Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2011
Publication Date: 4/10/2012
Citation: Ali, H., Alam, S., Attanayake, R., Rahman, M., Chen, W. 2012. Population structure and mating type distribution of the chickpea blight pathogen Ascochyta rabiei from Pakistan and United States. Journal of Plant Pathology. 94(1):99-108. Interpretive Summary: Ascochyta blight is a serious disease of chickpea in Pakistan, USA and elsewhere, and the disease is caused by the fungal pathogen Ascochyta rabiei. Despite extensive research on this pathogen, the population structure of the pathogen in Pakistan is unknown. This study compared 32 isolates of Ascochyta rabiei representing six geographic areas in Pakistan with a population of the pathogen from the US. Significant difference in mating type distribution were found between the two countries. There was a skewed (3 Mat1-2 : 1 Mat1-1) distribution in the Pakistani population, but an even 1:1 distribution in the US. The uneven distribution of mating types indicates that sexual reproduction among the Pak-isolates is likely rare or not occurring due to either unavailability of both mating types or lack of conducive environment. Genetic variation of the isolates was assessed at six microsatellite loci and each isolate was assigned to a microsatellite haplotype. Genetic structure analyses differentiated isolates into three distinct clusters, two clusters in Pakistan and one in the US. The two clusters of Pakistani isolates are not related to the geographic locations in Pakistan. This suggests frequent movement of isolates among different locations in Pakistan
Technical Abstract: Ascochyta blight caused by the fungus Ascochyta rabiei (AR) depresses chickpea production in Pakistan and worldwide. Thirty two AR isolates representing six geographical regions of Pakistan was compared with a US AR population for frequency of mating types and genetic variation. Mating type results showed that the Pak-population had a skewed (3 Mat1-2 : 1 Mat1-1) distribution, whereas the US-population showed a 1:1 distribution. The uneven distribution of mating types indicates that sexual reproduction among the Pak-isolates is rare or not occurring due to either unavailability of both mating types or lack of conducive environment. Genetic variation at six microsatellite loci was assessed and each isolate was assigned to a microsatellite haplotype. Population structure of the isolates was inferred using Bayesian analyses implemented in the Structure software, which differentiated isolates into three distinct clusters, two clusters in Pakistan and one in the US. However a few isolates from the US shared the same genetic background with one cluster of the Pak-isolates, providing a link of inter-continental migration of the pathogen. Additionally, the two clusters of Pakistani isolates are not strictly linked to the geographic locations in Pakistan, suggesting frequent gene flow of AR among different locations.