The diversification of Citrus clementina hort. ex tan., a vegetatively propagated crop species

Clementines, due to their high quality, are one of the most important cultivated citrus mandarins. As in the case of sweet orange and satsuma mandarins, genetic variability within this species is minimal when analyzed by molecular markers, because the existing varieties have not been obtained through hybridization, but through the selection of spontaneous mutations affecting traits of agronomic interest. This would explain, at least in part, the greater diversity for agronomic traits when compared to the variability for molecular markers. Another possible (nonexclusive) reason is that the types of molecular marker used are not focused on the kind of molecular change mainly involved in the origination of new clementine cultivars; i.e., are all sources of variation equally involved in the diversification of these plants? To answer this question, different kinds of markers based on primers of random sequence, simple sequence repeats, and retrotransposon sequences that may reveal point mutations, and somatic recombination and transposon activity, respectively, were used to compare the level of variability among 24 clementine varieties. Their ISSR, RAPD, and AFLP analysis provided only two polymorphic, bands, distinguishing just two varieties. No variability was found by SSRs, i.e., no new allele arising through somatic recombination was detected. Instead, the amplification of sequences adjacent to retrotransposons yielded a higher number of polymorphisms (14.6 vs 2.4% for the previous mentioned marker types). Two geographical distant groups, one from North Africa and the other from Spain, have evolved in agreement with polymorphisms based on IRA-P markers anchored to, at least, two different Copia-like retrotransposon sequences. Therefore, this study suggests that the DNA of this type of mobile elements is evolving faster than the DNA of other markers in this clonal lineage. (C) 2001 Academic Press.