Population genetic structure of Pyrenophora teres Drechs. the causal agent of net blotch in Sardinian landraces of barley (Hordeum vulgare L.)

Monoconidial cultures of Pyrenophora teres, the causal agent of barley net blotch, were isolated from leaves collected from six populations of the barley landrace "S'orgiu sardu" growing in five agro-ecological areas of Sardinia, Italy, and genotyped using AFLPs. The 150 isolates were from lesions of either the "net form" (P. teres f. sp. teres) or the "spot form" (P. teres f. sp. maculata) of the disease. Of 121 AFLP markers, 42%, were polymorphic. Cluster analysis resolved the isolates into two strongly divergent groups (F-ST = 0.79), corresponding to the net (45% of the isolates) and the spot (55% of the isolates) forms (designated the NFR and SFR groups, respectively). The absence of intermediate genotypes and the low number of shared markers between the two groups indicated that hybridization between the two formae is rare or absent under the field condition of Sardinia. Five of the barley populations hosted both forms but in different proportions. The SFR populations were similar in overall polymorphism to the NFR populations. However, compared to the SFR form, the NFR occurred in all fields sampled and showed a higher population divergence (F-ST = 0.43 versus F-ST = 0.09 with all isolates; F-ST = 0.37 versus F-ST = 0.06 with clone corrected samples) probably due to a lower migration rate. AFLP fingerprints resolved 117 distinct genotypes among the 150 isolates sampled (78%), 87% in SFR and 68% in NFR isolates. Although the absolute numbers may be a function of the number of AFLP markers assayed, the relative difference suggests that clonality is more prevalent among the NFR isolates (with 11 of 46 haplotypes observed more than once), compared with SFR isolates (7 of 71 haplotypes). Both digenic and multilocus linkage disequilibrium analyses suggested that sexual reproduction occurs at significant levels within the NFR and SFR populations, and that the relative contribution of sexual and asexual reproduction varies among different environments.