Population structure and mating system of Ascochyta rabiei in Tunisia: evidence for the recent introduction of mating type 2

Abstract

The population structure of Ascochyta rabiei (teleomorph: Didymella rabiei ) in Tunisia was estimated among five populations sampled from the main chickpea growing regions using simple sequence repeat markers (SSR) and a mating type ( MAT ) marker. Mating type 2 isolates ( MAT1-2 ) had reduced genetic and genotypic diversity relative to mating type 1 isolates ( MAT1-1 ). This result, coupled with previous observations of lower overall frequency and restricted geographical distribution of MAT1-2 in Tunisia, and recent (2001) observation of the sexual stage, support the hypothesis of a recent introduction of MAT1-2 . Despite the presence of both mating types in Nabeul, Kef and Jendouba, the hypothesis of random mating was rejected in these locations with multilocus gametic disequilibrium tests. Highly significant genetic differentiation ( θ = 0·32, G ST = 0·28, P < 0·001) was detected among populations and genetic distance and cluster analyses based on pooled allele frequencies revealed that populations from Nabeul and Kef were distinct from those in Beja, Bizerte and Jendouba. More than 70% of total gene diversity ( H T = 0·55) detected was attributable to variation within populations compared to 28% among populations. This result, coupled with the occurrence of private alleles in each population, suggests that gene flow is currently limited among populations, even those separated by short geographic distances. The presence of two main genetic clusters was confirmed using Bayesian model-based population structure analyses of multilocus genotypes (MLGs) without regard to geographic origin of samples. The presence of MAT1-2 isolates in both clusters suggests at least two independent introductions of MAT1-2 into Tunisia that are likely to be the result of importation and planting of infected chickpea seeds.

Aggressiveness Changes in Populations of Didymella pinodes over Winter and Spring Pea Cropping Seasons

“…Indeed, the H= value in our study ranged from 0.70 to 0.83, whereas it ranged from 0.39 to 0.47 in D. pinodes populations collected from different locations in Algeria (59). Moreover, the H= value was closely similar to those obtained for fungi showing great genetic diversity, such as Ascochyta rabiei (H= ϭ 0.58 [60]), Mycosphaerella fijiensis (H= ϭ 0.58 [61]), or Botrytis cinerea (H= values ranging from 0.401 to 0.518 [62]). This change in the diversity level observed during the growing season reinforced the fact that sexual reproduction was important in the life cycle of D. pinodes, and the predominance of this mode of reproduction seemed to affect the previous population structure.…”

An Update on Genetic Resistance of Chickpea to Ascochyta Blight

“…Sexual reproduction is controlled by a single regulatory locus referred to as a mating-type locus and alternate sequences at the mating-type locus are completely dissimilar and code for different regulatory genes. The presence of opposite mating types (MAT1-1 and MAT1-2) and the teleomorph have been reported from some chickpea-growing regions in the world [24][25][26][27][28][29]. The morphological characteristics of A. rabiei and Phoma medicaginis var.…”

“…rabiei shows a high degree of pathogenic and genetic variability, and AB-resistant chickpea cultivars have become susceptible in some countries [26][27][28]. The presence of a teleomorph (D. rabiei) in the A. rabiei life cycle contributes to variability within the pathogen population, which may generate a new combination of virulence genes and the development of new pathotypes [11].…”

“…Breeding materials at ICARDA are being screened against this new pathotype IV, and so far low levels of resistance have been observed [37,38]. High genetic diversity has also been reported from USA, Tunisia and Canada where popular varieties have become susceptible to new aggressive pathotypes [26][27][28]. Microsatellite markers revealed high levels of polymorphism among isolates from Tunisia, Australia, USA, Turkey, Pakistan, Syria and India [26,27,35,[39][40][41].…”

Ecological Genetic Divergence of the Fungal PathogenDidymella rabieion Sympatric Wild and DomesticatedCicerspp. (Chickpea)

“…Isolates were screened for allelic variation at six sequence-tagged microsatellite (STMS) loci (30), which have been demonstrated to be genetically unlinked (50,52; T. L. Peever, unpublished data). PCR amplifications were performed as described by Peever et al (50) with minor modifications.…”