Assessment of the genetic diversity of the Tunisian citrus rootstock germplasm
1 Horticultural Laboratory, Tunisian National Agronomic Research Institute (INRAT), Rue Hedi Karray, 2049 Ariana, Tunisia
2 Department BIOS, TGU AGAP, International Center for of Agricultural Research for Development (CIRAD), Avenue Agropolis, TA A-75/02, F-34398 Montpellier Cedex 5, France
3 Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigaciones Agrarias (IVIA), Crta Moncada-Naquera 5, Valencia 46113, Spain
4 Agronomy and Plant Biotechnology Department (INAT), Tunisian National Agronomic Institute (INAT), 43 Avenue Charles Nicolle, 1082 Tunis, Mahrajène, Tunisia
BMC Genetics 2012, 13:16 doi:10.1186/1471-2156-13-16Published: 19 March 2012
Citrus represents a substantial income for farmers in the Mediterranean Basin. However, the Mediterranean citrus industry faces increasing biotic and abiotic constraints. Therefore the breeding and selection of new rootstocks are now of the utmost importance. In Tunisia, in addition to sour orange, the most widespread traditional rootstock of the Mediterranean area, other citrus rootstocks and well adapted to local environmental conditions, are traditionally used and should be important genetic resources for breeding. To characterize the diversity of Tunisian citrus rootstocks, two hundred and one local accessions belonging to four facultative apomictic species (C. aurantium, sour orange; C. sinensis, orange; C. limon, lemon; and C. aurantifolia, lime) were collected and genotyped using 20 nuclear SSR markers and four indel mitochondrial markers. Multi-locus genotypes (MLGs) were compared to references from French and Spanish collections.
The differentiation of the four varietal groups was well-marked. The groups displayed a relatively high allelic diversity, primarily due to very high heterozygosity. Sixteen distinct MLGs were identified. Ten of these were noted in sour oranges. However, the majority of the analysed sour orange accessions corresponded with only two MLGs, differentiated by a single allele, likely due to a mutation. The most frequent MLG is shared with the reference sour oranges. No polymorphism was found within the sweet orange group. Two MLGs, differentiated by a single locus, were noted in lemon. The predominant MLG was shared with the reference lemons. Limes were represented by three genotypes. Two corresponded to the 'Mexican lime' and 'limonette de Marrakech' references. The MLG of 'Chiiri' lime was unique.
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. Finally, 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.