In the summer of 1983, immature embryos from 101 selfed inbred lines and germplasm stocks of Zea mays L. were examined for their ability to produce callus cultures capable of plant regeneration (regenerable cultures) using a medium with which some limited success had previously been obtained. Forty-nine of the genotypes (49%) produced callus which visually appeared similar to callus previously cultured and shown to be capable of plant regeneration. After five months, 38 of these genotypes were alive in culture and plants were subsequently regenerated from 35 (92%) of them. No correlation was observed between plant regeneration and callus growth rate, the vivipary mutation (genes vp1, 2, 5, 7, 8 and 9), or published vigor ratings based on K(+) uptake by roots. When F1 hybrid embryos were cultured, 97% of the hybrids having at least one regenerable parent also produced callus capable of plant regeneration. No regenerable cultures were obtained from any hybrid lacking a parent capable of producing a regenerable callus culture.In the summer of 1984, immature embryos from 218 additional inbred lines and germplasm stocks were plated and examined for their ability to produce regenerable callus cultures on media containing altered micronutrient concentrations, 3,6-dichloro-o-anisic acid (dicamba), glucose, and elevated levels of vitamin-free casamino acids and thiamine. Of these genotypes 199 (91%) produced callus that was regenerable in appearance. In the 1984 study, plant regeneration was noted in many commercially important inbreds, including B73, Mo17, B84, A632, A634, Ms71, W117, H99(3)H95 and Cm105. Thus tissue-culture techniques are now available to obtain callus cultures capable of plant regeneration from immature embryos of most maize genotypes.
Popcorn (Zea mays L.) hybrids grown in the United States are derived from narrow-based germplasm, and standard RFLP analysis detects relatively little polymorphism. Inter-simple sequence repeat (IS S R) amplification, a novel technique based on PCR amplification of inter-microsatellite sequences to target multiple loci in the genome, was employed to investigate its potential for detection of polymorphism among nineteen popcorn and eight dent corn inbred lines. IS SR yielded an average of 54 bands/primer/ inbred line, with over 98 ~o of the bands repeatable across DNA extractions and separate PCR runs. Ten primers based on di-and tri-nucleotide tandem repeats revealed 73?0 and 87~ polymorphism among popcorn and dent corn lines, respectively, with an overall 95?o polymorphism rate. Principal component and cluster analyses resulted in grouping of dent and popcorn lines corresponding to their heterotic breeding pools. ISSR amplification, in addition to being both simple and cost and time efficient, provides for rapid production of highly polymorphic markers which appear to correspond to known pedigree information. Therefore, the ISSR technique may have great potential for identifying polymorphism in species with narrow-based germplasm, and for use in DNA marker-assisted breeding approaches.
Callus cultures were initiated from immature embryos of three dent corn inbred lines: FR27rhm (B73 derivative), 79-R4443, and EXL1; and four sweet corn (sh-2) inbred lines: f84-267, f84-275, f84-278, and f84-700. Extensive qualitative variation was noted in self-pollinated progeny of plants regenerated from callus of FR27rhm, EXL1, and f84-700; quantitative variation was measured in self-pollinated progeny of plants regenerated from cultures of almost all inbreds. Most qualitative mutations were visible in the first self-pollinated generation; chimerism in regenerated plants was indicated by initial segregation of some mutants after two self-pollinations. All culture-induced mutant phenotypes were similar to known spontaneous mutations in maize; preliminary data indicate most have single-gene recessive inheritance. For qualitative variation, average mutation frequencies per regenerated plant were calculated as 0.18, 0.71, and 0.64 for FR27rhm, EXL1, and f84-700, respectively. Differences in mutation frequency among the four sweet corn lines were determined to have a genetic basis. Significant quantitative variation included alterations in date of pollen shed and silk emergence, silk emergence relative to pollen shed, plant height, ear, and kernel characteristics. These results show genotypic differences in the extent of qualitative somaclonal variation, with some quantitative variation having possible applications for conventional corn breeding programs.
Molecular markers can be used to detect alleles in donor genetic material for improvement of existing cultivars or hybrids. DNA restriction fragment length polymorphisms (RFLPs) were used as markers to search for favorable alleles at quantitative trait loci in the maize (Zea mays L.) population BS11(FR)C7 which were not in the hybrid 'FRB73 x FRMo17.' Thirty-four RFLP markers were used to determine RFLP 'fingerprints' for 220 [BS11(FR)C7 x FRMo17] F2 individuals; multiple morphs (bands) were observed at most markers. Statistical associations between RFLPs and trait expression in F2 x FRB73 progeny were found for grain yield, stalk and root lodging, plant and ear height, maturity, and seven grain yield component traits. Associations were found using linear contrasts among RFLP marker classes to estimate trait effects. Estimated effects for grain yield ranged from 213 to 538 kg ha(-1), 3.0-7.5% of the experimental mean, respectively. RFLP markers with greatest probability of association with grain yield were NPI234 (short arm of chromosome 1) and UMC16 (long arm of chromosome 3). Digenic epistasis appeared to be important in grain yield expression, as indicated by a 12% increase in the proportion of genotypic variation accounted for when significant di-marker interactions were added to a linear model, including all markers individually associated with grain yield. The majority of interactions associated with grain yield involved markers NPI234 and UMC21 (long arm of chromosome 6). Many RFLP markers were associated with multiple traits. At some markers, the same bands were associated with favorable effects for stalk lodging, grain yield, and yield components. RFLP bands unique to BS11(FR)C7 showed associations favorable over those from FRMo17 for at least one marker in all but one trait. The results of this study will be useful in future RFLP marker-assisted selection programs aimed at developing lines for improved performance in combination with FRB73.
Increased endosperm hardness of grain in maize (Zea mays L.) is desirable for dry milling, storage, and expert purposes. This study was conducted to evaluate changes in endosperm hardness resulting from four cycles of visual recurrent selection for increased hardness in two dent corn synthetics representative of U.S. Corn Belt germplasm, and to determine associated responses in other agronomic traits. Testcrosses of random S1 lines from each cycle of selection were grown in replicated trials at four locations in each of 2 yr for determination of grain hardness and evaluation of agronomic traits. Evaluation of grain hardness was through measurement of average kernel density and the Stenvert hardness test. Significant increase in endosperm hardness was limited to the first cycle of selection in both synthetics. No significant changes corresponding to altered endosperm hardness were detected for any of five agronomic traits measured in testcross hybrids from S1 lines representing each cycle of selection. Estimated genetic correlation coefficients indicated a negative, nonsignificant trend between grain yield and grain hardness. These results indicate increases in grain hardness can be achieved through selection in dent corn germplasm common to the U.S. Corn Belt. However, continual progress in recurrent selection may require quantification of hardness by measurements such as kernel density and the Stenvert hardness test, rather than simple visual judgment of relative hardness. Genetic correlation coefficients indicate selection progress can be simultaneously made for both grain yield and grain quality.
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