DISTRIBUTION OF 5S AND 18S-28S RDNA LOCI IN A TETRAPLOID COTTON (GOSSYPIUM-HIRSUTUM L) AND ITS PUTATIVE DIPLOID ANCESTORS

The most widely cultivated species of cotton, Gossypium hirsutum, is a disomic tetraploid (2n=4x=52). It has been proposed previously that e xtant A- and D-genome species are most closely related to the diploid progenitors of the tetraploid. We used fluorescent in situ hybridizati on (FISH) to determine the distribution of 5S and 18S-28S rDNA loci in the A-genome species G. herbaceum and G. arboreum, the D-genome speci es G. raimondii and G. thurberi, and the AD tetraploid G. hirsutum. Hi gh signal-to-noise, single-label FISH was used to enumerate rDNA loci, and simultaneous, dual-label FISH was used to determine the syntenic relationships of 5S rDNA loci relative to 18S-28S rDNA loci. These tec hniques provided greater sensitivity than our previous methods and per mitted detection of six new G. hirsutum 18S-28S rDNA loci, bringing th e total number of observed loci to 11. Differences in the intensity of the hybridization signal at these loci allowed us to designate them a s major, intermediate, or minor 18S-28S loci. Using genomic painting w ith labeled A-genome DNA, five 18S-28S loci were localized to the C. h irsutum A-subgenome and six to the D-subgenome. Four of the 11 18S-28S rDNA loci in G. hirsutum could not be accounted for in its presumed d iploid progenitors, as both A-genome species had three loci and both D -genome species had four. G. hirsutum has two 5S rDNA loci, both of wh ich are syntenic to major 18S-28S rDNA loci. All four of the diploid g enomes we examined contained a single 5S locus. In G. herbaceum (A(1)) and G. thurberi (D-1), the 5S locus is syntenic to a major 18S-28S lo cus, but in C. arboreum (A(2)) and G. raimondii (D-5), the proposed D- genome progenitor of G. hirsutum, the 5S loci are syntenic to minor an d intermediate 18S-28S loci, respectively. The multiplicity, variation in size and site number, and lack of additivity between the tetraploi d species and its putative diploid ancestors indicate that the behavio r of rDNA loci in cotton is nondogmatic, and considerably more complex and dynamic than previously envisioned. The relative variability of 1 8S-28S rDNA loci versus 5S rDNA loci suggests that the behavior of tan dem repeats can differ widely.