The Search for Molecular Markers, Parental Characterisation and inheritance Studies of Witchweed [Striga Asiatica (L.) Kuntze] Resistance in Sorghum [Sorghum Bicolor (L.) Moench].
Mutengwa, Charles Shelton
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Sorghum [Sorghum bicolor (L.) Moench] is ranked the third most important cereal crop in Zimbabwe, after maize and wheat. The major biotic constraint to sorghum production by resource poor farmers (RPFs) is attack by the parasitic weed Striga asiatica (L) Kuntze, or witchweed. Striga asiatica resistant sorghum cultivars could be a major component of integrated witchweed management, if resistance was available in adapted and productive germplasm. The objectives of this work were to; characterize available sorghum cultivars for resistance to witchweeds, study the inheritance of low S. asiatica seed germination stimulant production and identify molecular markers that are linked to the genes for S. asiatica resistance. Crosses were made between witchweed resistant (SAR 16, SAR 19 and SAR 29) and susceptible (SV-1) cultivars in a half-diallel arrangement. The F1s were selfed to generate F2 generation progeny. Parental lines and F2 progeny were screened for S. asiatica resistance using the pot culture and agar gel techniques. Combining ability analysis for witchweed counts and path coefficient analysis of sorghum grain yield and its components were conducted for parent materials that were grown under S. asiatica infestation in pots. The inheritance of low S. asiatica germination stimulant production was evaluated using seedlings of F2 progeny that were screened in water agar and using petri dishes. Parental and F2 genotypes were transferred from petri dishes into pots filled with clay. Deoxyribonucleic acid (DNA) was then extracted from the potted sorghum seedlings after two weeks for molecular marker analysis using random amplified polymorphic DNA (RAPD) and microsatellite or simple sequence repeat (SSR) markers. A total of 440 RAPD, 24 sorghum SSRs and six maize SSRs were used to screen SV-1 and SAR 29 for polymorphisms. Linkage analysis was conducted using the software Mapmaker/exp 3.0b. Cultivars SV-1 and SAR 16 were susceptible, while SARs 19 and 29 were resistant to witchweeds. Combining ability analysis revealed that GCA components of genotypic variance were significant. Additive genetic factors were therefore important in determining the response of a cultivar to witchweed infestation. Cultivars SAR 19 and SAR 29 were good general combiners for low S. asiatica counts. These resistant cultivars reduced the number of parasite counts in their F2 progeny, though this was more conspicuous for SAR 19 whose negative GCA effects were significantly different from zero. Cultivars SV-1 and SAR 16 had positive and highly significant GCA effects. These cultivars therefore increased parasite counts among progeny from crosses that involved them. Grain yield components that were important for the cultivars tested were head weight, 100 seed weight, plant height and days to 50 % flowering. However, the direct and indirect contribution of each of these parameters to yield was influenced by the type of cultivar (resistant or susceptible) and whether there was witchweed infestation or not. In general, head weight was the most important sorghum grain yield determinant, having moderate to high direct contributions. Direct effects of S. asiatica counts on sorghum grain yield were low. Striga asiatica indirectly caused yield reduction by affecting sorghum grain yield components, mostly head weight. A single recessive gene controlled low S. asiatica seed germination stimulant production in sorghum genotypes SAR 19 and SAR 29. A total of 199 markers (187 RAPDs; 10 sorghum SSRs and 2 maize SSRs) were polymorphic between cultivars SAR 29 and SV-1. Molecular markers that are linked to the gene(s) for low S. asiatica seed germination stimulant production could not be identified. Instead, a molecular marker linkage map was constructed and it consisted of 45 markers that were distributed over 13 linkage groups (LGs). The LGs consisted of 2 to 8 markers that were identified at a LOD grouping threshold of 4.0. The map spanned a total distance of 494.5 cM. Cultivars SAR 19 and SAR 29 are good sources of genes for resistance to witchweeds since they had negative GCA effects, which enabled them to reduce witchweed counts in progeny derived from them. Specifically, these resistant cultivars are a good source of the low S. asiatica seed germination stimulant trait. However, sustainable use of S.asiatica resistance can be achieved through pyramiding different mechanisms of resistance. This should be combined with the use an integrated Striga management package involving host-plant resistance and other appropriate technologies, to provide a long-term and effective way of combating witchweeds. Field screening for witchweed resistance requires independent and concurrent selection for low Striga counts and high yield under S. asiatica infested conditions. Improvement in sorghum grain yield can be primarily based upon selection for improved head weight, though 100 seed weight, plant height and days to 50 % flowering should also form part of the selection criteria. The molecular linkage map that was constructed in this investigation can be useful for practical plant breeding purposes, since the polymorphisms that were identified are within the cultivated gene pool of sorghum.