Analysis of genetic diversity in Tunisian citrus rootstocks

Breeding and selection of new citrus rootstocks are nowadays of the utmost importance in the MediterraneanBasin because the citrus industry faces increasing biotic and abiotic constraints. In Tunisia, citrus contributessignificantly to the national economy, and its extension is favored by natural conditions and economicconsiderations. Sour orange, the most widespread traditional rootstock of the Mediterranean area, is also themain one in Tunisia. In addition to sour orange, other citrus rootstocks well adapted to local environmentalconditions are traditionally used and should be important genetic resources for breeding. Prior to initiationof any breeding program, the exploration of Tunisian citrus rootstock diversity was a priority. Two hundredand one local accessions belonging to four facultative apomictic species (Citrus aurantium, sour orange; Citrussinensis, sweet orange; Citrus limon, lemon; and Citrus aurantifolia, lime) were collected and genotyped using20 nuclear SSR markers and four InDel mitochondrial markers. Sixteen distinct Multi-locus genotypes (MLGs)were identified and compared to references from French and Spanish collections. The differentiation of the fourvarietal groups was well-marked. Each group displayed a relatively high allelic diversity, primarily due to very highheterozygosity. The Tunisian citrus rootstock genetic diversity is predominantly due to high heterozygosity anddifferentiation between the four varietal groups. The phenotypic diversity within the varietal groups has resultedfrom multiple introductions, somatic mutations and rare sexual recombination events. This diversity studyenabled the identification of a core sample of accessions for further physiological and agronomic evaluations. These core accessions will be integrated into citrus rootstock breeding programs for the Mediterranean Basin.