from the TC means of their parents and F₁ crosses and four measures of parental genetic divergence: genetic distance (GD) determined by 194 RFLP or 691 AFLP^{TM 1} markers, mid-parent heterosis (MPH), and absolute difference between the TC means of parents (∣P₁−P₂∣). The experimental materials comprised six sets of crosses; each set consisted of four elite inbreds from the flint or dent germplasm and the six possible F₁ crosses between them, which were evaluated for mid-parent heterosis. Testcross progenies of these materials and 20 random F₃ plants per F₁ cross were produced with a single-cross tester from the opposite heterotic group and evaluated in two environments. The characters studied were plant height, dry matter content and grain yield. The genetic distance between parent lines ranged between 0.17 and 0.70 for RFLPs and between 0.14 and 0.57 for AFLPs in the six sets. Testcross-means of parents, F₁ crosses, and F₃ populations averaged across the six crosses in a particular set generally agreed well for all three traits. Bartlett’s test revealed heterogeneous TC variances among the six crosses in all sets for plant height, in four sets for grain yield and in five sets for dry matter content. Correlations among the TC means of the parents, F₁ crosses, and F₃ populations were highly significant and positive for all traits. Estimates of the TC variance among F₃ progenies for the 36 crosses showed only low correlations with the four measures of parental genetic divergence for all traits. The results demonstrated that for our material, the TC means of the parents or the parental F₁ cross can be used as predictors for the TC means of F₃ populations. However, the prediction of the TC variance remains an unsolved problem. Received: 4 August 1997 / Accepted: 17 November 1997     

QTL analysis of early stage heterosis for biomass in Arabidopsis

The main objective of this study was to identify genomic regions involved in biomass heterosis using QTL, generation means, and mode-of-inheritance classification analyses. In a modified North Carolina Design III we backcrossed 429 recombinant inbred line and 140 introgression line populations to the two parental accessions, C24 and Col-0, whose F ₁ hybrid exhibited 44% heterosis for biomass. Mid-parent heterosis in the RILs ranged from ?31 to 99% for dry weight and from ?58 to 143% for leaf area. We detected ten genomic positions involved in biomass heterosis at an early developmental stage, individually explaining between 2.4 and 15.7% of the phenotypic variation. While overdominant gene action was prevalent in heterotic QTL, our results suggest that a combination of dominance, overdominance and epistasis is involved in biomass heterosis in this Arabidopsis cross.     

Molecular marker assisted broadening of the Central European heterotic groups in rye with Eastern European germplasm

Broadening the genetic base of heterotic pools is a key to ensure continued genetic gains in hybrid breeding and extend hybrid cultivation to new areas. In the present study, two Central European heterotic pools (Carsten and Petkus) and five Eastern European open-pollinated varieties (OPVs, Pop-1 to Pop-5) were studied with the objectives to (1) investigate the genetic diversity in OPVs and the heterotic pools using molecular and field data, (2) evaluate the molecular diversity among OPVs, (3) examine the combining ability for grain yield of the OPVs when crossed with testers in field trials, and (4) develop a strategy for targeted introgression of OPV germplasm into the heterotic pools. In total, 610 S₀ plants, 347 from OPVs and 263 from heterotic pools, were developed. Clones of the S₀ plants of OPVs were crossed with two testers belonging to each heterotic pool, while clones of heterotic pools were crossed with only the opposite tester. Testcrosses were evaluated for grain yield in multi-location trials. In addition, 589 S₀ plants were fingerprinted with 30 SSR markers. The data revealed that the Carsten pool has a narrow genetic base and should be the primary target for broadening the established heterotic pattern. Mean and genetic variance suggested that Pop-2 and Pop-4 are good candidates for introgression in Petkus pool and Pop-5 in Carsten pool. Nevertheless, introgression of Pop-5 in Carsten could reduce the genetic diversity between heterotic pools. Therefore, we suggest that either selected plants of Pop-5 should be introgressed or more Eastern European germplasm should be fingerprinted and field evaluated to identify promising germplasm for broadening the established heterotic pattern.     

Broadening the genetic base of European maize heterotic pools with US Cornbelt germplasm using field and molecular marker data

Maize (Zea mays L.) breeders are concerned about the narrowing of the genetic base of elite germplasm. To reverse this trend, elite germplasm from other geographic regions can be introgressed, but due to lack of adaptation it is difficult to assess their breeding potential in the targeted environment. The objectives of this study were to (1) investigate the relationship between European and US maize germplasm, (2) examine the suitability of different mega-environments and measures of performance to assess the breeding potential of exotics, and (3) study the relationship of genetic distance with mid-parent heterosis (MPH). Eight European inbreds from the Dent and Flint heterotic groups, 11 US inbreds belonging to Stiff Stalk (SS), non-Stiff Stalk (NSS), and CIMMYT Pool 41, and their 88 factorial crosses in F₁ and F₂ generations were evaluated for grain yield and dry matter concentration. The experiments were conducted in three mega-environments: Central Europe (target mega-environment), US Cornbelt (mega-environment where donor lines were developed), and Southeast Europe (an intermediate mega-environment). The inbreds were also fingerprinted with 266 SSR markers. Suitable criteria to identify promising exotic germplasm were F₁ hybrid performance in the targeted mega-environment and F₁ and parental performance in the intermediate mega-environment. Marker-based genetic distances reflected relatedness among the inbreds, but showed no association with MPH. Based on genetic distance, MPH, and F₁ performance, we suggest to introgress SS germplasm into European Dents and NSS into European Flints, in order to exploit the specific adaptation of European flint germplasm and the excellent combining ability of US germplasm in European maize breeding programs.     

Temporal changes in allele frequencies in two European F2 flint maize populations under modified recurrent full-sib selection

Selection and random genetic drift are the two main forces affecting the selection response of recurrent selection (RS) programs by changes in allele frequencies. Therefore, detailed knowledge on allele frequency changes attributable to these forces is of fundamental importance for assessing RS programs. The objectives of our study were to (1) estimate the number, position, and genetic effect of quantitative trait loci (QTL) for selection index and its components in the base populations, (2) determine changes in allele frequencies of QTL regions due to the effects of random genetic drift and selection, and (3) predict allele frequency changes by using QTL results and compare these predictions with observed values. We performed QTL analyses, based on restriction fragment length polymorphisms (RFLPs) and simple sequence repeats (SSRs), in 274 F_2:3 lines of cross KW1265 × D146 (A × B) and 133 F_3:4 lines of cross D145 × KW1292 (C × D) originating from two European flint maize populations. Four (A × B) and seven (C × D) cycles of RS were analyzed with SSRs for significant allele frequency changes due to selection. Several QTL regions for selection index were detected with simple and composite interval mapping. In some of them, flanking markers showed a significant allele frequency change after the first and the final selection cycles. The correlation between observed and predicted allele frequencies was significant only in A × B. We attribute these observations mainly to (1) the high dependence of the power of QTL detection on the population size and (2) the occurrence of undetectable QTL in repulsion phase. Assessment of allele frequency changes in RS programs can be used to detect marker alleles linked to QTL regions under selection pressure. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.