Genetic variability, host specificity and resistance in Striga Asiatica-host plant interactions

Abstract

Genetic analyses were conducted on Striga asiatica populations collected from different host species and geographic areas throughout Zimbabwe. These collections had been suspected to differ in their virulence, leading to the breakdown of resistance among resistant sorghum cultivars. Discrete genetic markers analysed in this study included isozymes and arbitrary regions of the genome amplified by the polymerase chain reaction, commonly known as random amplified polymorphic DNA (RAPD). In all, twelve enzyme systems were evaluated; five of these producing well resolved bands and therefore being used for analysis. Polymorphisms were detected at 5 loci; glutamate oxaloacetate-transaminase (GOT), á -esterases (á-EST), leucine amino peptidase (LAP), glucose-6-phosphodehydrogenase (G-6-PDH) and peroxidases (PER). Isozyme variation was absent between populations collected from the same geographical areas but was present between populations from different regions. Much greater genetic variation was detected when populations were analysed with RAPD. Populations were analysed with 32 polymorphic bands amplified by five primers. Cluster analysis indicated strong similarities between populations from the same regions. There was a positive correlation between geographic and genetic distances.

Inter-host specificity of the different strains of Striga asiatica was also tested. Using a reciprocal cross-infection experiment, four host species (maize, pearl-millet, finger millet and sorghum) were exposed to Striga from the same hosts. The results showed evidence of host specialization as the most severe yield reductions in the respective cereal hosts were caused by Striga originating from the same host species. In maize, the Striga strain from a maize host caused the greatest yield reduction of 42.5% of the total dry weight and 80.6% of the head weight. In finger millet the strain from the same host caused the greatest grain yield reduction of 59 % as compared to the uninfested plots. There was the same trend in sorghum where the strain from the same host was the most virulent.

Intra-host specificity was investigated using sorghum as the host. Fifteen sorghum varieties were exposed to ten populations of Striga using the root exudate technique. There was significant interaction (P<0.05) between Striga population and sorghum variety. Populations SAR16 x SAR19 and SAR19 x SV-1 had stable resistance across all the Striga populations whilst the rest showed different reactions to different populations.

The major findings of this study are that there are physiological strains of the parasitic weed S. asiatica, which are adapted to specific hosts within and inter species. Another finding was that molecular techniques such as RAPDs and isozymes can be effectively used to differentiate strains of the parasitic weed, S asiatica however there is need to determine the relationship between molecular differences and differential virulence.