Distribution of Mating Types and Genetic Diversity of Ascochyta rabiei Populations in Tunisia Revealed by Mating-type-specific PCR and Random Amplified Polymorphic DNA Markers

Abstract

The distribution of mating types of Ascochyta rabiei (teleomorph: Didymella rabiei) was determined in Tunisia using a MAT-specific PCR assay. Among 123 isolates tested, 80% were MAT1-1 and 20%MAT1-2. Only MAT1-1 isolates were present in the Beja and Bizerte regions of Tunisia, whereas both mating types were present in Nabeul, Kef and Jendouba. In the latter three regions, the hypothesis of random mating could not be rejected based on chi-squared tests of mating-type ratios (P > 0.05). The lower frequency of the MAT1-2 coupled with the restricted distribution of this mating type in Tunisia may indicate a recent introduction of MAT1-2 in Tunisia. This speculation is consistent with the recent (2001) observation of D. rabiei pseudothecia on chickpea debris in Tunisia. Forty isolates representative of the five regions were genetically analysed using 10 random amplified polymorphic DNA (RAPD) primers to provide a preliminary estimate of genetic diversity of the pathogen in Tunisia. Among 129 putative RAPD loci amplified, 81% were polymorphic and 32 unique RAPD fingerprints were detected. A high level of genetic differentiation was detected among subpopulations (GST = 0.33). Cluster analyses revealed that isolates from Bizerte, Beja and Jendouba were genetically similar and distinct from isolates sampled in Nabeul and Kef. MAT1-1 isolates were clustered separately from MAT1-2 isolates in Jendouba and Nabeul suggesting that recombination may not yet be occurring in these regions despite the occurrence of both mating types in equal frequency in these regions. This lack of recombination between MAT1-1 and MAT1-2 also supports the hypothesis of a recent introduction of MAT1-2 into Tunisia.