Retrotransposon characterisation and fingerprinting of apple clones by S-SAP markers

Retrotransposons have been found to comprise the most common class of transposable elements in eukaryotes and to occur in high copy number in plant genomes. Several of these elements have been sequenced and were found to display a high degree of heterogeneity and insertional polymorphism, both within and between species. The dispersion, ubiquity and prevalence of retrotransposons in plant genomes provide an excellent basis for the development of marker systems and, hence, may be good molecular candidates in distinguishing among apple clones, when they represent bud mutations of the original variety, considering that the random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) used thus far in fingerprinting analyses have failed to meet discrimination expectations. The technique called sequence-specific amplified polymorphism (S-SAP), which makes it possible to identify dominant markers for the detection of variation in the DNA flanking the retrotransposon insertion site, was used in the present study to distinguish several clones of the cultivars 'Gala' and 'Braeburn' in apple fingerprinting. Moreover, our results suggest that the bud mutations, which have generated new patented varieties of 'Gala' and 'Braeburn', appear to derive from retrotransposon insertion.