Genetic analysis of tropical maize inbred lines for resistance to maize lethal necrosis disease

Maize lethal necrosis (MLN) disease is a recent outbreak in eastern Africa and has emerged as a significant threat to maize production in the region. The disease is caused by the co-infection of Maize chlorotic mottle virus and any member of potyviridae family. A total of 28 maize inbred lines with varying levels of tolerance to MLN were crossed in a half-diallel mating design, and the resulting 340 F₁ crosses and four commercial checks were evaluated under MLN artificial inoculation at Naivasha, Kenya in 2015 and 2016 using an alpha lattice design with two replications. The objectives of the study were to (i) investigate the magnitude of general combining ability variance (σ _GCA ² ) and specific combining ability variance (σ _SCA ² ) and their interaction with years; (ii) evaluate the efficiencies of GCA based prediction and hybrid performance by means of a cross-validation procedure; (iii) estimate trait correlations in the hybrids; and (iv) identify the MLN tolerant single cross hybrids to be used as female parents for three-way cross hybrids. Results of the combined analysis of variance revealed that both GCA and SCA effects were significant (P < 0.05) for all traits except for ear rot. For MLN scores at early and late stages, GCA effects were 2.5–3.5 times higher than SCA effects indicating that additive gene action is more important than non-additive gene action. The GCA based prediction efficiency for MLN resistance and grain yield accounted for 67–90% of the variations in the hybrid performance suggesting that GCA-based prediction can be proposed to predict MLN resistance and grain yield prior to field evaluation. Three parents, CKDHL120918, CML550, and CKLTI0227 with significant GCA effects for GY (0.61–1.21; P < 0.05) were the most resistant to MLN. Hybrids “CKLTI0227 × CML550”, “CKDHL120918 × CKLTI0138”, and “CKDHL120918 × CKLTI0136” ranked among the best performing hybrids with grain yield of 6.0–6.6 t/ha compared with mean yield of commercial check hybrids (0.6 t/ha). The MLN tolerant inbred lines and single cross hybrids identified in this study could be used to improve MLN tolerance in both public and private sector maize breeding programs in eastern Africa.     

Hybrid breeding for fall armyworm resistance: Combining ability and hybrid prediction

Fall armyworm (FAW, Spodoptera frugiperda) emerged as a major lepidopteran pest destroying maize in sub-Saharan Africa. A diallel mating design was used to generate 210 experimental hybrids from 21 lines. Experimental hybrids and four checks were evaluated in two locations. Commercial checks suffered higher foliar and ear damage compared to the top 15 hybrids. Mean squares associated with the genotypic variation were higher than genotype-by-environment interactions for foliar and ear damage traits. Heritabilities were moderate to high. Significant correlations were observed between grain yield (GY) with ear rot (−0.54) and ear damage (−0.45). Positive and significant GCA effects were observed for GY in seven parental lines, which were developed from multiple insect resistance breeding programmes. CKSBL10153 has the highest GCA value for GY and shows significant GCA effects for foliar and ear damage traits. These lines were identified as the ideal combiners for GY and FAW resistance and are therefore recommended for utilization as testers in the development of FAW-resistant three-way cross-hybrid maize with correlated response for increased GY. GCA and marker-based prediction correlations of GY were 0.79 and 0.96, respectively. Both GCA effects and marker-based models were effective in predicting hybrid performance for FAW resistance.     

Creation of new maize germplasm using alien introgression from <Emphasis Type="Italic">Zea mays</Emphasis> ssp. <Emphasis Type="Italic">mexicana</Emphasis>

Zea mays ssp. mexicana, an annual wild relative of maize, has many desirable characteristics for maize improvement. To transfer alien genetic germplasm into maize background, F₁ hybrids were generated by using Z. mays ssp. mexicana as the female parent and cultivated maize inbred line Ye515 as the male parent. Alien introgression lines, with a large range of genetic diversity, were produced by backcross and successive self-pollinations. A number of alien introgression lines with the predominant traits of cultivated maize were selected. Genomic in situ hybridization (GISH) proved that small chromosome segments of Z. mays ssp. mexicana had been integrated into the maize genome. Some outstanding alien introgression lines were evaluated in performance trials which showed 54.6% hybrids had grain yield greater than that of hybrid check Yedan12 which possessed 50% Ye515 parentage, and 17.1, 9.9% hybrids had grain yield competitive or greater than those of Nongda108 and Zheng958, which were elite commercial hybrids in China, respectively. The results indicated that some of the introgression lines had excellent agronomic traits and combining ability for maize cultivar, and demonstrated that Z. mays ssp. mexicana was a valuable source for maize breeding, and could be used to broaden and enrich the maize germplasm.     

Diallel analysis of grain yield and resistance to seven diseases of 12 African maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) inbred lines

Maize (Zea mays L.) is grown on 15 million ha in eastern and southern Africa. Several diseases are of common occurrence in the region and regularly result in significant yield losses. A collaborative regional disease nursery (REGNUR) project was initiated in 1998 to identify and increase access to disease resistant germplasm, generate and disseminate information on disease and insect resistance sources, and facilitate the development of resistant cultivars by project partners. A diallel among 12 elite inbred lines was formed with the specific objective of evaluating the combining ability of these inbred lines for grain yield and resistance to seven diseases. The trial was grown at six sites in 2001. Results showed that both general (GCA) and specific combining ability effects were significant for most diseases. On the average, GCA accounted for 69% of resistance to diseases and only 37% of variation for grain yield. Correlations between GCA effects for disease scores were generally non-significant, implying that it is possible to pyramid genes for disease resistance in inbred lines. This underscores the need for screening for resistance to prevailing diseases using artificial inoculation or reliable hot-spots. Based on GCA effects for grain yield and across diseases, P12 and P6 were the best inbred lines. The crosses P4 × P9 (6.7 t ha⁻¹) and P4 × P12 (6.9 t ha⁻¹) were the best hybrids in the earlier maturity group, while P3 × P9 (8.3 t ha⁻¹) and P2 × P8 (7.4 t ha⁻¹) were the best hybrids in the late maturity group.     

Development Traits Affecting Time to Low Ear Moisture in Maize

A diallel cross of six inbred lines of maize (Zea mays L.) from the Corn Belt of the USA and one inbred line (NZ1A) with characteristics of maize from the highlands of Mexico was grown in a cool, temperate environment. The objective was to investigate interrelationships and combining abilities of plant development traits likely to influence the time required to reach low ear moistures in the field. The traits were days to silking, days to physiological maturity, ear moisture at physiological maturity, grain filling duration and ear drying rate after physiological maturity. Grain yield and grain moisture at harvest were also considered. Differences among the hybrids of this diallel cross for ear and grain moisture at the usual harvest time were largely determined by differences in ear drying rate after physiological maturity, and not by differences in days to silking, days to physiological maturity, or in ear moisture at physiological maturity. General combining ability (GCA) effects were much larger than specific combining ability (SCA) effects for all traits. NZ1A had positive GCA effects for grain filling duration and negative GCA effects for ear moisture at physiological maturity which were outside the range of the Corn Belt lines suggesting that this line, or similar Mexican germplasm, could be useful for developing hybrids with high grain yields and low ear moistures at harvest in cool, temperate environments.     

Combining ability and heterosis of elite drought-tolerant maize inbred lines evaluated in diverse environments of lowland tropics

Estimates of combining abilities and heterosis of inbred lines are imperative for selection of suitable parents of maize hybrids. This study examined the combining ability of 24 drought-tolerant maize inbred lines, 12 each from International Centre for Maize and Wheat Improvement (CIMMYT) and International Institute of Tropical Agriculture (IITA). The lines were allotted into six groups each comprising four lines. The four lines in one group were used as females and crossed to the four lines in another group as males in six different sets using a North Carolina Design II mating scheme to generate 96 hybrids. The hybrids were evaluated together with four checks across six environments in the rainforest and savannah agro-ecologies of Nigeria between 2011 and 2012. The parental inbred lines were also evaluated in separate trial in each location. Significant hybrids × environment interaction was observed for grain yield and other measured traits. GCA effects accounted for 83.3% of the variation for grain yield at Bagauda, 78.1% at Saminaka, and 77.7% at Ikenne. GCA also contributed 91.1 and 80.0% to the variation observed for plant height and ear aspect, respectively, across the environments. Significant SCA × environment interaction detected for grain yield suggests that hybrids were not stable across test environments. Prediction of grain yield in hybrids using midparent values resulted in a R ² value of 0.13. Midparent heterosis for grain yield varied from 80 to 411%, with the top 36 hybrids recording >200%. Four CIMMYT (EXL02, EXL06, EXL04 and EXL16) and three IITA (ADL33, ADL41, and ADL32) inbred lines had positive and significant GCA effects for grain yield across environments. The novel alleles present in the CIMMYT lines will improve the adapted IITA germplasm in a new population for extracting new set of more productive inbred lines for developing adapted high yielding drought-tolerant maize hybrids.     

Genetic Analysis of Grain Yield and Other Traits of Early‐Maturing Maize Inbreds under Drought and Well‐Watered Conditions

Maize (Zea mays L.) is an important staple food crop in West and Central Africa (WCA). However, its production is constrained by drought. Knowledge and understanding of the genetics of hybrid performance under drought is invaluable in designing breeding strategies for improving maize yield. One hundred and fifty hybrids obtained by crossing 30 inbreds in sets using the North Carolina Design II plus six checks were evaluated under drought and well‐watered conditions for 2 years at three locations in Nigeria. The objectives of the studies were to (i) determine the mode of gene action controlling grain yield and other important agronomic traits of selected early inbred lines, (ii) examine the relationship between per se performance of inbreds and their hybrids and (iii) identify appropriate testers for maize breeding programmes in WCA. General combining ability (GCA) and specific combining ability (SCA) mean squares were significant (P < 0.01) for grain yield and other traits under the research environments. The GCA accounted for 64.5 % and 62.3 % of the total variation for grain yield under drought and well‐watered conditions, indicating that additive gene action largely controlled the inheritance of grain yield of the hybrids. Narrow‐sense heritability was 67 % for grain yield under drought and 49 % under well‐watered conditions. The correlations between traits of early‐maturing parental lines and their hybrids were significant (P < 0.01) under drought, well‐watered and across environments. Mid‐parent and better‐parent heterosis for grain yield were 45.3 % and 18.4 % under drought stress and 111.9 % and 102.6 % under well‐watered conditions. Inbreds TZEI 31, TZEI 17, TZEI 129 and TZEI 157 were identified as the best testers. Drought‐tolerant hybrids with superior performance under stress and non‐stress conditions could be obtained through the accumulation of favourable alleles for drought tolerance in both parental lines.     

Increased resistance to <Emphasis Type="Italic">Ustilago zeae</Emphasis> and <Emphasis Type="Italic">Fusarium verticilliodes</Emphasis> in maize inbred lines bred for <Emphasis Type="Italic">Fusarium graminearum</Emphasis> resistance

Preliminary field observations in our maize breeding nurseries indicated that breeding for improved resistance to gibberella ear rot (Fusarium graminearum) in maize may indirectly select for resistance to another ear disease, common smut (Ustilago zeae). To investigate this, we compared the disease severity ratings obtained on 189 maize inbreds, eight of which included our inbreds developed with selection for gibberella ear rot resistance after field inoculation and breeding for 8–10 years. No correlation was found between disease severities for the 189 inbreds but the eight gibberella-resistant lines were consistently more resistant to smut. To further examine this relationship and to determine if these eight inbreds would be useful for developing inbreds with either common smut or fusarium ear rot (F. verticilliodes) resistance, we conducted a Griffing’s diallel analysis on six inbreds of maize, four with high levels of gibberella ear rot resistance representing all of the pedigree groups in our eight gibberella lines, and two with very low levels. Our most gibberella ear rot resistant inbreds, CO433 and CO441, had the lowest disease ratings for all three diseases, the consistently largest general combining ability effects and several significant specific combining ability effects. It was concluded that some inbreds bred specifically for gibberella ear rot would also be useful in breeding for resistance to common smut and fusarium ear rot.     

Gene action and reciprocal effects for ear rot resistance in crosses derived from five tropical maize populations

Devastating maize grain yield and quality losses are caused by Aspergillus flavus, Fusarium verticillioides and Stenocarpella maydis ear rots especially in tropical countries. Therefore, combining ability of tropical maize populations for ear rot severity and ear rot-related traits was investigated. Ten full-sib progenies, comprising one resistant and one susceptible from each of the five populations, were selected for mating in a 10 × 10 full diallel. The full-sib progeny crosses were evaluated across two environments with two replications in Zambia. To determine resistance across three ear rots that occur together in Zambia, the crosses were artificially inoculated with a mixture of Aspergillus flavus, Fusarium verticillioides and Stenocarpella maydis isolates. There were marked differences between environment main effects and their interaction with GCA and SCA effects were highly significant, suggesting observation of genotype × environment interaction effects. Both additive and non-additive gene effects were significant for ear rot severity. Highly significant reciprocal differences were also revealed, suggesting that cytoplasmic gene effects and their interaction with nuclear genes were responsible in modifying resistance across the three ear rot diseases in the full-sib progenies that were derived from the five tropical maize populations.     

西南骨干玉米自交系主要农艺性状和产量正反交杂优效应分析

为了比较玉米自交系主要农艺性状和产量的正反交杂优效应,以9份西南地区骨干玉米自交系组配的72份单交种为材料,采用随机区组试验,对自交系及其正反交杂种F1代主要农艺性状和产量进行比较研究。结果表明,正反交效应对不同玉米自交系杂种F1代主要农艺性状和产量的影响不同;T检测表明,大多数玉米自交系正反交杂种F1代主要农艺性状和产量差异不显著。玉米自交系海9-21、5003、478正反交杂种F1代在穗粗、百粒重、单穗粒重、穗下茎节强度及穗位高等性状上差异显著或极显著。由此得出,不同自交系正反交杂种F1代在主要农艺性状和产量上具有不同的杂种优势,其中穗位高/株高具有负向杂种优势。由于玉米穗位高/株高具有负向杂种优势,即杂种F1代穗位高/株高值较自交系有所降低,因此在自交系选育时,不宜对亲本材料过分追求低的穗位高/株高。     

Hybrid reconstruction in maize

Summary The production of seed of the maize single cross hybrid F68^*NE2 is uneconomic because of the low grain yield of the maternal line. Therefore the aim was to produce it from newly developed inbred lines obtained by reshuffling the genes in the hybrid, accompanied by selection. Thus in open pollinating generations derived from this hybrid, i.e. in C0, C1, C2, C3 and C4, honeycomb selection for grain yield improvement was applied. Selfing of one ear and open pollination of another ear of selected prolific C4 plants yielded 20 pairs of S1/half sib progenies. Plants grown from remnant S1 seed corresponding to superior progeny pairs were selfed. In each S2-line a single plant was selected and selfed. The S3-lines were evaluated for yield. Two S3-lines, i.e. 6D and 2B, attracted attention because they yielded two and a half times as much as the best commercial inbred line B73.The S1-and S2-lines were tested for combining ability with the related inbred lines NE2 and F68 by means of honeycomb design experiments and for combining ability with unrelated, freely available inbred lines by means of randomized complete block designs. Two S2-lines, i.e. 5C and 6E, were selected for their good combining ability. The six single cross hybrids produced by crossing the four S3-lines 6D, 2B, 5C, and 6E were compared with the original hybrid F68^*NE2 in a honeycomb design at two sites. The grain yields of the single cross hybrids 6D^*6E and 5C^*6E were similar to that of F68^*NE2. However, these two reconstructed hybrids can be produced in a cheaper way because the new maternal inbred lines yield as good as B73 (line 5C) or much better (line 6D).     

Genetic analysis of resistance to nematodes in inbred maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) and maize hybrids

Nematodes cause substantial grain yield loss in susceptible maize (Zea mays L.) cultivars. This study was conducted to estimate general combining ability (GCA), specific combining ability (SCA) and genetic effects associated with nematode resistance in maize. The 30 F ₁ hybrids generated from a 6 × 6 diallel and two local checks were evaluated in 2009 at three sites in Uganda. A split plot design was used with nematode treatments serving as whole plots and the hybrids as subplots but arranged in an 8 × 4 spatially adjusted alpha-lattice design. The experiment was replicated three times. Results showed GCA to be important for the reduction of P. zeae and Meloidogyne spp. densities and increase of root mass, with a contribution of 72 to 93% of the phenotypic variance. Inbreds MP709 and CML206 had the highest GCA for Pratylenchus zeae resistance, whereas for grain yield, it was CML444, CML312 and CML395 that were outstanding. The SCA influenced plant height and grain yield under nematode infestation, contributing 43 and 58% of the phenotypic variance, respectively. Observed reciprocal differences due to maternal effects also played a role in influencing the grain yield under nematode infestation. Overdominance genetic effects explained the non-additive variance recorded for the plant height, grain yield, number of root lesions, and P. zeae and Meloidogyne spp. densities under nematode infestation. The parents MP709, CML206, 5057, and CML444 contributed most of the dominant genes for the P. zeae resistance in all their crosses. The parent CML444 contributed most of the dominant genes for improved grain yield in all of its crosses. The high GCA effects among some parents support their utility in breeding of widely adapted nematode-resistant cultivars. The dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure.     

Combining ability amongst CIMMYT’s early maturing maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) germplasm under stress and non-stress conditions and identification of testers

CIMMYT (International Maize and Wheat Research Institute) Zimbabwe’s early maturing maize program, which aims to supply seed to approximately 4 million hectares of maize area in eastern and southern Africa, lacks adequate information on heterotic relationships among early maturing germplasm and has no early maturing testers for hybrid development. Open-pollinated varieties (OPVs) and hybrids are the products targeted for this region. Among the hybrids, three-way and double-cross hybrids are desired. Thus the use of single crosses as testers would be an appropriate choice for such a breeding program as one could potentially identify three-way combinations during the early generation test. A twelve-parent diallel was formed and crosses evaluated to identify heterotic groups and single-cross testers. Crosses were evaluated under four different environments in Zimbabwe, two optimal, one low nitrogen stress and one drought stress. P5 (an early maturing line from heterotic group A) and CML 395 (a late maturing inbred line from heterotic group B) were used as reference parents to establish heterotic groupings of germplasm used in the diallel. The single cross (P7/P8) was identified as a potential group A tester because of: (a) co-classification of inbred lines into heterotic group A, (b) good yields-9.8 t/ha (optimal), 3.4 t/ha (low nitrogen) and 2.1 t/ha (drought); and (c) good GCA effects for grain yield (0.49 t/ha) of line P7 while line P8 contributed to reduced height and anthesis-silking interval.     

Genetic distance among doubled haploid maize lines and their testcross performance under drought stress and non-stress conditions

In contrast to conventional inbreeding that takes up to seven generations to develop inbred lines, the doubled haploid (DH) technology allows production of inbred lines in two generations. The objectives of the present study were to: (a) evaluate testcross performance of 45 doubled haploid lines under drought stress and non-stress conditions (b) estimate heritabilities for grain yield and other traits and (c) to assess the genetic distance and relationship among the DH lines using 163,080 SNPs generated using genotyping-by-sequencing (GBS). The 45 hybrid and five checks were evaluated using a 10 × 5 alpha lattice in six drought stress and nine well-watered environments in Kenya, Uganda, and Tanzania. Differences in trait means between the drought stress and well-watered conditions were significant for all measured traits except for anthesis date. Genetic variances for grain yield, grain moisture, plant height and ear height were high under well-watered environments while genetic variance for anthesis date, root lodging and stalk lodging were high under drought stress environments. Combined analyses across drought stress and well-watered environments showed that ten top hybrids produced 1.6–2.2 t/ha grain yield under well-watered condition and 1–1.4 t/ha under drought stress condition higher than the mean of the commercial checks. Genetic distance between pairwise comparisons of the 38 of the 45 DH lines ranged from 0.07 to 0.48, and the overall average distance was 0.36. Both cluster and principal coordinate analysis using the genetic distance matrix calculated from 163,080 SNPs showed two major groups and the patterns of group was in agreement with their pedigree. Thirteen (13) of the best hybrids are currently in National Performance Trials testing, an important step towards commercialization in Kenya, Tanzania and Uganda.     

Sclerotinia resistance in sunflower. II. Combining ability and midparent heterosis for reaction to ascospore infections at flowering

In Argentina, susceptibility of the sunflower crop to Sclerotinia sclerotiorum capitulum attacks can cause significant yield losses. We reported earlier two hybrids among a series obtained from crosses between inbred lines following a factorial mating design that had a sufficient level of resistance to be adapted to the main Argentina sunflower growing area. Results concerning the per se behavior of the parental lines involved in these crosses and the combining ability and heterosis effects of such parental genotypes are presented in this article. The lines showed different responses to white rot incidence and relative incubation period. There were significant effects for general and specific combining ability and also for midparent heterosis, for the two variables evaluated. Preponderance of additive gene effects on genetic control white rot resistance level was shown. The development of sunflower inbred lines to be used as parents in the production of hybrids for areas requiring moderately resistant hybrids is an important phase in breeding programs. To optimize the use of resources and time, our results suggest that testing general combining ability at early generations of selfing followed by testing of hybrids may be considered as an adequate strategy in breeding for white rot resistance.     

QTL for maize grain yield identified by QTL mapping in six environments and consensus loci for grain weight detected by meta‐analysis

Grain yield is the most important and complicated trait in maize. In this study, a total of 498 recombinant inbred lines (RIL) derived from a biparental cross of two elite inbred lines, 178 and P53, were grown in six different environments. Quantitative trait locus (QTL) mapping was conducted for three grain yield component traits (100 grain weight, ear weight and kernel weight per plant). Subsequently, meta‐analysis was performed after a comprehensive review of the research on QTL mapping for grain weight (100, 300 and 1000) using BioMercator V4.2. In total, 62 QTLs were identified for 100 grain weight, ear weight and kernel weight per plant in six environments. Forty‐three meta‐QTLs (MQTLs) were detected by meta‐analysis. A total of 13 candidate genes homologous to eight functionally characterized rice genes were found, and four candidate genes were located in the two hot spot regions of MQTL1.5 and MQTL2.3. Our results suggest that the combination of literature collection, meta‐analysis and homologous blast searches can offer abundant information for further fine mapping, marker‐assisted selection (MAS) breeding and map‐based cloning for maize.     

Combining ability,maternal, and reciprocal effects of elite early-maturing maize population hybrids

Choosing germplasm based on elite and diverse genetic sources is essential for the genetic improvement of maize (Zea mays L.) hybrids. The objectives of this research were to evaluate the agronomic and economic potential of maize population and single-cross hybrids and whether significant maternal (ME) and reciprocal effects (RE) reside in elite population hybrids for seed production purposes. Seven elite maize populations currently under recurrent selection at North Dakota State University (NDSU) [NDSCD(M-S)C11, NDSAB(MER-FS)C14, BS21(R)C7, BS22(R)C7, LEAMING(S)C4, CGL(S₁-S₂)C5 and CGSS(S₁-S₂)C5] were crossed in a diallel mating design to form 42 population hybrids, including their reciprocals. The 42 population hybrids with eight single-cross hybrids were evaluated at six U.S. North Central locations in 2005. Data collected across locations indicated that differences across genotypes were significant (P ≤ 0.05) for all traits observed, except for grain yield ear components. General combining ability (GCA) effects were on average larger than specific combining ability (SCA) effects. ME and RE were not significant for all traits, except for ear height. The large grain yield differences between macro-environments were reflected in the ranking of genotypes, with BS21(R)C7 × BS22(R)C7 being the top performer in eastern environments and CGSS(S1-S2)C5 × NDSAB(MER-FS)C14 being the top one across western environments where drought is the major limitation. The increased ethanol production and demand from maize make test weight (and grain quality), earliness, lodging resistance, and drought tolerance as important as grain yield for maintaining a sustainable maize-ethanol relationship. Part of the thesis submitted by McDonald B. Jumbo in partial fulfillment of the requirements for a MS degree at North Dakota State University.     

Determining resistance to <Emphasis Type="Italic">Fusarium verticillioides</Emphasis> and fumonisin accumulation in African maize inbred lines resistant to <Emphasis Type="Italic">Aspergillus flavus</Emphasis> and aflatoxins

Fusarium verticillioides and Aspergillus flavus cause Fusarium ear rot (FER) and Aspergillus ear rot (AER) of maize, respectively. Both pathogens are of concern to producers as they reduce grain yield and affect quality. F. verticillioides and A. flavus also contaminate maize grain with the mycotoxins fumonisins and aflatoxins, respectively, which has been associated with mycotoxicosis in humans and animals. The occurrence of common resistance mechanisms to FER and AER has been reported. Hence, ten Kenyan inbred lines resistant to AER and aflatoxin accumulation were evaluated for resistance to FER, F. verticillioides colonisation and fumonisin accumulation; and compared to nine South African lines resistant to FER and fumonisin accumulation. Field trials were conducted at three localities in South Africa and two localities in Kenya. FER severity was determined by visual assessment, while F. verticillioides colonisation and fumonisin content were quantified by real-time PCR and liquid chromatography tandem mass spectrometry, respectively. Significant genotype x environment interactions was determined at each location (P ≤ 0.05). Kenyan inbred CML495 was most resistant to FER and F. verticillioides colonisation, and accumulated the lowest concentration of fumonisins across localities. It was, however, not significantly more resistant than Kenyan lines CML264 and CKL05015, and the South African line RO549 W, which also exhibited low FER severity (≤5%), fungal target DNA (≤0.025 ng μL^?1) and fumonisin levels (≤2.5 mg kg^?1). Inbred lines resistant to AER and aflatoxin accumulation appear to be promising sources of resistance to F. verticillioides and fumonisin contamination.     

Combining ability,heterosis and genetic diversity in tropical maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) under stress and non-stress conditions

Drought and low soil fertility are considered the most important abiotic stresses limiting maize production in sub-Saharan Africa. Knowledge of the combining ability and diversity of inbred lines with tolerance to the two stresses and for those used as testers would be beneficial in setting breeding strategies for stress and nonstress environments. We used 15 tropical maize inbred lines to (i) evaluate the combining ability for grain yield (GY), (ii) assess the genetic diversity of this set of inbred lines using RFLP, SSR, and AFLP markers, (iii) estimate heterosis and assess the relationship between F₁ hybrid performance, genetic diversity and heterosis, and (iv) assess genotype × environment interaction of inbred lines and their hybrids. The F₁ diallel hybrids and parental inbreds were evaluated under drought stress, low N stress, and well-watered conditions at six locations in three countries. General combining ability (GCA) effects were highly significant (P < 0.01) for GY across stresses and well-watered environments. Inbred lines CML258, CML339, CML341, and CML343 had the best GCA effects for GY across environments. Additive genetic effects were more important for GY under drought stress and well-watered conditions but not under low N stress, suggesting different gene action in control of GY. Clustering based on genetic distance (GD) calculated using combined marker data grouped lines according to pedigree. Positive correlation was found between midparent heterosis (MPH) and specific combining ability (SCA), GD and GY. Hybrid breeding program targeting stress environments would benefit from the accumulation of favorable alleles for drought tolerance in both parental lines.     

Assessing the suitability of stress tolerant early-maturing maize (Zea mays) inbred lines for hybrid development using combining ability effects and DArTseq markers

Identification of hybrids for commercialization is crucial for sustainable maize production in sub-Saharan Africa (SSA). One hundred and ninety test crosses, 10 tester × tester crosses + 10 hybrid checks were evaluated across 11 environments, 2017 to 2019. Inheritance of grain yield under Striga infestation, optimal and across environments was influenced by additive genetic action, but there was greater influence of nonadditive gene action under drought stress conditions. Nine, seven and two inbreds had significant and positive general combining ability (GCA) effects for grain yield under Striga-infested, optimal and drought stress environments, respectively, and would contribute high grain yield to their progenies. Heterotic grouping methods based on specific and GCA, GCA effects of multiple traits and DArTseq markers classified the inbreds into five, three and two heterotic groups, respectively, across research conditions. The DArTseq markers method that classified the inbred lines into two major heterotic groups and was one of the most efficient methods should be adopted for practical purposes in maize breeding programmes in SSA. Hybrids TZEI 7 × TZdEI 352, TZEI 1238 × TZEI 7 and TZEI 1252 × TZEI 7 had outstanding grain yield under contrasting environments and should be tested on-farm for commercialization in SSA.