AbstractsBiology & Animal Science

Peronospora parasitica (Pers. ex. Fr.) Fr., pathotype, single spore isolate, Brassica spp., downy mildew, internally transcribed spacers, ITSI , 5.8S, ITS2 sequence, radial phylogram, phylogeny, peR, micro satellite markers, SSRs, DNA library, microsatellite primer sequences, allele, geographic and pathotype diversity, polyploidy The downy mildew disease caused by Peronospora parasitica (Pers. ex. Fr.) Fr. infects approximately lOO plant species in the family Cruciferae. It is primarily a foliage blight which causes 60-70% seedling losses to nurserymen and rural subsistence farmers in Kwazulu-Natal, South Africa who depend on this crop to maintain a healthy cash flow not provided by other products. The disease is also frequent in India, France, Portugal and in the UK. The pathogen is especially debilitating in winter when low temperatures and high humidity favour infection and spread of the disease. Control of the disease with the use of fungicides is inefficient since P. parasitica has developed resistance to metalaxyl-based fungicides. There is a need to establish patterns of genetic diversity in order to address the ambiguities surrounding species definition of P. parasitica. The present study focused on clarifying the relationships between different pathotypes of P. parasitica found on various Brassica species viz. B. oieracea, B. juncea, B. napus, B. rapa, as well as the Arabidopsis thaiiana pathotype, a wild host of P. parasitica. Co-evolution with plant hosts over long periods was suggested to have lead to the divergent forms of this pathogen adapted to different host taxa. Genetic analysis of host specificity was investigated, based on sequence analysis of intergenic spacer regions of ribosomal DNA and on micro satellite markers. The current study provides the first comprehensive ITS-based phylogeny of pathotypes of P. parasitica. Based on ITSl, 5.8S and ITS2 sequences all pathotypes of P. parasitica from Brassica species (viz. Brassica oieracea, Brassica napus, Brassica juncea and Brassica rapa) were monophyletic. Based on ITSl sequences, the pathotype of P. parasitica from the wild host, Arabidopsis thaliana, was found to be significantly different from the Brassica pathotypes (i.e. cultivated host pathotypes). Furthermore the genetic distance between the genus Peronospora and Phytophthora was closer compared to other taxa such as Pythium, supporting the observation that the downy mildews have derived from a Phytophthora ancestor. rDNA sequence analysis was unable to differentiate Brassica pathotypes of P. parasitica. The development of a powerful class of genetic markers known as microsatellites (SSRs) in the present study provided greater insight into the relationships between Brassica pathotypes of P. parasitica. A genomic DNA library highly enriched for various micro satellites was prepared and a large number of potential SSRs consisting primarily of dinuceotide repeats (CA)n and (CT)n were obtained. Sequence analysis of 351 clones yielded 120 clones containing SSR loci, and 29 (24%) potentially useful…