| Fluorescent in situ hybridization (FISH) was performed on metaphase chromosomes of the cotton AD-genome tetraploid Gossypium hirsutum and diploid species closely related to its ancestors. Four classes of probes were used: a bacterial artificial chromosome (BAC), 5S and 18S-28S rDNA repeats, a Tyl-copia-like retrotransposon and 20 uncharacterized interspersed repetitive elements. BAC-FISH demonstrated the reliable detection of single/low-copy genomic clones in plants, establishing a powerful tool for physical mapping. Eleven 18S-28S rDNA loci were detected in G. hirsutum; 5 in the A-subgenome and 6 in the D-subgenome. The G. hirsutum subgenomes each had 2 more 18S-28S rDNA loci than putative A-genome and D-genome diploid ancestors of the tetraploid. Dual-color FISH of 5S and 18S-28S rDNA loci allowed size comparisons between rDNA loci on homoeologous chromosomes, revealing significant changes in the sizes of Gossypium 18S-28S rDNA loci. Tyl-copia like retrotransposon clone A108 yielded strong FISH signal on all G. hirsutum chromosomes, roughly equal in intensity between the A- and D-subgenomes. Hybridization of A108 to G. arboreum gave similar results, whereas no signal was detected on chromosomes of the D-genome diploid G. raimondii, the putative diploid ancestor of the G. hirsutum D-subgenome. FISH of 20 G. hirsutum repetitive elements revealed that 17 of the 20 occurred in both subgenomes of G. hirsutum, while 3 were specific to the A-subgenome. Eight elements, 2 subgenome-specific and 6 non-subgenome specific elements, were selected for FISH to G. arboreum (A... |
