A detailed linkage map of Helianthus annuus was constructed based on segregation at 234 RFLP loci, detected by 213 probes, in an F2 population of 289 individuals (derived from a cross between the inbred lines HA89 and ZENB8). The genetic markers covered 1380 centiMorgans (cM) of the sunflower genome and were aranged in 17 linkage groups, corresponding to the haploid number of chromosomes in this species. One locus was found to be unlinked. Although the average interval size was 5.9 cM, there were a number of regions larger than 20 cM that were devoid of markers. Genotypic classes at 23 loci deviated significantly from the expected ratios (1∶2∶1 or 3∶1), all showing a reduction in the ZENB8 homozygous class. The majority of these loci were found to map to four regions on linkage groups G, L and P.

In maize (Zea mays L.) breeding programs identifying potentially high yielding hybrids, assigning new inbreds to heterotic groups, and determining the parental line in a hybrid to which a potential donor line is most closely related, are significant problems. Data from a diallel cross among 14 maize inbreds were analyzed to evaluate use of molecular marker date to (i) predict yield potential of hybrids, (ii) assign inbreds to heterotic groups, and (iii) determine to which of the two inbred parents of a single cross a donor line is most closely related. Four of the inbreds were from Iowa Stiff Stalk Synthetic background, five were from a Lancaster Sure Crop background and five were unrelated to either Stiff Stalk Synthetic or Lancaster Sure Crop. Allozyme genotypes at 14 loci and restriction fragment length polymorphism (RFLP) variant types for 52 cDNA clones were obtained for the 14 inbreds. Modified Roger's distance (MRD) values (a measure of genetic distance between inbreds based on marker data) were not significantly correlated with hybrid yields; however, a hybrid value based on the number of marker loci having the highest yielding genotype was significantly correlated with hybrid yield. Cluster analysis based on MRD values agreed well with pedigree information, but did not always agree with groupings based on yield. Where parents of hybrids were distantly related, a measure of relative relationship based on proportion of homomorphic marker loci was significantly correlated with a measure of relationship based on yield. Where parents of a hybrid were more closely related, however, such correlations were low.

Increased seed oil percentage is an important objective when breeding for high oil yield in sunflower (Helianthus annuus L.). Although some researchers have investigated the genetics and heritability of sunflower oil percentage, most analyses were conducted on the oil percentage in the whole seed through conventional breeding and biometric procedures. The primary objective of this research was to identify restriction fragment length polymorphisms (RFLPs) linked to quantitative trait loci affecting seed oil percentage, kernel oil percentage, and kernel percentage. An F2 population consisting of 289 individuals was produced by crossing two inbred lines that differ for the traits. The RFLP and trait data were obtained directly from self‐pollinated F2 plants. The RFLP markers (identifying 201 codominant loci) located six regions representing 57% of the genetic variation of seed oil percentage. Two of these regions were associated with kernel oil percentage, two with kernel percentage, and two with both components. Additive gene action was predominant for seed oil percentage and its components.

To be useful to plant breeders, gains made from molecular marker‐assisted selection (MAS) must be more cost‐effective than gains made through traditional breeding. The objective of this research was to evaluate the effectiveness of MAS in an applied maize (Zea mays L.) breeding program. Maize population BS11 (FR) C7 was chosen to improve hybrid FRMo17 × FRB73. FRMo17 and BS11 were crossed, then random‐mated to create F2 plants (families). F2 plants were testcrossed to FRB73 and genotyped with 34 molecular markers. F2 families were selfed to create F2:S4 families. Based on F2 testcross yield, the top 20 families were selected. A second set of 20 families was selected based on a marker‐derived selection index. Marker genotypes were obtained for 20 plants within each of the marker selected families. Index selections were made for favorable and unfavorable genotypes within the 20 families. F2:S4 testcrosses to FRB73 were grown to compare the different selection methods. Selection among F2:S4 families using either method resulted in similar testcross performance. However, neither method selected families that performed significantly better than FRMo17 × FRB73 or the So unselected population testcross. Within‐family index selection was not effective. The selection index, developed with F2 testcross data, was not correlated with F2:S4 testcross performance. Marker‐yield associations were determined using F2:S4 genotypic and testcross performance data. Ten (31%) markers were significantly associated with yield. One marker‐allele combination was favorable (and significant) in both F2 and F2:S4 testcross performance.

Breeding maize for gray leaf spot (GLS) resistance has been hindered by the quantitative nature of the inheritance of GLS resistance and by the limitations of selection under less than optimumal disease pressure. In order to identify the quantitative trait loci (QTLs) controlling GLS resistance, a cross was made between B73 (susceptible) and Va14 (resistant) to generate a large F2 population. Six GLS disease assessments were made throughout the disease season for over 1000 F2 plants in 1989, and for 600 F2-derived F3 lines replicated in two blocks in 1990. RFLP analysis for78 marker loci representing all ten maize chromosomes was conducted in 239 F2 individuals including those with the extreme GLS disease phenotypes. The GLS disease scores of the three field evaluations, each averaged over six ratings, were separately used for the interval mapping in order to determine the consistency of the QTL effects. The heavy GLS disease pressure, meticulous disease ratings, and large population size of this study afforded us the sensitivity for detecting QTL effects. QTLs located on three chromosomes (1, 4, and 8) had large effects on GLS resistance, each explaining 35.0-56.0%, 8.8-14.3%, and 7.7-11.0% of the variance, respectively. These three QTL effects were remarkably consistent across three disease evaluations over 2 years and two generations. Smaller QTL effects were also found on chromosomes 2 and 5, but the chromosome-5 effect might be a false positive because it was not repeatable even in the same location. The chromosome-1 QTLs had the largest effect or highest R(2) reported for any quantitative trait to-date. Except for the chromosome-4 gene, which was from the susceptible parent B73, the resistance alleles at all QTL were derived from Va14. The resistance QTLs on chromosomes 1 and 2 appear to have additive effects, but those on chromosomes 4 and 8 are dominant and recessive, respectively. Significant interaction between the QTLs on chromosomes 1 and 4 was detected in all three evaluations. Cumulatively, the four QTLs identified in this study explained 44, 60, and 68% of the variance in F2, and in F3 replications 1 and 2, respectively.