Bogotá - Ciencias Agrarias - Doctorado en Ciencias Agrarias · 2016

Genetic and molecular analysis of the inmmunity to cassava bacterial blight through genetic mapping and RNA-seq approaches

Cassava is one of the most important crops world-wide. One of the diseases compromising its production is the cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam). The best way to control this disease is growing resistant varieties obtained through traditional breeding or by genetic transformation. Nevertheless, currently there are no reports of the cloning of immunity related gene to CBB. One important step toward the isolation of genes is the construction of high dense genetic maps allowing the positional cloning. Here we present the development of one the highest dense genetic maps of cassava. This map was obtained through the use of the Genotyping by Sequencing (GBS) approach allowing the generation of thousands of Single Nucleotide Polymorphisms (SNPs). These SNPs were evaluated in the F1 segregating progeny resulted from cross between TMS30572 and CM2177-32. A total of 78,854 SNPs were identified covering 87% (463.2 Mb) of the cassava genome. The total set of SNPs was evaluated for mapping parameters and high quality SNPs were selected to construct the linkage map. The map covered 2,571 cM distributed in 18 linkage groups and includes 2,141 SNPs with an average distance of 1.26 cM between markers. This map was used to perform QTL (Quantitative Trait Loci) detection for CBB resistance. The F1 mapping population was tested for resistance to two Xam strains (Xam318 and Xam681) at two locations in Colombia: La Vega (Cundinamarca) and Arauca (Arauca). The evaluation was conducted during rainy and dry seasons. A third evaluation was conducted on greenhouse conditions. Additionally, the population was evaluated under natural infection conditions at Puerto López (Meta) during a rainy season. Through QTL mapping, 18 strain-specific QTLs were detected, explaining between 10.9 and 22.1% of the phenotypic variance. From these QTL, nine showed stability between the evaluated seasons. A significant QTL x Environment interaction was detected for ten QTL. Within the QTL intervals were described a repertoire of 151 CBB candidate defense-related genes (CDRGs), from which thirteen correspond to genes coding for proteins containing domains representative of the immunity proteins. Four CDRGs show differentially expression during Xam681 infection in the resistant parental TMS30572. The repertoire of CDRGs co-localizing with the QTL reported here, represents a source of novel genomic regions involved in CBB resistance to be explored and validated for its future use into cassava breeding programs.

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