Evaluating testcross performance and genetic divergence of lines derived from reciprocal tropical maize composites

The development of hybrids with enhanced expression of heterosis depends on the genetic background of the source population and the effectiveness of a breeding scheme used to identify and use divergent inbred lines with good combining ability. The present study was conducted to examine the potential of improved tropical reciprocal composites as sources of inbred lines for developing productive hybrids. Thirty-six S lines derived from the third RRS cycle of each composite were crossed in pairs to form 36 testcrosses, which were evaluated along with com- monly grown commercial hybrids in Nigeria across nine test environments. Results showed consistent ranking of testcrosses for grain yield and other traits across the diverse test environments. The best 22 testcrosses produced 21% to 51% more grain yields than the highest yielding commercial hybrid. Several testcrosses combined high grain yields with other desirable agronomic traits. The SSR and SNP markers used in the present study detected a broad range of genetic diversity among the 72 S lines, which was structured along the two composites. The two markers portrayed similar trends in determining genetic distance estimates and detecting composite-specific al- leles in the S lines. These results highlight the potential of improved reciprocal composites as sources of new and divergent parents for developing productive hybrids and as sources of novel alleles for broadening and diversifying the genetic base of adapted germplasm to sustain genetic gain in productivity of hybrids in WCA

Impact of Moisture Deficit on Physiological Quality of Maize Seeds

There is limited empirical information on the physiological basis of the development of seed quality despite the awareness that vigorous seeds provide a basis for solving cropestablishment problems. To address this problem, seeds of 16 maize (Zea mays L.) genotypes were planted in the dry season and the crop was grown entirely under irrigation. In one of the blocks, irrigation was suspended from five weeks after planting to harvest, whereas the control plot received irrigation from planting to physiological maturity. The seeds produced under the irrigated and moisture deficit conditions were harvested and subjected to seed viability and vigor tests as well as to seedling-evaluation tests. High-yielding genotypes did not necessarily produce seeds with high vigor. Moisture deficit resulted in 34% loss in germination potential, whereas vigor loss was between 40% and 220%, depending on the type of vigor test used. The seeds produced under moisture deficit had slow and non-uniform germination and seedling growth, with poor storage potential. Water availability to the crop affected seed vigor more than the genotypic effect, suggesting an inelastic limit to the extent that genotype can compensate for inadequacy of moisture of a maize plant

Impact of Moisture Deficit on Physiological Quality of Maize Seeds

There is limited empirical information on the physiological basis of the development of seed quality despite the awareness that vigorous seeds provide a basis for solving cropestablishment problems. To address this problem, seeds of 16 maize (Zea mays L.) genotypes were planted in the dry season and the crop was grown entirely under irrigation. In one of the blocks, irrigation was suspended from five weeks after planting to harvest, whereas the control plot received irrigation from planting to physiological maturity. The seeds produced under the irrigated and moisture deficit conditions were harvested and subjected to seed viability and vigor tests as well as to seedling-evaluation tests. High-yielding genotypes did not necessarily produce seeds with high vigor. Moisture deficit resulted in 34% loss in germination potential, whereas vigor loss was between 40% and 220%, depending on the type of vigor test used. The seeds produced under moisture deficit had slow and non-uniform germination and seedling growth, with poor storage potential. Water availability to the crop affected seed vigor more than the genotypic effect, suggesting an inelastic limit to the extent that genotype can compensate for inadequacy of moisture of a maize plant

Using biotechnology to enhance host resistance to aflatoxin contamination of corn

Host resistance is the most widely explored strategy for eliminating aflatoxin contamination by Aspergillus flavus. Breeding strategies for developing resistant corn germplasm have been enhanced by the development of new screening tools for field inoculation and for laboratory screening. RFLP analysis of corn populations has highlighted the possibility that different resistance traits can be successfully pyramided into agronomically useful germplasm, while proteomics has impacted the identification of proteins associated with resistance (RAPs). The identification of RAPs has also been enhanced by the discovery of near-isogenic corn lines in progeny generated in a West African breeding program. The characterization of genes of the aflatoxin biosynthetic pathway has provided a foundation for a genomics investigation aimed at understanding the biochemical function and genetic regulation of aflatoxin biosynthesis. Successful inhibition of aflatoxin elaboration may require not only the action of antifungal compounds, but of compounds that block biosynthesis of toxins as well. Key words: Aflatoxin contamination, corn, molecular biology, biotechnology, host resistance. African Journal of Biotechnology Vol. 2 (12), pp. 557-562, December 200

Agronomic response of drought-tolerant and Striga-resistant maize cultivars to nitrogen fertilization in the Nigerian Guinea savannahs

In additon to drought and Striga hermonthica parasitism, nitrogen deficiency is a major constraint to maize pro¬duction in the Guinea savannahs of Nigeria. The use of mineral fertilizers is limited because of unavailability and high costs. The use of maize cultivars that perform well under sub-optimal N conditons is therefore desirable. Breeders at IITA have selected maize cultivars that combine tolerance to drought and Striga hermonthica parasit¬ism. This study evaluated six of these cultivars under a range of N application rates and compared these with a widely grown variety, TZB-SR. The study was conducted in Zaria and Samaru Kataf in Kaduna State of Nigeria. Results showed N application significantly reduced days to flowering and increased yield and yield components. Cultivars selected for combined tolerance to drought and Striga recorded lower number of days to flowering under N stress and higher dry matter, higher grain yield, higher number of grains m-2 and higher 500-seed weight at all N rates. This confirms earlier reports that maize cultivars selected for tolerance to drought will perform well under N-limited conditions. Grain yield was significantly associated with dry matter, number of grains m-2 and 500-seed weight at all N levels suggesting that these agronomic traits are significant determinants of maize yield at all N rates. Two cultivars (DT STR SYN-W/IWD C3 SYN and IWD C3 SYN/DT-SYN-1-W) were particularly outstanding at all added N levels probably due to long term improvement for drought tolerance. These two cultivars can be recommended for large-scale demonstration and release to the farmers in the West African savannas

Performance-based grouping of adapted and exotic drought-tolerant maize (Zea mays L) inbred lines under stressed and non-stressed conditions

Knowledge of the heterotic responses of elite adapted and exotic maize inbred lines can facilitate their utilization for population improvement and hybrid development. In the present study, a line x tester mating design was used to determine the combining ability of 20 elite drought-tolerant maize inbred lines developed at CIMMYT and IITA and to classify them into heterotic groups under diverse growing conditions. The 20 lines were crossed each with two inbred line testers representing the tropical and temperate heterotic pattern established in West and Cental Africa (WCA), to generate 40 testcrosses. A trial comprising the 40 testcrosses along with the cross between the two testers and three hybrid checks were evaluated at two environments in the dry season and at six environments in the rainy season. GCA effects were more important than SCA effects in controlling grain yield in both seasons. Two exotic lines in the dry season and four exotic lines in the rainy season had significantly positive GCA effects. Only EXL22 was identified as a superior line in the two seasons. Only two adapted lines had significantly positive GCA effects in either or both seasons while three adapted lines consistently had significantly negative GCA effects in both seasons. Hybrid between EXL22 and tester 9071 showed broad adaptation to all test environments. The two testers separated some of the lines into two main heterotic groups. the lines in each heterotic group and the good combiners will be utilized for developing populations for extracting new improved inbred lines

Development of Maize Host Resistance to Aflatoxigenic Fungi

United States Agency for International DevelopmentUnited States Department of AgriculturePeer Revie

Managing vulnerability to drought and enhancing livelihood resilience in sub-Saharan Africa: Technological, institutional and policy options

AbstractAgriculture and the economies of Sub-Saharan Africa (SSA) are highly sensitive to climatic variability. Drought, in particular, represents one of the most important natural factors contributing to malnutrition and famine in many parts of the region. The overall impact of drought on a given country/region and its ability to recover from the resulting social, economic and environmental impacts depends on several factors. The economic, social and environmental impacts of drought are huge in SSA and the national costs and losses incurred threaten to undermine the wider economic and development gains made in the last few decades in the region. There is an urgent need to reduce the vulnerability of countries to climate variability and to the threats posed by climate change. This paper attempts to highlight the challenges of drought in SSA and reviews the current drought risk management strategies, especially the promising technological and policy options for managing drought risks to protect livelihoods and reduce vulnerability. The review suggests the possibilities of several ex ante and ex post drought management strategies in SSA although their effectiveness depends on agro-climatic and socio-economic conditions. Existing technological, policy and institutional risk management measures need to be strengthened and integrated to manage drought ex ante and to minimize the ex post negative effects for vulnerable households and regions. A proactive approach that combines promising technological, institutional and policy solutions to manage the risks within vulnerable communities implemented by institutions operating at different levels (community, sub-national, and national) is considered to be the way forward for managing drought and climate variability

Genetic diversity assessment and relationship among tropicalyellow endosperm maize inbred lines using SSR markers

Maize is the most important food crop in sub-Saharan Africa. Tropical-adapted yellow maize inbred lines can be used as source of provision of pro-vitamin A, needed by millions of African children that suffer from vitamin A deficiency. A substantial variation of pro-vitamin A content is known to exist among available inbred lines. The objective of this study was to assess the diversity and relationship among 38 tropical-adapted maize inbred lines. The lines were investigated using 87 SSR markers. Seventy-five (75) SSR markers were polymorphic across the 38 inbred lines and generated a total of 297 alleles. The polymorphic information content (PIC) values obtained for the polymorphic SSR markers varied from 0.17 to 0.84 with an average of 0.56 .The number of polymorphic alleles per locus ranged from 2 to 11 with a mean of 3.96. Genetic distance (GD) values among all pairs of inbred lines varied from 0.007 to 0.59 with an average of 0.45. Cluster and principal coordinate analyses based on genetic distance from SSR data showed two well defined main groups of the 38 inbred lines, consistent with their pedigrees. The study has revealed a considerable level of genetic diversity among the inbred lines. The genetic potential that exists will facilitate the selection of parents with diverse alleles for development of new maize genotypes with higher pro-vitamin A level