Characterization of Ascochyta rabiei for population structure, mating type and pathogenic variability from Pakistan and United States

Abstract

Chickpea production is greatly hampered by blight causing fungal pathogen Ascochyta rabiei (AR) in chickpea growing regions of the world. Genetic variability and mating type frequency of thirty two AR isolates from six geographical regions of Pakistan were compared with a US-AR population. 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 and the US-population showed a 1:1 distribution. The results showed that sexual reproduction is rare in PAR due to either unavailability of both mating types or lack of conductive environment but statistical analysis showed panmixia which may be due to past recombinational events. Genetic variation at six microsatellite loci was assessed and each isolate was assigned to a microsatellite haplotype. Population structure using Bayesian analyses differentiated isolates into three distinct clusters, two clusters of PAR and one of the US isolates. However, few isolates from US shared same genetic background with one cluster of the PAR isolates, providing a link of inter-continental migration of the pathogen due to import of seeds. Additionally, the two clusters of Pak-isolates are not strictly linked to the geographic locations in Pakistan, suggesting frequent gene flow of AR among different locations. Pathogenic variability of nineteen PAR collected from two different provinces was assessed. The results based on the reaction of isolates with differential lines showed that aggressive and highly aggressive pathotypes II and III respectively are prevalent in Pakistan as compared to least aggressive pathotype I. It is interesting to note that highly aggressive pathotypes III and IV have only beenreported from Syria and Pakistan where we assume less frequency of sexual reproduction due to predominance of one mating type, in contrast to other countries where both mating types are present in equal ratio hence, this issue needs further investigations.