Evidence of recombination between net- and spot-type populations of Pyrenophora teres as determined by RAPD analysis
The genetic structure of Pyrenophora teres, the causal agent of net blotch of barley, was examined in two fields 30 km apart in the south-western Cape of South Africa. The two fields respectively represented a net- and spot-type population, the two types being distinguished on the basis of symptom expression on differentially susceptible cultivars. The number of isolates sampled from each field was 36 for the net-type population and 29 for the spot-type population. Samples were collected from infected barley leaves from two separate quadrants in each field, the two quadrants positioned in corners of the fields, diagonal to one another. Of the 40 10-mer random oligonucleotide primers screened, five produced scorable, reproducible DNA bands suitable for the determination of population structure. A total of 65 loci were produced of which 54 were polymorphic. Genetic analysis of bands produced by one of the primers has revealed single locus segregation in a mating between a net- and spot-type isolate, indicating that RAPD bands can be interpreted as alleles at genetic loci. Total gene diversities determined for all loci resulted in mean indices of 0.063 and 0.082 being obtained respectively for the net- and spot-type populations. Genetic diversity among the two populations was divided into within- (variation between sampling quadrants) and among population components using Nei's GST. A coefficient of genetic differentiation (GS) of 0.0149 was obtained between quadrants within populations while a coefficient (GT) of 0.63 was obtained between the two populations. Genotypic variation revealed 13 distinct multilocus genotypes (haplotypes) in the net-type population while there were 12 in the spot-type population. UPGMA cluster analysis of the two populations together with six progeny from a mating between a net- and spot-type isolate resulted in three main clusters being produced, one for each population and one for the progeny. One isolate collected from the net-type population that did not cluster with the other net-type isolates clustered directly next to the cluster containing the sexual progeny. This isolate also contained a unique spot-type DNA band. This suggested that sexual recombination may be occurring between net- and spot-type isolates under field conditions.