Analysis of genetic diversity in Tunisian citrus rootstocks

Breeding and selection of new citrus rootstocks is very important in the Mediterranean Basin because citrus faces increasing biotic and abiotic constraints. In Tunisia, citrus contributes significantly to the national economy, and increases in the citrus production area are favored by natural conditions and economic considerations. Sour orange, the most widespread rootstock of the Mediterranean area, is also the main one in Tunisia. In addition to sour orange, other citrus rootstocks well adapted to local environmental conditions are traditionally used and should be important genetic resources for breeding. Prior to initiating any breeding program, determining the genetic diversity of Tunisian citrus rootstocks was a priority. Two hundred and one local accessions belonging to four facultative apomictic species (C. aurantium, sour orange; C. sinensis, sweet orange; C. limon, lemon; and C. aurantifolia, lime) were collected and genotyped using 20 nuclear SSR markers and four indel mitochondrial markers. Sixteen distinct multi-locus genotypes (MLGs) were identified and compared to reference genotypes from French and Spanish collections. The differentiation of the four varietal groups was well-marked. Each group displayed a relatively high allelic diversity, primarily due to very high heterozygosity. The Tunisian citrus rootstock genetic diversity is predominantly due to high heterozygosity and differentiation between the four varietal groups. The phenotypic diversity within the varietal groups has resulted from multiple introductions, somatic mutations and rare sexual recombination events. This diversity study enabled the identification of a core sample of accessions for further physiological and agronomical evaluations. These core accessions will be integrated into citrus rootstock breeding programs for the Mediterranean Basin.