Résumé
Previous studies with molecular markers (ISSR, RAPD, SCAR, AFLP and SSR) have shown that most of the genetic diversity of cultivated Citrus (except C. aurantifolia) comes from the recombination between three main species: C. medica (citron), C. reticulata (mandarin) and C. maxima (pummelo). However, the precise contribution of these basic species to the genome constitution of secondary species (C. sinensis, C. limon, C. aurantium, C. paradisi) and recent hybrids is not known. In this study, 58 nuclear and four mitochondrial markers were used to investigate the genetic diversity among 106 Citrus accessions, representing the three main ancestors groups, secondary species and several hybrids from the 20th century breeding programs. For the nuclear analysis, 50 simple sequence repeats (SSRs) developed from genomic libraries and ESTs databases were used. Moreover, 10 Insertion-Deletion (INDEL) markers were developed from genomic sequences of some primary and secondary metabolites determining the citrus fruit quality (sugars, acids, flavonoids and carotenoids). All the SSR markers and one INDEL are included in a consensus genetic map of clementine and pummelo Chandler and are distributed along the nine linkage groups, representing positively the global genome of Citrus. Genetic diversity statistics were calculated for each SSR and INDEL marker, within the entire population and within and between the different specified Citrus groups. The organizations of the genetic diversity among all the accessions were determined by constructing neighbor-joining trees for the different sets of primers. INDEL markers are less polymorphic than SSRs, display a higher organization of genetic diversity and appear to be better phylogenetic markers to trace the contribution of the three ancestral species. Population structure was studied using the Structure software, version 2.2.3, (http://cbsuapps.tc.cornell.edu/structure) which implements a model-based clustering method for inferring population structure using genotype data. The relative proportion of ancestral taxa genomes in the secondary species and recent hybrids was assigned. Mitochondrial markers revealed a maternal phylogeny of citrus germplasm accessions in agreement with previous studies with chloroplastic markers. This analysis allowed a better understanding of the genetic diversity organization among citrus cultivars, opening the way for a better management of citrus germplasm banks and breeding programs.(Texte intégral)