Cytogenetic evidence for the origin of teosinte (Zea mays ssp. Mexicana)

Summary Cytogenetic evidence has shown that teosinte (Zea mays ssp. mexicana (Schrad.) Iltes) and maize (Zea mays L. ssp. mays) are conspecific. They hybridize readily and their offspring are generally fertile. Teosinte could not have originated as a byproduct of maize-Tripsacum hybridization. Such introgression gave rise to plants that are phenotypically maize or Tripsacum, depending on which parent was used as a pollen donor. Compartive morphological and genetical studies indicated that it is more probable that maize originated from a teosinte-like ancestor under domestication, than that a maize-like plant gave rise to teosinte through a series of mutations.Reseach supported financially by the Illinois Agricultural Experiment Station, and Grants GB-40136-X and BM573-01034 A02 from the National Science Foundation.     

基于SNP标记的玉米株高及穗位高QTL定位

为进一步弄清玉米株高和穗位高的遗传机理,为育种生产提供服务,本研究以K22×CI7、K22×Dan3402个F₂群体为作图群体,利用覆盖玉米10条染色体的SNP标记构建了2个连锁图谱。并将这2个F₂群体衍生的分别含237和218个家系的F_2:3群体用于田间性状的鉴定。用复合区间作图模型对2个群体的株高、穗位高表型进行QTL定位分析,结果显示,在武汉和南宁两种环境条件下共定位到21个株高QTL和27个穗位高QTL;单个QTL表型变异贡献率的变幅为4.9%~17.9%;株高和穗位高QTL的作用方式以加性和部分显性为主;第7染色体上可能存在控制株高和穗位高的主效QTL。     

QTL mapping of maize (Zea mays) stay-green traits and their relationship to yield

A maize genetic linkage map derived from 115 simple sequence repeat (SSR) markers was constructed from an F₂ population. The F₂ was generated from a cross between a stay-green inbred line (Q319) and a normal inbred line (Mo17). The map resolved 10 linkage groups and spanned 1431.0 cM in length with an average genetic distance of 12.44 cM between two neighbouring loci. A total of 14 quantitative trait loci (QTL) were detected for stay-green traits at different postflowering time intervals and identified by composite interval mapping. The respective QTL contribution to phenotypic variance ranged from 5.40% to 11.49%, with trait synergistic action from Q319. Moreover, maize stay-green traits were closely correlated to grain yield. Additional QTL analyses indicated that multiple intervals of stay-green QTL overlapped with yield QTL.     

Genetic regulation of root traits for soil flooding tolerance in genus Zea

Flooding stress caused by excessive precipitation and poor drainage threatens upland crop production and food sustainability, so new upland crop cultivars are needed with greater tolerance to soil flooding (waterlogging). So far, however, there have been no reports of highly flooding-tolerant upland crop cultivars, including maize, because of the lack of flooding-tolerant germplasm and the presence of a large number of traits affecting flooding tolerance. To achieve the goal of breeding flooding-tolerant maize cultivars by overcoming these difficulties, we chose highly flooding-tolerant teosinte germplasm. These flooding-tolerance-related traits were separately assessed by establishing a method for the accurate evaluation of each one, followed by performing quantitative trait locus (QTL) analyses for each trait using maize × teosinte mapping populations, developing introgression lines (ILs) or near-isogenic lines (NILs) containing QTLs and pyramiding useful traits. We have identified QTLs for flooding-tolerance-related root traits, including the capacity to form aerenchyma, formation of radial oxygen loss barriers, tolerance to flooded reducing soil conditions, flooding-induced adventitious root formation and shallow root angle. In addition, we have developed several ILs and NILs with flooding-tolerance-related QTLs and are currently developing pyramided lines. These lines should be valuable for practical maize breeding programs focused on flooding tolerance.     

QTL analysis of flooding tolerance in soybean at an early vegetative growth stage

Soybean cultivars are sensitive to flooding stress and their seed yields are substantially reduced in response to the stress. This study was conducted to investigate the genetic basis of flooding tolerance at an early vegetative growth stage. Sixty recombinant inbred lines derived from a cross between a relatively tolerant cv. ‘Misuzudaizu’ and a sensitive cv. ‘Moshidou Gong 503’ were grown in pots in a vinyl plastic greenhouse in 2002 and 2003. At the two‐leaf stage, half of the pots were waterlogged by water placed in plastic containers and adjusted to 5 cm above the soil surface. After 3 weeks of treatment, the pots were returned to the greenhouse and grown until maturity. Flooding tolerance was evaluated by dividing the seed weight of the treated plants by that of the control plants. Quantitative trait loci (QTL) analysis using 360 genetic markers revealed three QTLs for flooding tolerance, ft1 to ft3 in 2002. The ft1 (molecular linkage group C2) was reproducible and an additional four QTLs, ft4 to ft7, were found in 2003. The ft1 had a high LOD score in both years (15.41 and 7.57) and accounted for 49.2% and 30.5% of the total variance, respectively. A large QTL for days to flowering was consistently observed across treatments and years at a similar position to ft1. Comparing the relative location with markers, the maturity gene probably corresponds to E1. Late maturity may have conferred a longer growth period for recovery from flooding stress.     

Identification of RAPD and SCAR markers linked to northern leaf blight resistance in waxy corn (Zea mays var. ceratina)

Exserohilum turcicum causes northern corn leaf blight (NCLB), an important disease occurring in maize producing areas throughout the world. Currently, the development of cultivars resistant to E. turcicum seems to be the most efficient method to control NCLB damage. Marker-assisted selection (MAS) enables breeders to improve selection efficiency. The objective of this work was to identify random amplified polymorphic DNA (RAPD) and sequence characterized amplified region (SCAR) markers associated with NCLB resistance. Bulked segregant analysis (BSA) was used to search for RAPD markers linked to NCLB resistance genes, using F₂ segregating population obtained by crossing a susceptible inbred ‘209W’ line with a resistant inbred ‘241W’ line. Two hundred and twenty-two decamer primers were screened to identify four RAPD markers: OPA07₅₂₁, OPA16₄₅₇, OPB09₅₂₀, and OPE20₅₃₆ linked to NCLB resistance phenotype. These markers were converted into dominant SCAR markers: SCA07₄₉₆, SCA16₄₂₀, SCB09₄₆₄, and SCE20₄₂₉, respectively. The RAPD and SCAR markers were developed successfully to identify NCLB resistant genotypes in segregating progenies carrying NCLB resistant traits. Thus, the markers identified in this study should be applicable for MAS for the NCLB resistance in waxy corn breeding programs.     

Structural observations during androgenic microspore culture of the 4c1 genotype of Zea mays L.

Summary The capacity of the maize genotype 4c1 to regenerate microcalli and embryos from cultured microspores has been examined by comparing various cold pretreatments and culture media, using microspores and pollen at different stages of development. Viability of cultured cells was tested with FDA and their development was traced with light and fluorescence microscopy using DAPI as a nuclear dye.It was found that a pre-incubation of dissected flowers floating in a liquid nutrient medium at 8°C during 10–14 days was most successful for the induction of cell division. Among the developmental stages tested only the microspores appeared to regenerate. Subculture at 25°C in the same liquid medium, supplemented with 0.1 mg/l TIBA, gave highest rates of microspore division, i.e. up to 70% at 4 to 6 days of culture.All pathways described earlier for maize androgenic embryogenesis were observed within the 4c1 genotype. Symmetric divisions occurred in cultured microspores but most frequently asymmetric divisions lead to the formation of microcalli within 12 days of culture. In at least 60% of all dividing microspores cells were derived from the generative nucleus. Microcalli further developed either into loose or compact calli. Compact calli formed embryo-like structures.Abbreviations DAPI 4,6-diamidino-2-phenylindole - Dicamba 3,6-dichloro-2-methoxy benzoic acid - 2,4D 2,4 dichlorophenoxyacetic acid - FDA fluorescein diacetate - PAA phenylacetic acid - TIBA 2,3,5-triiodobenzoic acid - YP medium Yu-Pei basal salt medium     

Cytoplasmic-nucleargenic effects in the C121 (Athens) Zea mays L. inbred with three cytoplasms

Summary Seeds of the long-time maize (Zea mays L.) inbred CI 21 (Athens) were prepared with the following cytoplasms: GA 199 and GT 112. Each source of cytoplasm as the female parent was corssed to the CI 21 (A) as the pollen parent. The F₁, as the female, was backcrossed successively to the CI 21 (A) genotype until the CI 21 (A) nucleus was substituted into each of the cytoplasms. Thus, each cytoplasmic source of seed assumed to have the same nuclear genotype.The cytoplasms were compared in all possible combinations for one year with 10 replications. Among the characters studied, significant differences were obtained for the following: effect of herbicide, reaction to Fusarium sp., silking on a given day, tasseling, plant height, ear height, lodging, number of ears, and yield.These results revealed a favorable interaction of GA 199 cytoplasm with the CI 21 (A) genotype for desirable characters. Apparently, plasmon-sensitive effects caused the unfavorable results. Results from these experiments also provide further support for cytoplasmic effects on agronomic characters in maize.Part of the dissertation submitted by the senior author to the Dep. of Agronomy, University of Georgia, Athens, Georgia 30602, USA.     

Storage of maize (Zea mays L.) pollen at −196°C in liquid nitrogen

Summary The water content of pollen has a decisive influence on its storability in liquid nitrogen. Pollen with an initial high water content cannot be stored successfully at extremely low temperatures, so a certain degree of drying must be carried out before storage. Provided the viability of the pollen is not significantly reduced during drying, the pollen remains viable and fertile when kept at –196°C.     

Glyphosate tolerance in maize (Zea mays L.). 1. Differential response among inbred lines

Summary Variation in susceptibility to the safe broad-spectrum herbicide glyphosate was investigated in maize. Eleven inbred lines, grown in a growth chamber, were evaluated for their tolerance to the herbicide at 2.4 mM (0.2 kg a.i. in 400 I ha^-1 of water). Following treatment with glyphosate at the three-leaf stage, significant variation in damage, expressed as visual injury ratings scored 7, 14 and 21 days after the application of the herbicide, was found. Effects on dry weight and shoot height were consistent with visual scores and the carbon-exchange rate was found to be a sensitive index of differential injury.Biochemical characterization of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase, the main target of the herbicide, ruled out the possibility that this differential susceptibility was due to variations in the sensitivity of the enzyme. On the contrary, a positive correlation was found between in vivo tolerance and EPSP synthase levels, measured at different stages during seedling growth. This result suggests that a naturally occurring difference in EPSP synthase levels in the tissues may contribute to the differential response observed in vivo in maize inbreds.Abbreviations CER carbon-exchange rate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - ID₅₀ concentration causing 50% inhibition     

Maturity interaction and black layer occurrence in opaque-2 and normal hybrids in maize (Zea mays L.)

Summary Six opaque-2 lines and their normal counterparts were crossed in diallel crosses. The crosses were grown in 1970 in a split plot randomized complete block design. Harvests were made at 7-day intervals starting at 28 days after pollination and countinuing through 63 days.The average kernel weight of opaque-2 hybrids was inferior to that of the normal. Nevertheless, the opaque-2 gene performed differently in different hybrids. In B14 x B37 single cross the mutant had similar kernel weight as its normal counterpart in the first and second harvests. In contrast a wide difference was found between the opaque-2 and the normal, both at early and late stages of development in W64A x A545 background.The normal hybrids had greater cob weight, ranging from 9'7 to 11.8% more than the opaque-2. The difference in cob weight of the opaque-2 and the normal remained constant over the different harvest dates.At physiological maturity, the opaque-2 hybrids averaged 3.5% higher moisture content than the normal. In general, a slower accumulation of dry matter in the kernels was accompanied by a retention of more moisture.Shelling percentage was higher for the normal hybrids. Black layer, an indicator of physiological maturity, was formed at about the same time in the opaque-2 and normal.Journal paper No. 6094.     

Genetic variation in maize (Zea mays L.) for germination and emergence at 10°C

Summary Full-sib, paternal half-sib and S1 families were derived from a population of maize, Pool 5, with a wide germplasm base. These families were used to study genetic variation for germination percentage, emergence percentage and time to emergence at 10°C.Genotypic variances were large for all characters, and the best families from the population were markedly superior to hybrids used as checks. Maternal effects appeared to be of overwhelming importance for germination percentage, but variances due to males were significant for emergence percentage and time to emergence. Inbreeding depression at the seedling level was also important for these two emergence characters. Germination and emergence characters were not correlated with seed weight.We concluded that Pool 5 should be a valuable source of genes for improving germination and emergence of maize at low temperatures.     

Glyphosate tolerance in maize (Zea mays L.). 2. Selection and characterization of a tolerant somaclone

Summary The progeny of 104 regenerated maize plants were screened for tolerance to the safe broad-spectrum herbicide glyphosate during seed germination and early growth. Seven somaclones showed varying degrees of resistance to the application of the herbicide at 1.2 mM (0.1 kg a.i. in 400 1 ha^-1 of water). Plants capable of a normal growth following treatment with 2.4 mM (0.2 kg ha^-1) glyphosate at the three leaf stage were selfed, and their progeny analyzed. A family able to tolerate the exposure to glyphosate at 2.4 mM was isolated and shown to maintain a photosynthetic rate comparable with control after the application of the herbicide.The selfed progeny of the tolerant somaclone was characterized as to the properties of two targets of glyphosate, the shikimate pathway enzymes 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase and 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase. In vitro tests ruled out the possibility that the tolerance was due to altered forms of these enzymes. Families showed significant variability with regard to EPSP and DAHP synthase levels, measured at different stages during seedling growth; however, not even these traits were correlated with in vivo response to glyphosate. The possible role of other physiological processes in determining the increased tolerance to the herbicide is discussed.Abbreviations DAHP 3-deoxy-D-arabino-heptulosonate-7-phosphate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - CER carbon-exchange rate - ID₅₀ concentration causing 50% inhibition     

Mapping QTLs in breeding for drought tolerance in maize (Zea mays L.)

Summary Grain yield in the maize (Zea mays L) plant is sensitive to drought in the period three weeks either side of flowering. Maize is well-adapted to the use of restriction fragment length polymorphisms (RFLPs) to identify a tight linkage between gene(s) controlling the quantitative trait and a molecular marker. We have determined the chromosomal locations of quantitative trait loci (QTLs) affecting grain yield under drought, anthesis-silking interval, and number of ears per plant. The F₃ families derived from the cross SD34(tolerant) × SD35 (intolerant) were evaluated for these traits in a two replicated experiment. RFLP analysis of the maize genome included non-radioactive DNA-DNA hybridization detection using chemiluminescence. To identify QTLs underlying tolerance to drought, the mean phenotypic performances of F₃ families were compared based on genotypic classification at each of 70 RFLP marker loci. The genetic linkage map assembled from these markers was in good agreement with previously published maps. The phenotypic correlations between yield and other traits were highly significant. In the combined analyses, genomic regions significantly affecting tolerance to drought were found on chromosomes 1,3,5,6, and 8. For yield, a total of 50% of the phenotypic variance could be explained by five putative QTLs. Different types of gene action were found for the putative QTLs for the three traits.     

Identification of pedigrees of hybrid maize (Zea mays L.) cultivars by isozyme electrophoresis and reversed-phase high-performance liquid chromatography

Summary Determination of pedigree by laboratory means can provide an initial and rapid check of hybrid identity in studies designed to compare genotypes. In order to test the accuracy of such determinations, isozymic and chromatographic data were used in an attempt to ascribe pedigrees to 25 hybrids that were identified only by a letter code from a list of 30 inbred lines. The pedigrees of 9 single-cross hybrids were correctly deduced as was the direction in which each cross was made. The pedigrees of 2 hybrids could only be determined ambiguously, however these hybrids were reciprocals and the biochemical data were unable to distinguish between 2 related candidate inbred lines. Pedigrees of 3 hybrids were either incompletely determined or could not be determined from the list of 30 candidate inbred lines and were thus correctly identified as non-Pioneer^® brand hybrids. Pedigrees of 9 3-way hybrids could be determined accurately but the pedigrees of 2 non-Pioneer^® brand hybrids could not be deduced completely. The ability to correctly ascribe pedigrees of hybrids shows that these data provide a rapid means of determining whether additional field and laboratory tests would be warranted in tests designed to compare genotypic similarity of hybrids.     

Detection of linkage between RFLP markers and genes affecting anthocyanin pigmentation in maize (Zea mays L.)

Summary Linkages between molecular markers and genes involved in the expression of agronomical traits have already been described in all of the major crops. In most cases, the genetic model underlying the Quantitative Traits Loci (QTL) is discussed. Here, Restriction Fragment Length Polymorphisms (RFLPs) and Mapmaker-QTL have been used to pinpoint seven regions of the genome significantly correlated with four pigmentation qualitative traits of maize (Zea mays L.). Two of these, located on chromosomes 2 and 10, explain most of the variation of these traits. The R and B gene loci known to be involved in the regulation of the anthocyanin pathway map to the same regions and we suggest that these loci could be the candidate genes involved in the correlations detected with RFLPs. This type of result is in accordance with the hypothesis of the candidate gene which supposes that, if we have a very high density map of randomly-selected cDNA clones, it should theoretically be possible to associate a cloned genic sequence with a phenotypic trait where correlations are found.     

玉米出籽率、籽粒深度和百粒重的QTL分析

为研究玉米出籽率、籽粒深度、百粒重的遗传机制,以豫82×沈137组配的229个F_2:3家系为试验材料,采用复合区间作图法进行QTL定位分析。在3个环境下共检测到10个QTL。其中,控制出籽率、籽粒深度、百粒重相关QTL分别为3个、3个和4个,它们的联合贡献率分别为35.5%、28.1%和39.0%。位于第1染色体上介于标记umc1335与umc2236之间控制出籽率的QTL qKR1b和位于第9染色体上介于标记bnlg1209–umc2095之间控制百粒重QTL q100-KW9b,分别解释18.9%和11.7%的表型变异,且作用方式为加性效应,分析表明这些区域可能包含调控玉米籽粒性状关键基因,对剖析玉米产量形成机制具有重要的参考价值。     

Heterosis in relation to genetic divergence in maize (Zea mays L.)

Summary The magnitude of heterosis was examined in relation to genetic divergence among 10 parents in a 10 × 10 diallel cross of maize. The 10 parents (varieties) were selected on the basis of multivariate analysis of quantitative characters using Mahalanobis' D²-statistic from a lot of 64 varieties of maize belonging to different geograshical regions of the world. The magnitude of heterosis for grain yield and its components was found to be higher with moderate (intermediate) parential diversity than with extreme ones. The results indicated that not only the genetic diversity but the high per se performance with respect to specific characters of interest should be taken into consideration also while selecting parents for hybridization.     

Genetic improvement of cell-wall digestibility in forage maize (Zea mays L.). I. Performance of inbred lines and related hybrids

Summary The current study deals with genetic improvement of the nutritive value of forage maize. In separate field trials, maize inbred lines without the brown midrib trait and derived hybrids were evaluated for stalk quality as well as some other agronomic traits. The aim was to relate the performance of lines and hybrids. Quality traits studied were the contents of ash and cell walls expressed as percentage of dry matter and the digestibilities of organic matter and cell walls (stalk-dv% and stalk-dcw%, respectively). The performance of hybrids was established in a trial at two locations with three replicates per location and the performance of lines at one location in an unreplicated trial.The range for stalk-dcw% was about 10 percentage units between hybrids and 15 percentage units between inbred lines. Stalk-dcw% had of all quality traits of hybrids the highest broad-sense heritability (h ²=0.74), and determined about 80% of the variation in stalk-dv%. The only stalk quality trait where a significant correlation was found between the mean hybrid performance and the corresponding midparent value was stalk-dcw% (r=0.70, P<0.01).In conclusion, stalk-dcw% proved to be the only stalk quality trait worth evaluating at the inbred line level in a breeding programme aimed at producing commercial hybrid varieties of forage maize.     

Selection and characterization of a 5-methyltryptophan resistant mutant in Zea mays L.

Summary A maize plant resistant to 5-methyltrytophan (5MT) was selected from M₂ seeds (Zea mays L. Danggin, inbred line) originating from ears treated with ethylmethane sulfonate (0.2%) at 6 hr after self-pollination. Genetic analysis of the progeny of plants selected from a medium containing 50 ppm 5MT showed that 5MT resistance was inherited as a single dominant nuclear gene. This resistance was also expressed in callus and seedling. Analysis of the free amino acids in kernels and calli showed that homozygous resistant plants (MR1) contained higher levels of total free amino acids than sensitive plants and calli. In particular, the their kernels the levels of tryptophan, threonine and serine were, respectively, 4.5, 5.9 and 6.3 times higher than those of the sensitive plants. From the results, it may be expected that mutants resistant to amino acid anologs will be useful not only for studying amino acid biosynthesis but also for improving the nutritional quality of maize.Abbreviations EMS Ethylmethane sulfonate - 5MT 5-methyltryptophan - 2,4-D 2-4-dichlorophenoxyacetic acid