Genetic studies and a search for molecular markers that are linked to Striga asiatica resistance in sorghum

Abstract

Sorghum [Sorghum bicolor (L.) Moench] is important both as a food and feed crop in Zimbabwe. Its yield losses can be up to 100% when the crop is heavily infested by witchweeds [Striga asiatica (L.) Kuntze]. Witchweed resistant cultivars offer the most practical control option under smallholder (SH) farmer conditions and could become part of a sustainable integrated control strategy. Development of S. asiatica resistant cultivars by conventional breeding is slow and has been hampered by the lack of efficient and reliable screening techniques in breeding programs. Molecular markers that are linked to witchweed resistance can expedite the development of resistant cultivars through adoption of appropriate markerassisted selection (MAS) strategies. The objectives of this investigation were to study the inheritance or low germination stimulant (lgs) production in cultivar SAR 29 and to identify molecular markers that are linked to this trait. Low germination stimulant production is one of the recognised mechanisms of witchweed resistance. A segregating F2 population derived from crosses between cultivars SV-1 (high germination stimulant producer, Striga-susceptible) and SAR 29 (low germination stimulant producer, Striga-resistant) was used for this purpose. Parental and F2 genotypes were screened for lgs production using the agar gel technique (AGT). Maximum germination distance (MGD) was used as the index of
resistance. Deoxyribonucleic acid was extracted from agar gel-screened F2s, and DNA bulks were createdfrom 16 resistant (MGD<10 mm) and 25  extremely susceptible (MGD>25 mm) progeny. Bulked segregant
analysis (BSA) was done using random amplified polymorphic DNA (RAPD) and simple sequence repeat (SSR) markers. Ninety-nine of the primers that were polymorphic between parent genotypes (10 SSR and 89 RAPD) were then used to screen a total of 77 segregating F2 progeny. Linkage analysis was performed using the computer software MAPMAKER 3.0b. Segregation ratios of high to low F2 stimulant producers did not differ significantly (P³0.05) from the expected ratio of 3:1. It was therefore deduced that a single recessive gene controlled lgs production in cultivar SAR 29. No molecular marker was found to be linked to the lgs locus. Instead, linkage analysis resulted in the construction of a molecular marker linkage map
consisting of 45 markers that were distributed over 13 linkage groups (LGs). The other fifty-four loci, including the locus for lgs production, were completely unlinked and could not be assigned to any linkage group. The LGs consisted of 2-8 markers, identified at a LOD grouping threshold of 4.0. The map spanned a total distance of 494.5 cM, Haldane.