Morpho-phenologic vs molecular genetic variation in Tunisian Brachypodium spp populations

Brachypodium distachyon belongs to the Poaceae grass subfamily. It has a close genetic relationship with temperate cereal crops which makes it a model for temperate cereals and grass crop. In order to bring a better knowledge on the genetic diversity of this species, 180 lines of B. distachyon representative of 9 populations and 6 eco-regions of Tunisia were characterized on the basis of 18 morpho-phenologic features and 15 microsatellites. Morphological analysis showed a considerable variation between populations and eco-regions in all studied traits. Variation was relatively higher for reproductive traits (53.2 %) than for vegetative ones (32.8 %). Differentiation between populations (Qst) varied from 0.033 for average length of spikes to 0.56 for heading date with an average of 0.27 which confirm the wide intra-population variation in Tunisian natural populations of B. distachyon. At the DNA level, we show that variation patterns was strongly higher within populations than among populations (98% vs 2%; Fst = 0.021). In addition, almost all characters displayed a significant higher Qst than Fst, indicating occurrence of phenotypic plasticity and local adaptation. The latter is explicable as there is no reason to expect that natural selection would affect in similar fashion all traits and affect all populations at a similar level. We also found a non significant correlation between genetic variation in molecular marker loci and quantitative traits at the multitrait scale (r = 0.016; P = 0.926). This finding attests that the evolution of these two markers in B. distachyon was not paralleled.