INTROGRESSION OF QUANTITATIVE TRAIT LOCI FOR STRIGA RESISTANCE INTO SORGHUM VARIETIES ADAPTED IN NIGERIA
ABSTRACT
Striga hermonthica, is a major constraint to sorghum production in the Northern region of Nigeria causing high yield losses due to the increasing infestation. The introgression of resistance genes into locally adapted cultivars could be a cost effective control strategy for Striga. A series of studies were conducted with the overall goal of developing new sorghum genotypes that are resistant to Striga and also possessing farmers’ desired qualities. First, molecular characterization using SNP markers was conducted on 157 accessions to determine the genetic diversity, genetic differentiation and the level of homozygosity of the accessions. Subsequently, forty different sorghum accessions were evaluated to determine phenotypic and genotypic variability for Striga resistance and to detect appropriate parental lines to be used in breeding for new Striga resistant sorghum genotypes. Third, a marker assisted backcrossing program was initiated to introgress Striga resistance QTLs from two resistant genotypes into three farmer-preferred varieties using SSR markers. There were significant levels of genetic variation within the germplasm as revealed by the gene diversity of 0.29. Polymorphism information content (PIC) value was 0.25 across the entire accessions which also indicates a moderate diversity. Variability among the populations was low at 6% but amounted to 94% within populations. Clustering analysis based on the genetic distance matrix, grouped the 157 sorghum accessions into 3 distinct clusters. Information obtained from this study could be used to select parents for the development of hybrids, to get the best of yield and also to generate segregating populations for identification of genes influencing specific quantitative traits of economic importance in sorghum. The identification of Striga resistant lines in sorghum germplasm through field evaluation of forty accessions and the use of the area under Striga number progress curve (AUSNPC) as the best discriminative measure for Striga resistance, revealed significant genotypic variability among the screened accessions. Both phenotypic and genotypic components influenced the variation observed in the sorghum accessions with respect to Striga resistance and agronomic performance. The values for genetic coefficient of variation, heritability and genetic advance obtained in this study showed that genetic gain for resistance to Striga could be realized through selection centered on the individual Striga counts, the AUSNPC and also the area under Striga severity progress curve (AUSVPC). The sorghum accessions N13, CSR-02, Sepon82, MaceDaKunya and SRN39 were found to be resistant to Striga hermonthica based on the low AUSNPC and AUSVPC values. They are, therefore, recommended as good sources for the introgression of Striga resistance genes in Nigeria. Marker Assisted Backcrossing (MABC) was successfully used to introgress Striga resistance from the resistant genotype, N13, into important genetic backgrounds (SAMSORG39 and DANYANA) and also from the resistant genotype SRN39 into the background of SAMSORG14. Backcross populations BC1F1 and BC2F1 were generated and genotyped using Simple Sequence Repeat (SSR) markers. Forty two DANYANA-N13 BC2F1 progenies (4 QTLs in 3 progenies, 3 QTLs in 10 progenies and the other 28 with 1 to 2 QTLs) have been selected for the presence of the N13 QTLs. Forty three SAMSORG39-N13 BC2F1 progenies (3 QTLs in 2 progenies while the other 41 with 1 to 2 QTLs) were also selected for the presence of the N13 QTLs. Also, 54 SAMSORG14-SRN39 BC2F1 progenies were heterozygous at two flanking markers for the lgs gene. These selected lines need to be screened with SSR markers for the recovery of the recurrent parent background and then advanced to the next generation for the introgression of more QTLs. Following this, field evaluation needs to be conducted at multiple locations to identify elite genotypes for possible release as varieties. These selected lines could be used to improve farmers’ sorghum production thereby contributing to food security status in Nigeria.