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United States Department of Agriculture

Agricultural Research Service

Title: Distribution of 5s and 18s-Rdna Loci in a Tetraploid Cotton (Gossypium Hirsutum L.) and Its Putative Diploid Ancestors

Authors
item Hanson, Robert - TEXAS A&M UNIVERSITY
item Islam-Faridi, M - TEXAS A & M UNIVERSITY
item Percival Jr, Albert
item Crane, Charles - TEXAS A & M UNIVERSITY
item Ji, Yuanfu - TEXAS A & M UNVIERSITY
item Mc Knight, Thomas - TEXAS A & M UNIVERSITY
item Stelly, David - TEXAS A & M UNIVERSITY

Submitted to: Chromosoma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 28, 1996
Publication Date: N/A

Interpretive Summary: Gossypium hirsutum is a disomic tetraploid (2n=4x=52. Previously it was found that existing A- and D-genome species are the most closely related to the diploid ancestors of the tetraploids. We used fluorescent in situ hybridization (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 species G. raimondii and G. thurberi, and the AD tetraploid G. hirsutum. High signal-to-noise, single-label FISH was used to count rDNA loci; dual label FISH was used to determine the syntenic relationship of 5S relative to 18S-28S rDNA loci. These techniques permitted detection of six new G. hirsutum 18S-28S rDNA loci, totaling 11 observed loci. Differences in the intensity of the hybridization signal at these loci allowed designating them as major, intermediate, or minor 18S-28S loci. Using genomic painting with labeled A-genome DNA found that five 18S-28S loci were localized to the G. hirsutum 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 the presumed diploid ancestors. A-genome species had 3 loci and D-genome species had 4. G. hirsutum has two 5S rDNA loci; both are syntenic to major 18S-28S rDNA loci. All 4 of the diploid genomes we examined contained a single 5S locus. In G. herbaceum (A1) and G. thurberi (D1), the 5S locus is syntenic to a major 18S-28S locus. But in G. arboreum (A2) and G. raimondii (D5), the implied D-genome ancestor of G. hirsutum, the 5S loci are syntenic to minor and intermediate 18S-28S loci respectively. The multiplicity, variation in size and site number, and lack of additivity between the tetraploid species and its supposed diploid ancestors indicate the behavior of rDNA loci in cotton is nondogmatic, and more complex and dynamic.

Technical Abstract: Gossypium hirsutum is a disomic tetraploid (2n=4x=52). It was proposed that extant A- and D-genome species are the diploid progenitors of the tetraploid. We used fluorescent in situ hybridization (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 species G. raimondii and G. thurberi, and the AD tetraploid G. hirsutum. High signal-to-noise, single label FISH was used to enumerate rDNA loci; dual label FISH was used to determine the syntenic relationships of 5S rDNA loci relative to 18S-28S rDNA loci. Different intensities of the hybridization signal at these loci allowed designation as major, intermediate, or minor 18S-28S loci. Using genomic painting with labeled A-genome DNA, five 18S-28S loci were localized to the G. hirsutum 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 diploid progenitors, as both A-genome species had 3 loci and both D-genome species had 4. G. hirsutum has two 5S rDNA loci, and both are syntenic to major 18S-28S rDNA loci. All 4 diploid genomes we examined contained a single 5S locus. In G. herbaceum (A1) and G. thurberi (D1), the 5S locus is syntenic to a major 18S-28S locus, but in G. arboreum (A2) and G. raimondii (D5), the proposed D-genome progenitor of G. hirsutum, the 5S loci are syntenic to minor and intermediate 18S-28S loci, respectively. The multiplicity, variation in size and site number, and lack of additivity between the tetraploid species and its putative diploid ancestors indicate behavior of rDNA loci in cotton is nondogmatic, more complex and dynamic than previously thought. Relative variability of 18S-28S rDNA loci versus 5S rDNA loci suggests that behavior of tandem repeats can differ widely.

Last Modified: 10/24/2014
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