EVOLUTION OF INTERSPERSED REPETITIVE ELEMENTS IN GOSSYPIUM (MALVACEAE)

Very little is known regarding how repetitive elements evolve in polyp loid organisms. Here we address this subject by fluorescent in situ hy bridization (FISH) of 20 interspersed repetitive elements to metaphase chromosomes of the cotton AD-genome tetraploid Gossypium hirsutum and its putative A- and D-genome diploid ancestors. These elements collec tively represent an estimated 18% of the G. hirsutum genome, and const itute the majority of high-copy interspersed repetitive elements in G. hirsutum. Seventeen of the elements yielded FISH signals on chromosom es of both G, hirsutum subgenomes, while three were A-subgenome specif ic. Hybridization of eight selected elements, two of which were A-subg enome specific, to the A(2) genome of G. arboreum yielded a signal dis tribution that was similar to that of the G, hirsutum A-subgenome. How ever, when hybridized to the D-5 genome of G. raimondii, the putative diploid ancestor of the G. hirsutum D-subgenome, none of the probes, i ncluding elements that strongly hybridized to both G. hirsutum subgeno mes, yielded detectable signal. The results suggest that the majority, although not all, G. hirsutum interspersed repetitive elements have u ndergone intergenomic concerted evolution following polyploidization a nd that this has involved colonization of the D-subgenome by A-subgeno me elements and/or replacement of D-subgenome elements by elements of the A-subgenome type.