IDENTIFICATION OF COPIA-LIKE RETROTRANSPOSABLE ELEMENT BY APPLE

Abstract

Until now attempts to use the existing molecular techniques (RFLP, RAPD or AFLP) for the identification of numerous apple fruit colour mutants (from Jonagold or Gala varieties for instance) always failed. In previous reports, the AFLP technique allowed us to detect occasional polymorphisms between some Jonagold mutants and their mother-variety. However, it was demonstrated that such variations, although not artefactual, were not correlated with the fruit colour genotype and are likely to be linked to a certain level of genetic instability within vegetatively propagated lines. In order to isolate cDNA fragments that might be differentially expressed in relation with the fruit colour phenotype, a differential display technique was used to compare mRNA populations extracted from the fruit epidermis of mutants versus the original Jonagold variety. Candidate fragments were obtained using the differential display protocol of Sokolov and Prockop (1994). After their recovery from gels, those fragments were cloned and sequenced. One of them appeared to share strong sequence similarities with the reverse transcriptase gene of several copia-like retrotransposons. These transposable elements have been previously described in diverse plant species like cotton, pea, tobacco, maize or wheat. A Southern blot analysis performed on DNA from several apple varieties clearly demonstrated that the corresponding fragments were present in numerous copies within the genome of all tested cultivars. A genomic library was screened using the reverse transcriptase fragment in order to isolate fulllength copies of this element. Their characterization and sequence analysis are in progress. The demonstration of the existence of a transposable element in the apple genome could be a major step in the understanding of the origin of the genetic variation in this species. This information could also be used to develop new molecular marker methodologies, that could be particularly useful for mutant identification and cultivar conformity testing in apple.