The diversity of retroelements in diploid and allotetraploid Brassica species

Using universal PCR primers, some 80 fragments of retroelement reverse transcriptase genes were isolated from 16 accessions of the three diploid and three derived allotetraploid species of Brassica in the triangle of U. Sequence analysis showed that the Ty1/copia and LINE-like elements were distinct, while a third clade could be sub-divided into Ty3/gypsy, Athila and virus-like branches, providing evidence that there are multiple sub-lineages within this group normally considered to be gypsy-like elements in plants. The parsimony trees showed no branches correlating with the known genome relationships for the six diploid and allotetraploid Brassica species, probably because members of the element families were present in the common ancestor of the Brassica and, unlike other repetitive sequences, there is no evidence for genome-wide homogenization, although convergent evolution or horizontal transfer cannot be ruled out. Southern hybridization suggested some sub-families were amplified in individual species. The data show that retroelement sequence data do not allow inference of phylogeny, but knowledge of evolution of such abundant sequences assists in exploitation and interpretation of data from other species including models with much smaller genomes and may provide markers.