Restriction fragment length polymorphisms (RFLPs) have been proposed for investigating relationships among inbred lines and predicting heterosis and performance of single-cross hybrids in maize (Zea mays L.). Such use was evaluated in 20 maize inbreds classified as first-cycle, second-cycle, good, and poor lines, and in diallel crosses within types. Eight generations (parents, F 1 , F 2 , F 3 , backcrosses, and backcrosses selfed) from 67 crosses were evaluated for grain yield in five Iowa environments. Genetic effects were estimated from generation means by ordinary diallel analyses and the Eberhart-Gardner genetic model. Poor inbreds showed significantly greater average heterosis than did good lines. Estimates of additive ✕ additive epistatic effects were negative and highly significant in all except first-cycle lines. Using two restriction enzymes and 82 genomic) DNA probes distributed throughout the maize genome, all but one probe revealed polymorphisms with at least one enzyme (~4.5 variants per RFLP locus). Genetic distances between lines within types were estimated as Rogers' distances (RD). Within diallel sets, RD values were partitioned into general (GRD)and specific (SRD). All four types of lines showed similar means and substantial variation for RD; GRD explained 40% of the variation among RD values. Cluster analyses revealed associations among lines generally consistent with expectations based on known pedigrees. Correlations of RD and SRD with F 1 yield heterosis, specific heterosis, and specific combining ability were positive but small (r = ≤ 0.50) when combined for all crosses. Results indicated that RFLPs can be used to investigate pedigree relationships among maize inbreds, but also suggest that RFLP-based genetic distance measures are of limited use in predicting heterotic performance of single crosses between unrelated lines. Disciplines

Sugarcane borer (SCB), Diatraea saccharalis Fabricius, is a serious pest in tropical maize production areas in the Americas. Little is known about the genetic resistance of maize genotypes to this pest. In this study, we mapped and characterized quantitative trait loci (QTL) affecting resistance to the leaf feeding generation ofSCB (1SCB), grain yield under both protection (GYP) and infestation (GYI) with SCB larvae, and plant height (PITT). A total of 171 F2 genotypes derived from cross CML131 (susceptible) × CML67 (resistant) 93 RFLP marker loci were used in QTL analyses. F3 lines were evaluated for the above traits and grain yield reduction (GYR) in field experiments with two replications at two or three tropical environments. Resistance was assessed by rating leaf feeding damage after artificial infestation with SCB larvae. The method of composite interval mapping with selected markers as cofactors was used for detection and characterization of QTL. Resistance to 1SCB was significantly affected by 10 putative QTL on Chromosomes 1, 2, 5, 7, 8, 9, and 10. These showed predominantly additive gene action and explained 65.0% of the phenotypic variance and 93.5% of the genetic variance in a simultaneous fit. Six QTL for GYP, five QTL for GYI with primarily dominant genetic effects, and four QTL for PHT with primarily additive genetic effects were identified, explaining in total about one third of the phenotypic variance for the respective trait. No more than one putative QTL was found to be common between different characters. QTL × environment interaction was found to be significant for 1SCB ratings only. Based on these data, prospects for improving 1SCB resistance by marker-assisted breeding are promising. 'N CENTRAL AND LATIN AMERICA the sugarcane borer .(SCB), Diatraea saccha.ralis Fabricius, can cause extensive damage in maize production fields. In the smallscale, low-input farming systems prevailing in this region, low yield stability is partly due to the highly variable damage caused by SCB. Moths of SCB deposit their eggs on leaves of maize plants during the whorl stage. After hatching, larvae migrate into the whorl and feed on the epidermis for 9 to 10 d. Subsequently, larvae move outside of the stalk and start penetrating into the stalk for further development and pupation. After 35 to 50 d, the second-generation adults appear. In tropical environments, up to seven generations can develop annually. Total yield loss due to maize borer infestation can be divided into several components. Up to 20% yield loss can be attributed to the first-generation leaf feeding (Scott

selection criterion. Under a constant selection intensity for all crosses, R ij depends on the genetic variance The efficiency of breeding programs could be increased by pre-( 2 gij ) of the homozygous lines and the heritability (h 2 ij ) dicting the prospects of crosses for line development before producing of the trait under study in the specific cross i ϫ j. Other and testing lines derived from them. In this study, we examined the performance of F 4:n (n ϭ 7 or 8) lines randomly derived from 30 winter criteria closely related to usefulness are the "varietal wheat (Triticum aestivum L.) crosses produced by a factorial mating of ability" (Gallais, 1979) and the "probability of obtaining five high yielding with six high baking quality cultivars. Our objectives transgressive segregants" (Jinks and Pooni, 1976). Prewere to (i) contrast the midparent value m ij with ĉ ij for each cross cise estimates of U ij and its components c ij and 2 gij are and apply tests for the presence of epistasis, (ii) compare the estimates difficult to obtain especially for small grains because of the variance between means of crosses ( 2 c ) with the average segreseed shortage prohibits intensive testing in plots and gation variance within crosses ( 2 g ), (iii) determine the variation in across multiple environments in early breeding genera-2 gij among crosses, (iv) evaluate the use of various parameters for tions. Therefore, reliable information can be obtained predicting c ij , 2 gij , and U ij , and (v) briefly describe quantitative-genetic only in the parental and late generations. theory for interpretation of our experimental findings. Twenty-two Under a model with purely additive gene effects, the lines per cross and the respective parents were evaluated in four midparent value m ij is a perfect predictor of c ij . The environments for seven agronomic and quality traits. Additionally, 44 F 2:4 lines per cross were tested in hills in two environments. On value of parents may be impaired only in the presence the basis of first-degree statistics, parental means were good predictors of epistasis and linkage disequilibrium (Kearsey and of cross means, yet significant epistatic effects were observed for most Pooni, 1996). Phenotypic correlations between m ij and traits. In agreement with quantitative-genetic expectations, the genetic ĉ ij for grain yield in wheat were usually moderate for variance between 2 c was of the same size as the variance between data from a single environment (Bhatt, 1973; Fowler parental means and the average 2 g of F 4:n lines within crosses for all and Heyne, 1955) but fairly high (0.7 Ͻ r p Ͻ 0.8) for traits but sedimentation. Here, the two parental groups differed sigdata from multiple environments (Busch et al., 1974; nificantly in their mean, and 2 g was significantly larger than 2 c . Lupton, 1961). Results from a recent study with barley Estimation of the segregation variance ( 2 gij ) for individual crosses is(Hordeum vulgare L.) confirmed these findings (Schut not recommended because they (i) have...

duced the concept of breeding for different MEs. A ME is defined as a large, not necessarily contiguous area, Improved bread wheat (Triticum aestivum L.) cultivars for diverse which usually encompasses more than one country and agroecological environments are important for success in the effort to increase food production. In the 1980s, CIMMYT introduced the S. Dreisigacker and A.E. Melchinger, Inst. of Plant Breeding, Seed recent approach allows researchers to shift from the Science, and Population Genetics, Univ. of Hohenheim, 70593 Stuttuse of anonymous markers with unknown effect on the gart, Germany; M. Bohn,

Single crosses and backcrosses from established lines are importeat parental sources for developing new inbred lines in hybrid maize (Zea mays L.) breeding. The objectives of this study were (i) compare, both theoretically and experimentally, the testcross properties of F2 vs. first backcross populations; and (ii) to outline criteria for choosing between these two types of base populations in recycling breeding programs. Two homozygous lines (P1, P2) and individual plants from their F2 and first backcross (BC1, BC2) generations were mated to an unrelated single‐cross tester. A total of 186 testcross progenies were evaluated for grain and forage traits at three and two locations, respectively. The five generations (P1, P2, F2, BC1, BC2) differed significantly (P < 0.05) in their testcross means (X̄) for grain (GY) and forage (FY) yield. A nonepistatic model accounted for 92 to 99% of the variation among testcross generation means for five of the six traits examined, yet the presence of epistasis was indicated in several instances. In accordance with theory, the ratio of BC (the pooled estimate of BC1 and BC2) to F2 genotypic variance among testcross progenies (σ2g) was close to 0.50 for all traits but one. Heterogeneous σ2g estimates in BC1 and BC2 were found for GY, FY, and grain dry matter content, suggesting the presence of epistasis. Predicted selection response for testcross performance (ΔG) was greatest in the F2. Testcross progenies of the F2, BC1, and BC2 generations displayed similar phenotypic and genotypic correlations as expected by theory. The “usefulness” criterion U = x̄ + ΔG was employed to assess the breeding prospects of each generation. With weak selection, the better backcross had superior usefulness for four traits, whereas with strong selection the F2 had highest usefulness for all traits except GY.