POPULATION STRUCTURE AND MATING TYPE DISTRIBUTION OF THE CHICKPEA BLIGHT PATHOGEN ASCOCHYTA RABIEI FROM PAKISTAN AND THE UNITED STATES
H. Ali, S.S. Alam, R.N. Attanayake, M. Rahman, W. Chen
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 were compared with a US-AR population for mating type frequency and genetic variation. Mating type results showed that the Pakistani AR (PAR) population had an apparent skewed (3 Mat1-2: 1 Mat1-1) distribution, although Chi-square tests showed non-significant deviation from equal distribution due to small sample sizes. The US population showed a 1:1 distribution of the two mating types. The uneven distribution of mating types indicates that sexual reproduction among the PAR is rare due to either unavailability of both mating types or lack of conducive environment, but statistical analysis showed that panmixia is there reflecting past recombinational events. 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 of PAR and one of the US isolates. However, few isolates from the US shared the same genetic background with one cluster of the PAR isolates, providing a link of inter-continental migration of the pathogen. Additionally, the two clusters of PAR-isolates are not strictly associated with geographic locations in Pakistan, suggesting frequent gene flow of AR among different locations. Future studies should extend the sampling of representative populations to overcome the limitations of the small sample size for more accurate assessment of population structure.