with a small number of key lines as the center of one cluster. This genotypic information provided abundant insight into maize genetic diversity around the world. However, most lines used in previous studies were pub- licly available, representing historical recombinants dur- ing the maize domestication and improvement progress. Along with the development of private commercial maize breeding, more and more new elite inbred lines were produced and widely used in modern maize breed- ing practice. This was especially true for the Suwan population, which came from tropical regions, and de- rived many more elite lines widely used in tropical/sub- tropical maize breeding program, such as Ki32, QR273, and QB446. Therefore, collecting one naturally diverse panel that contained the latest lines derived from tem- perate and Suwan resources that are widely used in modern breeding programs is better for in-depth study of the genetic character of maize germplasm. Based on this panel, we dissect the genetic relationship between pair-lines by using the high-throughput genotyping method, which is important not only for expanding gene pools to accelerate maize breeding progress but also for providing additional information for evaluating whether this panel is suitable for genome-wide study (GWAS) in analyzing variation of complex traits.
The United States hybrid maize crop is based almost entirely on crosses between proprietary commercial inbred lines (Mikel and Dudley, 2006). Many of the commercial inbred lines are derived from older publicly developed inbred lines, representing a relatively narrow sampling of the available maize gene pool (Mikel and Dudley, 2006; Nelson et al, 2008). Therefore, because commercial hybrids do not appear to provide adequate levels of resistance, it is reasonable to search for higher levels of resistance in germplasm not closely related to the progenitors of modern commercial hybrids. Two important alternative sources of diverse maize germplasm are older public inbred lines and tropical germplasm. Older public inbred lines are often better adapted to typical USA corn production environments, but tend to have lower inherent yield potential, and often poor stalk strength. Tropical germplasm may have higher yield potential, but it is poorly adapted to temperate production environments, limiting its immediate utility to breeding programs in the United States (Goodman et al., 2000).
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In the similar context, the present study was conducted to establish correlations between yield and its components complemented with heritability estimates in exotic low- nitrogen responsive maize germplasm. Path analysis was used to dissect the correlations into direct and indirect effects. These analyses were done when analysis of variance established significant differences among genotypes for most of the traits studied. The correlation between 100-kernel weight and grain yield was significantly positive and its direct effects were also positive towards yield. Rahman et al. , Kumar et al.  and Hefny  also reported positive direct effects for yield component as opposed to Moradi and Azarpour  who observed minute role of 1000-grain weight towards grain yield. Moreover, the values for correlation and
Southern leaf blight (SLB), caused by Cochliobolus heterostrophus, is another hemibiotrophic Ascomycete (though SLB is more necrotropic compared to GLS or NLB) foliar fungal-disease of maize that occurs worldwide, but is most common and destructive in warm temperate and tropical maize growing regions (Hooker, 1972). Primary infection occurs when spores are dispersed by wind currents and water droplets. The source of new inoculum for SLB comes from overwintering mycelium on debris left in the field from the previous year (Hesseltine et al, 1971). Disease development is favored by warm moist weather conditions, and therefore it is one of the most prevalent biotic stresses in the Southeast U.S.A. and the humid tropics. C. heterostrophus has a short life cycle of 60 to 72 hours under ideal conditions, making it one of the faster-progressing maize pathogens. Reproduction occurs in both asexual and sexual stages, with asexual reproduction occuring repeatedly within a single growing season and providing inoculum for increased disease pressure during a growing season. The sexual stage has rarely been observed in the field, and requires a strain of the opposite mating type in order to induce sexual reproduction (Turgeon and Lu, 2000).
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proportion of the approximately 800 inbreds in the PVPA system and are not necessarily a reflection of the germplasm that was available to private seed companies at the time these lines entered protection; many inbreds may be protected via means other than PVPA such as trade secrets, license agreements, and patents. Some companies, such as Illinois Foundation Seeds (Champaign, IL) and Thurston Genetics (Olivia, MN), have essentially opted out of the PVPA program. Third, mergers and acquisitions within the maize seed industry in the last decade have drastically changed the maize breeding landscape. Some of the proprietary boundaries that once limited the flow of germplasm between breeding programs have now been dissolved. For example, when lines from Asgrow, DEKALB, and Holden‘s, all presently owned by Monsanto (Monsanto Company, St. Louis, Mo), are considered collectively, Monsanto ranks second in allelic diversity, behind the public subset. Despite these sampling limitations, the ex-PVPA inbreds are the only publicly available
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Backcrossing is most effective when the donor parent is not seriously deficient in traits other than those of interest (Fehr, 1987). Although GE440 is rather exotic and possesses many poor agronomic characteristics, it was chosen as the resistance donor because similar levels of resistance have not been identified in more adapted germplasm (Clements et al., 2004). Maize as a whole is a highly genetically diverse crop, but elite germplasm in the United States is derived from a relatively narrow genetic base, which may leave maize production vulnerable to new or emerging pests and pathogens (Smith, 2007). Gouesnard et al. (1996) and Goodman (2004) both highlighted the importance of
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ABSTRACT:- During spring 1999, on the basis of survived plants, a total of 250 accessions including 150 exotic out of 350 and 100 indigenous out of 900 were selected. During autumn 1999, out of 250 accessions (progenies) 35 exotic and 10 indigenous could survive under artificial infestation. Under natural infestation, 100 progenies performed little better. The best plants among these progenies were advanced by self pollination. A total of 400 self pollinated cobs (as progenies) were screened during spring 2000 under natural infestation. The borer attack started at a very initial stage of the crop and swept down many of the progenies. Only 88 exotic and 7 indigenous progenies showed some resistance. The survived plants, among these progenies, were advanced by self pollination. During autumn 2000, only 9 progenies out of 95 showed some resistance with 35-66% plants survived under artificial infestation. Under natural infestation, 10 other progenies had more than 50% survived plants. The best survived plants among these 19 progenies were advanced by selfing. During spring 2001, 8 of the comparatively resistant germplasm showed no leaf damage while 9 others had 7.10 - 44.7% infestation. In checks, the infestation was from 27.9 to 100.0%. The selected materials were again maintained by self pollination to make pure lines. During autumn 2001, selected material was screened under double artificial infestation, first at 3-4 leaf stage of the crop and the second 10 days after first infestation with 15 newly hatched larvae per plant every time. Despite of double artificial infestation, 40.0-66.7% plants survived in the resistant germplasm but none survived in susceptible genotypes. Under natural infestation, 5 progenies had no damage while other showed 10.0-22.9% damage with intensity of 2-3 in resistant germplasm. The susceptible germplasm showed 30.0-66.6% infestation with intensity of 5-8. The dead hearts in resistant and susceptible germplasm were 0.0-7.8% and 19.6-25.4%, respectively. The performance of the resistant germplasm was far better than susceptible ones. The best plants in resistant germplasm were maintained by selfing to make them more uniform for resistance. During pollination, three resistant composite genotypes (BR-1, BR-2 and BR-3) were constituted by bulk pollination.
The U.S. maize (Zea mays L.) germplasm base is narrow. While maize is a very diverse species, that diversity is not represented in U.S. maize production acreage. Most elite U.S. maize inbreds can be traced back to a small pool of inbreds that were developed decades ago. Increased genetic diversity can be obtained through breeding with exotic germplasm, especially tropical-exotic sources. However, setbacks are often encountered when working with tropical germplasm due to adaptation barriers. Furthermore, the pool of available tropical germplasm is large and diverse, making choices of tropical parents difficult. The maize breeding program at North Carolina State University has begun a large-scale screening effort to evaluate elite exotic maize inbreds, most of which are tropical-exotic in origin. The purpose of this research was to: 1) generate comparative yield-trial data for over 100 elite exotic maize inbreds, 2) determine the relative effectiveness of various testcross regimes, 3) identify sources of gray leaf spot (GLS) resistance among these elite exotic inbreds, and 4) promote the use of exotic maize germplasm to broaden the genetic base of U.S. maize.
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especially organic producers, have an interest in hybrids with selective pollinating systems. The previously identified alleles conferring this selective action have been problematic to work with from a production standpoint, leaving a gap between identification and utilization of these systems. In this study, we first screened exotic maize germplasm for dominant gametophyte factors (DGFs) conferring selective pollination phenotypes. From this screen, we identified several lines that putatively contain DGFs and were able to demonstrate that 1) the ability to backcross these factors into adapted material is variable, but that some factors appear to be successfully introgressed, 2) that topcrosses of lines putatively containing DGFs have sufficient yield to be viable economically, as well as performing competitively with the available commercial alternative with a similar system and 3) that Tcb-1 homozygous lines exist that overcome the yield drag associated with these alleles. We believe these DGF lines have potential to be a viable alternative to current specialty production protocols, as well as being a valuable tool in protecting a producer’s right to choose their own production system.
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Genetic distances within crop species are measures of average genetic divergence between populations and it provides an index for parental selection. This study was undertaken to identify diverse inbreds from a group of 38 maize inbreds using 27 Random Amplified Polymorphic DNA (RAPD) primers. The data obtained was subjected to genetic diversity analysis by Sequential Agglomerative Hierarchical Non-overlapping (SAHN) clustering using Dice’s coefficient and Unweighted Paired Group Method with Arithmetic Average (UPGMA). Genotypes were broadly classified into seven clusters. Similarity coefficient at molecular level was highest between UMI-852 and UMI-752. Based on the study, 11 inbreds were selected for use in heterosis breeding.
and grain development in maize production (Adediran et al., 1995). The increased plant height with increasing levels of nitrogen was recorded by Muthukrishnan and Subramanian (1980). Plant height and leaf area index in winter maize increased with increasing levels of nitrogen application upto 100 kg ha -1 (Prasad et al., 1985). The maximum growth rate at 150 DAS with 200 kg N ha -1 in winter maize was reported by Nandal and Agarwal (1991). Leaf growth, leaf appearance and photosynthetic capacity in maize increased with higher levels of N fertilizer (Vos et al., 2005). Nitrogen deficiency in corn delays the development of ears relative to the tassel and may thus affect the coincidence of pollen shed and silking of individual spikelets (Jacob and Pearson, 1991). Nitrogen is the most limiting nutrient for maize production. Maize is an exhaustive crop and requires high quantities of nitrogen. The practice of fertilizer recommendation on the basis of individual crop is becoming less relevant because individual crop is a component of cropping system and cannot be grown in isolation. Therefore fertilizer recommendation should be made by giving due considerations to nature of preceding crops or in other words the cropping system as a whole besides the soil condition.
Although good progress has been made internationally to understand yellow spot resistance, only one gene ( tsn1 on chromosome 5BL) is in general and known use in Australian breeding programs. While tsn1 , is an important YS resistance gene, it doesn’t explain the full spectrum of resistance in the Australian germplasm. Furthermore, Faris et al. ( 2012 ) have shown that the amount of variation explained by tsn1 can vary considerably (5–30 %) with different isolates and suggest possible variability in ToxA gene regulation amongst isolates.
Traditional breeding efforts are time consuming, therefore the use of molecular tools that will allow breeders to screen germplasm at the seedling stage and expedite cultivar development. Using conventional breeding methods in raspberry, the breeding cycle begins with the crossing of complementary parents to combine traits that are more suitable for the growing environment than either parent alone (1 year). The resulting seedlings are planted in the field, grown for 2 to 3 seasons and then evaluated. At the end of this cycle, typically less than 5% of those seedlings are “selected” based on combinations of desirable traits. The “selections” are propagated and placed in 2 to 3 different locations for further evaluation. In subsequent years, the selections are rated on plant characteristics, berry weight and quality, climatic adaptation, and disease and pest resistance. This stage can take 3 to 5 or more years. From this initial group of “selections”, the most promosing are placed in replicated trials and placed in 2 or more sites for 3 to 5 years. An “advanced selection” which consistently
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We then compared levels of heterozygosity in sorghum, maize and barley across time, Figure 3. Maize and teosinte show a pattern that is initially counterintuitive, with some maize accessions from the US southwest and northern Mexico showing higher heterozygosity than teosinte (Figure 3B). This runs counter to the expectation of lower diversity in maize—approximately 83% relative to teosinte based on nucleotide diversity analyses (Hufford et al. 2012). This result may reflect structured populations in teosinte, which when considered together give rise to relatively high nucleotide diversity values because of the extent of inter-population differentiation, making nucleotide diversity a poor estimator of heterozygosity in this case. Conversely, the maize of central Mexico and South America show heterozygosity similar to that found in teosinte populations (Figure 3C). It is likely the higher maize heterozygosities of northern Mexico and the US represent accessions in which multiple teosinte populations have input genetic material augmenting the primary gene pool of domestication. Consequently, teosinte should not be considered as a single unit of origin of maize, but a series of differentiated populations, not all of which will have contributed to the maize gene pool, nor which have contributed to the diverse lineages of domestic maize uniformly. The ancient maize genomes of Turkey Pen show an elevated heterozygosity range relative to teosinte, which suggests a general reduction in heterozygosity in the central Mexican and South American maize, if continuity can be assumed, and maintenance of genetic diversity in the North Mexican and US maize.
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The increasing on the levels of carotenoids in staple foods of broad human consumption is one of the strategies of food biofortification programs, mainly due to the importance of these compounds to human health on the prevention of vitamin A deficiency. Maize is a major staple food due to its high consumption in regions where problems of Vitamin A deficiency are of great relevance. Maize biofortification programs have made progress in determining the amounts of carotenoids in grain of thousands of accesses. This work aimed at studying the influence of the color of the grains in the profile of carotenoids in four different Brazilian genotypes. The selection of ears within the same genotype was based on a color scale, considering the lighter (lightest yellow) in one group and the most colorful (darkest orange) in another group. Significant interactions (p < 0.05) between the color of the grains and the genotypes for all the variables were detected in addition to genetic va- riability for both groups (lightest yellow and darkest orange). The colored ears of corn showed a high level of total carotenoids (TC) and fractions in RS 535 and RS 445, and the colorful ears of genotype RS 535 showed 300% more α + β carotenes (μg·g −1 ) in relation to lighter of this same
found to be only intermediate in resistance in their screening studies. Staub et al. (1989) screened the germplasm collection for reaction to six pathogens. They found that 6.2% of the 753 accessions tested were resistant to downy mildew and 7.2% were susceptible. Of the resistant accessions, 34% came from China, 28% from Japan and 3% from India. Dhillon et al. (1999) tested 217 cultigens in northern India, using natural infestations in the field, for downy mildew resistance. They reported that five of the nine most resistant cultigens were of Japanese origin, two were Indian landraces and two were European. Neykov and Dobrev (1987) also reported that the most resistant cultivars were of Asian origin, mostly from Japan followed by India and P.R. China. In 1992 (Lebeda, 1992b) and 1994 (Lebeda and Prasil, 1994), 303 and 155 cucumber cultigens respectively, were tested under controlled conditions for downy mildew resistance. Little resistance was reported for these tests. However, they suggested that some cultivars despite doing poorly in the greenhouse tests, have a high degree of field resistance.
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Twenty eight rice germplasm (Oryze sativa L.) Including eighteen course candidate varieties, nine fine candidate varieties and one elite commercial variety were screened. The seeds of all rice germplasm were collected from Rice Program, Crop Research Institute (CSI), National Agricultural Research Centre (NARC) Islamabad.
Only one of the nine plants within which mastrevirus sequences were detected (a S. officinarum from the Miami germplasm collection, NG28-020) displayed any discernable chlorotic streak- or striation-like symptoms such as those caused by many of the known monocoty- ledonous plant-infecting mastreviruses (Fig. 1). Total DNA was extracted from the samples from Florida/ Miami (three plants of Saccharum spontaneum Iranspon, one of S. barberi Ketari, one of S. officinarum NG28-020, and one from S. officinarum Pundia), Guadeloupe (one plant of noble cane, S. officinarum EK2, and one plant each of commercial sugarcane culti- vars TC3 and TC9), and Réunion (one plant of S. barberi Sararoo 1492 and one plant from S. sinense UBA Aust). Total DNA was enriched for mastrevirus full genomes by rolling circle amplification (RCA) using Phi29 DNA polymerase (TempliPhi™, GE Healthcare, USA) as previously described by Shepherd et al. . The RCA products were either used as templates for polymer- ase chain reaction (PCR)-based amplification using a set of primers designed based on the VANA contigs (Table 3), or were restricted using either BamHI or PstI. The amplified products were ligated to pJET1.2 (Thermo Fisher USA), whereas the restricted ~2.8Kb mastrevirus genome-length fragments were ligated to pBlueScript (Agilent, USA). The resulting recombin- ant plasmids were Sanger sequenced by primer walk- ing at Beckman Coulter Genomics (plasmids from the Florida/Miami site) and Macrogen Inc. (plasmids from the Guadeloupe site).
conditions for the assay developed in one laboratory to another. When this has been achieved the marker has to be validated in relevant germplasm for that breeding program. In Australia the major breeding programs have quite different germplasm bases, and for linked markers a marker that may work in the germplasm of one program may not be applicable in another breeding program due to lack of polymorphism at the marker site. Another consideration at this stage is that it may be necessary to convert a marker to a more appropriate marker type for the large scale implementation used in that laboratory, or to facilitate the opportunities for multiplexing.
important disease of maize (Zea mays L.) in the United States. Yield losses due to SLB race O have been reported up to 40% (White, 1999). SLB became a main concern for growers and breeders due to SLB race T, the cause of the SLB epidemic of 1970-71. In 1970, the Race T SLB outbreak started in Florida in early spring and inoculum moved westward via wind into Texas and eventually northward into Canada (Hooker, 1972). Race T devastated yields on highly susceptible Texas-male sterile cytoplasmic (cms-T) hybrids, causing losses upwards of 50% throughout the U.S. and nearly complete yield loss in southern states. This was due to the prevalent use of cms-T in hybrids seed production. The T-toxin produced by race T is specifically toxic to the mitochondria of cms-T maize and is responsible for the host specificity of race T (Yoder and Gracen, 1974). Race O also produces a toxin but in low quantity and with no apparent host-specificity (Hooker, 1972), and is the current strain of concern for breeders, geneticists, and pathologists. Methodologies for controlling SLB include using resistant cultivars, crop rotation, fungicides, and conventional tillage.
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