Genetic diversity of Tunisian date palms (Phoenixdactylifera L.) revealed by nuclear microsatellite polymprohism

摘要

Date-palm (Phoenix dactylifera L), a long-lived dioecious monocotyledon (2n=36), is the major factor of oases environmental and economic stability (Rhouma 1994). In Tunisia, this important subtropical fruit crop is currently in danger due to severe genetic erosion as a consequence of the predominance of the elite cultivar Deglet Nour in modern cultures (Rhouma 1994). This tendency led to the disappearance of many cultivars with medium and low fruit qualities. It is therefore imperative to elaborate a strategy aiming at the evaluation of the genetic diversity and the preservation of the Tunisian date-palm germplasm. Many studies have been designed on this matter and described the use of either morphological traits or isozyme makers to identify the Tunisian date-palm varieties (Reynes et al. 1994; Rhouma 1994; Bouabidi et al. 1996; Ould Mohamed Salem et al. 2001). However, most morphological traits are highly influenced by environmental conditions or vary with the development stage of plants, and isozymes are limiting due to low levels of polymorphism. Consequently, DNA based techniques have been developed and proved effective to assess genetic diversity, because they provide a nearly unlimited potential of markers to uncover differences at the molecular level. Data based on molecular markers such as RFLPs, RAPDs and ISSRs have been performed to characterize date-palm genotypes (Cornicquel and Mercier 1994, 1996; Sedra et al. 1998; Ben Abadallah et al. 2000; Trifi et al. 2000; Trifi 2001; Zehdi et al. 2002, Sakka et al. 2004). These studies have permitted to identify markers suitable for identification of date-palm varieties. However, the search of many other markers is required to obtain a deeper comprehension of the genetic organization in Tunisian date-palm germplasm.Microsatellites or simple sequence repeats (SSRs) consist of variable numbers of tandemly repeated units each one of 1 to 6 bp, and represent a class of repetitive DNA commonly found in eukaryotic genomes (Tautz and Renz 1984). They are characterized by their great abundance (Condit and Hubbell 1991; R?der et al. 1995), high variability (Schug et al. 1998) and large distribution throughout different genomes (Liu et al. 1996; Taramino and Tingey 1996; R?der et al. 1998). Microsatellites are typically multi-allelic loci since more than five alleles per locus are commonly observed either in populations of plants (Innan et al. 1997; Senior and Heun 1998) or in animals (MacHugh et al. 1997). In addition, automated polymerase chain reaction (PCR) based techniques, which enable high throughput data collection and good analytical resolution at a low cost, have been developed for microsatellites (Kresovich et al. 1995; Mitchell et al. 1997). Hence, taking into account these advantages, microsatellites are actually one of the preferred genetic markers in plants and animals. They are exploited as tools to assess genetic distances and genetic diversity in evolutionary studies (Bruford and Wayne 1993; Goldstein and Pollock 1997).In the present article, we report the use of the SSR markers to survey polymorphisms and identify genotypes within Tunisian date-palms.