Chloroplast DNA variations in Mediterranean olive

Abstract

SummaryOver ten years, several European teams have characterised polymorphism in the olive chloroplast genome (cpDNA). A particularly low level of cpDNA polymorphism was detected; but, in a recent study based on 14 Mediterranean trees, an intergenic (trnD-trnT) spacer was reported to be sufficiently variable to distinguish at least six haplotypes in cultivated olive. Compared to previous studies, the level of polymorphism described (nine substitutions and seven short indels on a segment of approx. 1 kbp) was particularly high. In order to enlarge upon the sample analysed, new trnD-trnT sequences were generated from 18 olive trees originating from the Mediterranean Basin, Macaronesia, S. Algeria, Kenya, Yemen, Reunion Island, Hawaii, Iran, and China. Only two base substitutions were detected in this enlarged sample, one of which discriminated between Olea europaea subsp. cuspidata and other olive sub-species. Length variation at a (poly-T) microsatellite motif was also found. The relatively low variation revealed between distant populations of olive suggests that sequencing problems might be the origin of the high polymorphism previously reported in the trnD-trnT spacer. Furthermore, length polymorphism at the poly-T motif was investigated in 55 Mediterranean olive trees which had previously been analysed for other cpDNA polymorphisms. Two alleles were found in Mediterranean olive, but they did not permit detection of new cpDNA haplotypes. It is proposed that proper analyses of cpDNA polymorphism in Mediterranean olive should use a combination of seven markers in order to detect its correct phylogeographic structure.