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 culti vated citrus mandarins. As in the case of sweet orange and satsuma mandarin s, genetic variability within this species is minimal when analyzed by mole cular markers, because the existing varieties have not been obtained throug h hybridization, but through the selection of spontaneous mutations affecti ng 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 type s 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 a ll sources of variation equally involved in the diversification of these pl ants? 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 a ctivity, 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 w as found by SSRs, i.e., no new allele arising through somatic recombination was detected. Instead, the amplification of sequences adjacent to retrotra nsposons yielded a higher number of polymorphisms (14.6 vs 2.4% for the pre vious mentioned marker types). Two geographical distant groups, one from No rth Africa and the other from Spain, have evolved in agreement with polymor phisms based on IRA-P markers anchored to, at least, two different Copia-li ke retrotransposon sequences. Therefore, this study suggests that the DNA o f this type of mobile elements is evolving faster than the DNA of other mar kers in this clonal lineage. (C) 2001 Academic Press.