Analysis of the genetic diversity of Tunisian natural populations of Brachypodium

Over the last decade, Tunisia has been invaded by recurrent drought periods which caused an extension of desertification along the North-South axis. Accompanied with various anthropogenic factors, the low and erratic rainfall results in frequent periods of serious drought alternating with periods of floods have caused major damages in the natural forage resources. Face to this situation, The Tunisian Government has recently engaged a vast program for the management, conservation and mobilization of natural genetic resources: national strategies for soil and water conservation, water resources, forage resources and rangelands rehabilitation. Because of its great ecological, agronomic and economic properties, the genus Brachypodium, has been developed as a model for temperate cereals, biofuel and grass crop. In order to define a strategy for improving and managing genetic resources of Brachypodium species in Tunisia, we have characterized the morpho-phenologic and the genetic variation of 180 lines of the complex of species B. distachyon, B. stacei and B. hybridum through 18 quantitatives traits and a large set of SSR and InDels markers. The first results of our analysis showed that the species allotetraploid B. hybridum is the predominant species in Tunisia, while the two diploid species B. distachyon and B.stacei are less frequent. The analysis of morpho-genetic diversity of this complex of species by the 18 morpho-phenologic traits and 15 SSR markers showed that the morphogenetic variation between species dominate that within species. However, the allotetraploid species was found to be more morphologically and genetically variable its two parental species. The detailed analysis of the morpho-phenologic diversity of the Tunisian B. hybridum germplasm (9 natural populations) showed a considerable variation between populations and eco-regions in all studied traits Variation was relatively higher for reproductive (34.8 %) than for vegetative (32.8 %) traits. The majority of traits showed very low (<0.3) to high (> 0.3) heritability with low border value for average length of spikelet and an average value of 0.64. Analysis of the morpho-phenologic between populations revealed that populations originated from Ain Drahem and Jbel Zaghouan (0.61) are the most differenciated while those Haouria and Raoued are the most morphologically closes (Qst = 0.1) (Overal Qst = 0.32). Moreover, the results showed also that the morphological differentiation between populations is not affected neither by geographic distances nor altitudinal differences. The genetic variation the Tunisian B. hybridum germplasm analyzed by SSR and InDels markers showed that the studied populations of B. hybrdium are characterized by high genetic variability. The results of SSR analysis revealed 100% of polymorphism and an average of genetic diversity (expected heterozygosity) of 0.79 across populations, while those revealed by InDels markers showed that the studied populations exhibited 90% of polymorphism and 35% of genetic variation. The extensive intra-population genetic variation is accompanied by a recurrent inter-populational gene flow (Nm> 2) promoting a homogeneous intra-population genetic variation a relatively low inter-populations genetic differentiation. Indeed, the analysis of molecular variance (AMOVA) based on SSR and Indels data revealed that most of genetic variation was attributed to the within populations component. The level of genetic differentiation among populations was doesn’t exceeds 2% for SSR markers (PT = 0.023) and 20% for InDels markers (PT = 0.196). Moreover, the spatial genetic structure established through Cluster analysis (UPGMA) principal co-ordinate analysis (PCoA) and Bayesian approach grouped populations from distinct geographic area and altitudinal classes, suggesting that neither geographical distances nor altitudinal differences constrain the relationships among the populations. Taken together, the results the morpho-phenologic and molecular approaches suggest that the genetic structure of the Tunisian natural populations of B. hybridum is influenced by both, a gene flow via long seed dispersal and the natural selection.