Morpho-phenological diversity among Tunisian natural populations of Brachypodium hybridum

Brachypodium hybridum belongs to the Poaceae grass subfamily. It has a close genetic relationship with temperate cereal crops, which means that it can be used as a model for temperate cereal and grass crops. In order to improve knowledge on the genetic diversity of this species, 145 lines of B. hybridum representative of nine populations and all the ecoregions of Tunisia were characterized on the basis of 18 morpho-phenologic features. The results show a considerable variation between populations and ecoregions in all traits studied. Variation was relatively higher for reproductive than vegetative traits. The majority of traits showed very low to high heritability with low border value for average length of spikelet (ALS) and an average value of 0·64. It is noticeable that high values of heritability were observed for most vegetative descriptors, with low values for reproductive ones. Differentiation between populations (Q<inf>ST</inf>) varied from 0·02 for ALS to 0·78 for average length of leaves with a mean value across traits of 0·4, which confirms the wide intra-population variation in Tunisian natural population of B. hybridum. Pairwise Q<inf>ST</inf> showed that the greatest differentiation among populations was registered between Ain Drahem and Jbel Zaghouan and the smallest between Haouria and Raoued. Overall, the Ain Draham population showed the largest differentiation from the rest of the populations. To infer the effect of geographic distribution of the species, a Mantel test was applied between observed pairwise differentiation and geographic distance between populations and between ecoregions: the results show a positive, but not significant, relationship. In addition a significant negative relationship was found between phenotypic diversity and altitude, indicating that genetic diversity decreased with increasing altitude. Taken together, the high levels of intra-population variation and the lack of correlation between genetic differentiation and geographic distribution suggest a potentially important rate of long-distance seed dispersal and confirm the role played by natural selection in the population structure of Tunisian natural populations of B. hybridum.