Current climate change models predict an increased frequency and intensity of drought for much of the developing world within the next 30 years. These events will negatively affect maize yields, potentially leading to economic and social instability in many smallholder farming communities. Knowledge about the genetic resources available for traits related to drought tolerance has great importance in developing breeding program strategies. The aim of this research was to study a maize landrace introgression panel to identify chromosomal regions associated with a drought tolerance index. For that, we performed Genome-Wide Association Study (GWAS) on 1326 landrace progenies developed by the CIMMYT Genetic Resources Program, originating from 20 landraces populations collected in arid regions. Phenotypic data were obtained from early testcross trials conducted in three sites and two contrasting irrigation environments, full irrigation (well-watered) and reduced irrigation (drought). The populations were genotyped using the DArTSeq® platform, and a final set of 5,695 SNPs markers was used. The genotypic values were estimated using spatial adjustment in a two-stage analysis. First, we performed the individual analysis for each site/irrigation treatment combination. The best linear unbiased estimates (BLUEs) were used to calculate the Harmonic Mean of Relative Performance (HMRP) as a drought tolerance index for each testcross. The second stage was a joint analysis, which was performed using the HMRP to obtain the best linear unbiased predictions (BLUPs) of the index for each genotype. Then, GWAS was performed to determine the marker-index associations and the marker-Grain Yield (GY) associations for the two irrigation treatments. We detected two significant markers associated with the drought-tolerance index, four associated with GY in drought condition, and other four associated with GY in irrigated conditions each. Although each of these markers explained less than 0.1% of the phenotypic variation for the index and GY, we found two genes likely related to the plant response to drought stress. For these markers, alleles from landraces provide a slightly higher yield under drought conditions. Our results indicate that the positive diversity delivered by landraces are still present on the backcrosses and this is a potential breeding strategy for improving maize for drought tolerance and for trait introgression bringing new superior allelic diversity from landraces to breeding populations.
One use of soybean [Glycine max (L.) Merr.] as food is as a vegetable, also known as edamame, in which the green pods with seeds fully developed are harvested, cooked briefly, and consumed. Few efforts have been devoted to developing new soybean cultivars with high agronomic performance and characteristics suitable for human consumption. In the present research, crosses between six soybean lines were performed in a diallel scheme without reciprocals to determine the combining ability and the type of gene action that controls the inheritance of development and yield component traits for edamame production. The results showed the existence of variability among the evaluated genotypes and indicated that the progenies presented superior performance in comparison to the parental lines for important traits. The cultivar BRS267 and the inbred line USP-13-19-007 had consistently high estimates of general combining ability for pod and seed size, indicating their potential as parents in breeding for edamame. Most progenies had positive specific combining ability for pod and seed size, showing that generating novel genotypic combinations through new crossings is a promising strategy for improvement. Additive effects played a major role in the inheritance for maturity traits and for pod size, seed size, and yield; both additive and nonadditive effects were important. The magnitude and significance of the correlations indicated that it is possible to improve vegetable soybean traits without affecting grain yield.
Association mapping to exploit maize diversity for drought tolerance: landraces and early testcross as genetic resources Maize (Zea mayz L.) production worldwide have been facing a tremendous obstacle in the past years, drought events are increasing in frequency and severity, mainly in tropical and subtropical regions. Moreover, climate change forecasts this as a trend for the next few years as well. The maize production can be highly affected by water deficiency stress, resulting in losses in grain yield. Mexico is the center of origin of maize, and there are many diversity centers across Latin America. This diversity should be exploited by breeding programs once it is a source of new alleles that can be responsible for the needed genetic improvements to face the forecasting challenges. This doctoral thesis addresses the theme in two sections. First, we present a review of maize genetic diversity, the use of landraces introduction in breeding programs to improve for drought tolerance, highlighting the importance to develop improved tropical germplasm to face the drought issue. In this review we also discuss the opportunity to apply Genome-Wide Associations Studies (GWAS) and Marker-Assisted Selection (MAS) in this context. In the second section we present an original GWAS application in a pre-breeding program using selected landraces as genetic sources for drought tolerance maize improvement. The aim was to study the genetic resources of a landrace panel to identify maize chromosomal regions associated with drought tolerance. For that, we performed the GWAS in 1306 landraces progenies originated from 20 landraces populations selected due to its agricultural performance in dry regions of Latin America. Phenotypic data were obtained from early testcross trials of two generations (BC1S1 in 2016 and BCS2 in 2017) conducted in two water regimes, irrigated and drought condition, in three locations in Mexico. Harmonic Mean of Relative Performance (HMRP) of grain yield in both water regimes was used as a measure of drought tolerance of the genotypes. The genotypic values were estimated using a spatial adjustment in a two-stage analysis. A final set of 5,695 single-nucleotide polymorphism (SNPs) markers was considered for GWAS. We were able to detect a total of 10 significant markers associated with grain yield and drought tolerance index, and we suggest two putative genes mapped close to two of these markers that can be part of the plant's response to drought stress. Besides, for two associated SNPs, the alleles from landraces provided a slightly higher yield under drought conditions. Our results indicate that the diversity delivered by these landraces combined with commercial lines is an exciting strategy to improve maize for drought tolerance.
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