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Identifying efficient testers for separating maize (Zea mays L.) inbred lines into heterotic groups can facilitate the development of superior hybrids. Striga-resistant yellow-maize inbred lines de...

Rice yellow mottle virus (RYMV) is the most harmful virus that affects irrigated and lowland rice in Africa. The RBe24 isolate of the virus is the most pathogenic strain in Benin. A total of 79 genotypes including susceptible IR64 (Oryza sativa) and the resistant TOG5681 (O. glaberrima) as checks were screened for their reactions to RBe24 isolate of RYMV and the effects of silicon on the response of host plants to the virus investigated. The experiment was a three-factor factorial consisting of genotypes, inoculation level (inoculated vs. non-inoculated), and silicon dose (0, 5, and 10 g/plant) applied as CaSiO3 with two replications and carried out twice in the screen house. Significant differences were observed among the rice genotypes. Fifteen highly resistant and eight resistant genotypes were identified, and these were mainly O. glaberrima. Silicon application did not affect disease incidence and severity at 21 and 42 days after inoculation (DAI); it, however, significantly increased plant height of inoculated (3.6% for 5 g CaSiO3/plant and 6.3% for 10 g CaSiO3/plant) and non-inoculated (1.9% for 5 g CaSiO3/plant and 4.9% for 10 g CaSiO3/plant) plants at 42 DAI, with a reduction in the number of tillers (12.3% for both 5 and 10 g CaSiO3/plant) and leaves (26.8% for 5 g CaSiO3/plant and 28% for 10 g CaSiO3/plant) under both inoculation treatments. Our results confirm O. glaberrima germplasm as an important source of resistance to RYMV, and critical in developing a comprehensive strategy for the control of RYMV in West Africa.

Yam (Dioscorea spp.) is a monocotyledonous herbaceous vine plant grown in the tropics and subtropics. It is a multi-species plant with varied intra- and interspecific ploidy levels. Of the 600 species, 11 are cultivated staple supporting the livelihood of over 300 million people. The paucity of information on ploidy and the genomic constitution is a significant challenge to the crop’s genetic improvement through crossbreeding. The objective of this study was to investigate the ploidy levels of 236 accessions across six cultivated and two wild species using chromosome counting, flow cytometry and genotyping-based ploidy determination methods. Results obtained from chromosome counting and genotyping-based ploidy determination were in agreement. In majority of the accessions, chromosome counting and flow cytometry were congruent, allowing future rapid screening of ploidy levels using flow cytometry. Among cultivated accessions, 168 (71%) were diploid, 50 (21%) were triploid, and 12 (5%) were tetraploid. Two wild species included in the study were diploids. Resolution of ploidy level in yams offers opportunities for implementing successful breeding programmes through intra- and inter-specific hybridization.

A hemiparasitic plant, Striga hermonthica (Del.) Benth and soil nitrogen stress are the key constraints to maize (Zea mays L.) productivity in sub-Saharan Africa, where commonly cultivated maize is the normal endosperm type that is deficient in provitamin A, tryptophan and lysine (PVATL). Seventy-six extra-early maize inbreds with provitamin A, tryptophan, and lysine qualities (TZEEIORQ) were developed to address these constraints, and four checks were assessed under Striga, low and high nitrogen conditions at three locations in Nigeria. The inbreds were further genotyped with two beta-carotene hydroxylase 1 (crtRB1) markers, and their seeds were quantified for provitamin A content. Significant (P &lt, 0.01) genetic variations were observed for grain yield and other agronomic attributes of the inbreds under varying environmental conditions. Levels of PVATL for the inbreds ranged from 2.21–10.95 µg g−1, 0.04–0.08%, and 0.19–0.39%, respectively. Beta-carotene marker, crtRB1-3′TE, was polymorphic and grouped the inbreds into two. The marker was effective in identifying inbreds with moderate provitamin A content. Inbreds TZEEIORQ 5, TZEEIORQ 52, and TZEEIORQ 55 exhibited resistance to Striga, tolerance to nitrogen stress with moderate levels of PVATL and could be invaluable sources of favorable alleles for breeding nutritionally improved maize varieties with resistance/tolerance to Striga and soil nitrogen stress.

Yam (Dioscorea spp.) is a nutritional and medicinal staple tuber crop grown in the tropics and sub-tropics. Among the food yam species, water yam (Dioscorea alata L.) is the most widely distributed and cultivated species worldwide. Tuber dry matter content (DMC) and oxidative browning (OxB) are important quality attributes that determine cultivar acceptability in water yam. This study used a single nucleotide polymorphism (SNP) assay from a diversity arrays technology (DArT) platform for a genome-wide association study (GWAS) of the two quality traits in a panel of 100 water yam clones grown in three environments. The marker&ndash, trait association analysis identified significant SNPs associated with tuber DMC on chromosomes 6 and 19 and with OxB on chromosome 5. The significant SNPs cumulatively explained 45.87 and 12.74% of the total phenotypic variation for the tuber DMC and OxB, respectively. Gene annotation for the significant SNP loci identified important genes associated in the process of the proteolytic modification of carbohydrates in the dry matter accumulation pathway as well as fatty acid &beta, oxidation in peroxisome for enzymatic oxidation. Additional putative genes were also identified in the peak SNP sites for both tuber dry matter and enzymatic oxidation with unknown functions. The results of this study provide valuable insight for further dissection of the genetic architecture of tuber dry matter and enzymatic oxidation in water yam. They also highlight SNP variants and genes useful for genomics-informed selection decisions in the breeding process for improving food quality traits in water yam.

Yam (Dioscorea spp.) is a monocotyledonous herbaceous vine plant grown in the tropics and subtropics. It is a multi-species plant with varied intra- and interspecific ploidy levels. Of the 600 species, 11 are cultivated supporting the livelihood of over 300 million people. The paucity of information on ploidy and the genomic constitution is a significant challenge to the crop’s genetic improvement through crossbreeding. The objective of this study was to investigate the ploidy levels of 236 accessions across six cultivated and two wild species using chromosome counting, flow cytometry and genotyping-based ploidy determination methods. Results obtained from chromosome counting and genotyping-based ploidy determination were in agreement. In majority of the accessions, chromosome counting and flow cytometry were congruent, allowing future rapid screening of ploidy levels using flow cytometry. Among cultivated accessions, 168 (71%) were diploid, 50 (21%) were triploid, and 12 (5%) were tetraploid. Two wild species included in the study were diploids. Resolution of ploidy level in yams offers opportunities for implementing successful breeding programmes through intra- and interspecific hybridization.

Poor milling and grain appearance is a common feature of locally produced rice (Oryza sativa L.) in West Africa. Development of genotypes with high yield and uniform milled grain size will enhance the market for the locally produced rice. One hundred rice accessions were evaluated to assess genetic variability, heritability and correlations for 11 milling and appearance quality traits and grain yield. The field was laid as a 10 × 10 alpha lattice design with three replications following standard cultivation practices. ANOVA revealed significant differences for the traits studied. The genotypic and environmental variances showed significant differences for all characters studied. Phenotypic coefficient of variation was greater than its corresponding genotypic coefficient of variation for each studied trait. Heritability at broad-sense varied from 14.1% for milling degree to 73.8% for milled grain length-to-width ratio (L/W). Genetic advance as percent of mean ranged from 2.2% for brown rice recovery to 129.6% for percentage of grain with chalkiness (PGWC). In general, genotypic correlations ranged higher than their corresponding phenotypic correlations. Brown rice recovery had significant positive phenotypic and genotypic correlations with milling recovery and head rice recovery. Consequently, brown rice recovery, milling recovery and L/W could be used as selection criteria for the improvement of head rice recovery. Genotypes BETIA and CRI-AMANKWATIA had the desirable PGWC and degree of chalkiness, therefore, they are recommended for the improvement of high yielding varieties with high amount of chalkiness.

Knowledge of genebank and on-farm genetic diversity, particularly in an introduced crop species, is crucial to the management and utilization of the genetic resources available. Microsatellite markers were used to determine genetic diversity in 574 accessions of cacao, Theobroma cacao L., representing eight groups covering parental populations in West Africa, genebank, and farmers’ populations in Nigeria. From the 12 microsatellite markers used, a total of 144 alleles were detected with a mean allelic richness of 4.39 alleles/locus. The largest genetic diversity was found in the Upper Amazon parent population (Hnb = 0.730), followed by the 1944 Posnette’s Introduction (Hnb = 0.704), and was lowest in the Local parent population (Hnb = 0.471). Gene diversity was appreciably high in the farmers’ populations (Hnb = 0.563–0.624); however, the effective number of alleles was lower than that found in the genebank’s Posnette’s population. Fixation index estimates indicated deficiency of heterozygotes in the Upper Amazon and the Local parent populations (Fis = 0.209 and 0.160, respectively), and excess of heterozygotes in the Trinitario parent population (Fis = −0.341). The presence of inbreeding in the Local parent populations and substructure (Wahlund effect) in the Upper Amazon were suggested for the deficiency of heterozygotes observed. Non-significant genetic differentiation observed between the genebank’s and farmers’ populations indicated significant impact of national breeding programs on varieties grown in farmers’ plantations. From this study, we showed that appreciable genetic diversity was present in on-farm and field genebank collections of cacao that can be exploited for crop improvement in West Africa. Suggestions for future conservation of on-farm genetic diversity and local landraces are further discussed.

Maize (Zea mays) is a staple food crop in sub-Saharan Africa, but its production is threatened by Striga hermonthica (Del.) Benth. Transfer of resistance genes from wild relatives may increase resistance to S. hermonthica in tropical maize. The objective of this study was to determine the combining ability of resistance to S. hermonthica among lines containing Zea diploperennis and tropical germplasm. Forty-five diallel crosses of 10 inbred lines were evaluated in an alpha-lattice design with and without artificial Striga infestation at two locations each in the Republic of Benin and Nigeria for 3 years. Results of analyses showed that only general combining ability (GCA) mean square was significant (P = 0.01) for number of emerged Striga plants (NESPP), while both GCA and specific combining ability (SCA) mean squares were significant for host damage score (HDS) and grain yield under Striga infestation. The ratio of GCA to SCA mean squares for the three traits varied from 3.5 to 57.5. Although GCA x environment interaction was significant for the three traits, two inbred lines containing Z. diploperennis (ZD 551) and tropical (TZL TC 87) germplasm had negative and significant GCA effects for NESPP and HDS and positive GCA effects for grain yield under Striga infestation in the two countries. Correlation between NESPP and HDS was strong and significant (r = 0.87, P = 0.01). Our results highlight the importance of harnessing useful genes from wild relatives to improve resistance to S. hermonthica in adapted maize germplasm.

Twelve maize genotypes with different degrees of resistance were evaluated in plots inoculated with seeds of the phytoparasite Striga hermonthica (Del.) Benth. and in Striga-free (control) plots for three seasons between 1991 and 1993. Resistant genotypes, although showing similar levels of underground infection as susceptible 9 weeks after maize planting (WAP), had significantly fewer emerged parasitic plants and sustained lower damage. Host damage was not determined by the severity of infection. Percent height and dry matter reductions increased from 3 to 6 WAP for resistant and susceptible genotypes. While percent height and dry matter reductions for resistant genotypes declined at 9 and 12 WAP, susceptible genotypes either maintained or increased their levels of damage. Correlation between the severity of height and dry matter reductions at the vegetative (3 and 6 WAP) and reproductive (9 and 12 WAP) stages of maize growth were not significant, indicating that damage at early stages of growth cannot be used to predict mature plant response. Maize damage score (1–9) at 8 WAP, a non destructive and composite shoot indicator of host performance under Striga infestation, was significantly correlated (r = 0.88, r = 0.82; p < 0.01) to the level of shoot reduction at mature plant stages. Emerged Striga count at 8 WAP was significantly correlated (r = 0.98, p < 0.01) to the count at 10 WAP, the time when parasite emergence was highest. Under high and uniform infestation, mature plant resistance can be detected at 8 WAP, the onset of flowering in maize.

A parasitic plant, Striga hermonthica (Del.) Berth.,infests millions of hectares of arable land in sub-Saharan Africa, and it threatens production of major cereal crops including maize (Zea mays L). Erratic natural infestation is considered a major constraint to assess host plant resistance to the parasite. The objective of this study was to evaluate the effects of different S. hermonthica seed inoculum rates on a tolerant and a susceptible maize genotype. Individual maize plants of the tolerant (8322-13) and the susceptible maize hybrid (8338-1) were inoculated with six rates of germinable Striga seeds: 750, 1500, 2250, 3000, 3750, and 4500. Uninfested plots served as the control. The experiment was conducted at two locations, Mokwa (two seasons) and Abuja (one season), in Nigeria. Increasing Striga inoculum level increased Striga emergence and the amount of damage to the host plant, plant height, and grain yield of the host crop. The tolerant hybrid showed less host plant damage (P < 0.01) and that fewer S. hermonthica plants emerged (P < 0.05) than the susceptible hybrid. Among the four traits studied, host plant damage score was most correlated with grain yield (r = -0.65**). Average grain yield of the tolerant hybrid, 8322-13, was 97% higher than that of the susceptible hybrid, 8338-1, across the six inoculum rates. The tolerant hybrid produced similar grain yields under all levels of the inoculum; the yield reductions ranged from 27% (2250 seeds) to 35% (4500 seeds), while the yield reduction of the susceptible hybrid ranged from 43% (750 seeds) to 74% (3750 seeds). Based on the results of this study, we conclude that host plant tolerance is the key element for stabilizing crop yield under S. hermonthica infestation.

A phytoparasite, Striga hennonthica (Del.) Benth., infests millions of hectares of cultivated fields of cereal crops in sub-Saharan Africa. Yield losses are often 70 but can be as high as 100% and farmers often abandon infested fields and move to new areas. The objectives of this study were to investigate the effects of nitrogen on S. hennonthica infestation and the subsequent grain yield and agronomic traits of tolerant and susceptible maize hybrids (Zea mays L.). Two tolerant and two susceptible hybrids were grown under six levels of N (0-150 kg ha -1 ), with and without Striga seed infestation. The trials were conducted for 3 yr at Mokwa, Nigeria. Each plant was infested with approximately 3000 germinable Striga seeds, and the density of Striga that emerged, Striga damage on the host plant, plant height, stalk lodging, and grain yield were measured. Striga infestation (emergence and host damage), maize grain yield, and plant height were significantly affected by N rates. Two tolerant hybrids showed significantly lower Striga emergence and host plant damage symptoms than the susceptible hybrids (P < 0.001). Interactions between infested and uninfested plots and hybrid (tolerant vs. susceptible) for grain yields and stalk lodging were significant (P < 0.001). Striga infestation reduced grain yields of two susceptible hybrids by 49%, and of two tolerant hybrids by 24%. The two tolerant hybrids produced on average 87% greater grain yields than the two susceptible hybrids under low N rates (0-60 kg ha -1 ) and 51% greater yields under high N (90-150 kg ha -1 ). Among the plant traits measured, Striga damage score had the highest correlation with grain yield (r = -0.60**).

Seeds of the obligate parasite Striga hermonthica , germinate in response to stimulants in the root exudate of maize and other host crops. Under laboratory conditions, conditioning (defined as treatment with moisture) of Striga seeds for one to three weeks before exposure to germination stimulant is required for optimum germination. The objective of this study was to determine if maize performance and Striga emergence, both indices of resistance, were influenced by the interval between Striga seed infestation and maize planting, under high parasite inoculum (3000–6000 germinable seeds) as practiced in maize resistance breeding field nurseries. Four conditioning treatments were investigated: maize planted immediately after Striga seed infestation, and maize planted after one, two and three weeks after infestation. Experimental design was randomized complete block with four replications. Differences among the four conditioning treatments were not significant for maize damage score, height, stalk lodging, number of ears harvested, grain yield and number of emerged Striga plants. Thus, maize planted on the same day as Striga infestation showed similar response as maize planted one to three weeks after infestation. Using high seed inoculum, maize can be planted immediately after Striga infestation in maize breeding nurseries. Combining Striga seed infestation and maize planting into a single field operation is convenient and reduces cost of screening.