Genetic diversity revealed by EST-SSR markers in carob tree (Ceratonia siliqua L.)

Carob is a caesalpinoid tree species widespread in the Mediterranean. In this paper, carob EST-SSR (Expressed Sequence Tags-Simple Sequence Repeat) markers were used to assess the level of genetic diversity among 71 cultivars and accessions collected in three Mediterranean countries (Italy, Malta and Spain). Starting from 20 microsatellite loci, we identified a set of 9 polymorphic EST-SSRs, with a number of alleles per locus ranging from 2 to 7, and a number of genotypes from 3 to 13. The nine polymorphic EST-SSRs discriminated most of the analyzed genotypes with the exceptions of some cultivated clones having similar phenotypic features and probably belonging to cultivar-populations, and clarified some cases of homonymy.A neighbor joining dendrogram generated three main clusters which did not show a correlation with the distribution areas of the analyzed genotypes. A Bayesian analysis evidenced a high degree of admixture among the gene pools from the three countries. The low level of diversification among geographical areas reveals that the asexual propagation of selected clones played a major role in the diffusion of the species in the western Mediterranean. The work described in this paper represents the first report of carob genotyping achieved through microsatellite markers.