Chromosomal Locations of 5S and 45S rDNA in Gossypium Genus and Its Phylogenetic Implications Revealed by FISH

Abstract

We investigated the locations of 5S and 45S rDNA in Gossypium diploid A, B, D, E, F, G genomes and tetraploid genome (AD) using multi-probe fluorescent in situ hybridization (FISH) for evolution analysis in Gossypium genus. The rDNA numbers and sizes, and synteny relationships between 5S and 45S were revealed using 5S and 45S as double-probe for all species, and the rDNA-bearing chromosomes were identified for A, D and AD genomes with one more probe that is single-chromosome-specific BAC clone from G. hirsutum (A1D1). Two to four 45S and one 5S loci were found in diploid-species except two 5S loci in G. incanum (E4), the same as that in tetraploid species. The 45S on the 7th and 9th chromosomes and the 5S on the 9th chromosomes seemed to be conserved in A, D and AD genomes. In the species of B, E, F and G genomes, the rDNA numbers, sizes, and synteny relationships were first reported in this paper. The rDNA pattern agrees with previously reported phylogenetic history with some disagreements. Combined with the whole-genome sequencing data from G. raimondii (D5) and the conserved cotton karyotype, it is suggested that the expansion, decrease and transposition of rDNA other than chromosome rearrangements might occur during the Gossypium evolution.

Figure 1. The number of rDNA in AD, D, A, B, E, F and G genomes.

5S rDNA: green fluorescence signals marked with green arrows; 45S rDNA: red fluorescence signals marked with red arrows. a: A₁D₁, b: A ₃D₃, c: A ₄D₄, d: D₅, e: D₆, f: D₉, g: D₁₁, h: A₁, i: A_1-a, j: B₁, k: B₃, l: E₂, m: E₃, n: E₄, o: F₁, p: G₃, q: G₁. Bar =5 µm.

Figure 2. Locations of 5S and 45S rDNA in four tetraploid and four D-genome species.

green and weak fluorescence signals with green arrow; 5S rDNA: green fluorescence signals; 45S rDNA: red fluorescence signals. For D-genome species and D-subgenome, the short arm and the long arm were distinguished by the location of 150D24 with red fluorescence signals on intercalary chromosomes. Marked chromosomes with green arrow were enlarged at the top-right corner with the short arm on the top, and the 45S or 5S signals were marked with white arrow. Bar =5µm. a–c: FISH images with 45S on chromosomes A _h09, D _h07 and D _h09 and that with 5S on chromosomes A _h09 and D _h09 for A ₁D₁, respectively. d–f: FISH images with 45S on chromosomes A _tt09, D _tt07 and D _tt09 and that with 5S on chromosomes A _tt09 and D _tt09 for A ₃D₃, respectively. g–j: FISH images with 45S on chromosomes A _m07, A _m09 and A _m08 (g, h, i) and that with 5S on chromosomes A _m09 and D _m09 (h, j) for A ₄D₄, respectively. k: FISH images with 45S and 5S on chromosome A _b09 of A ₂D₂. l–o: FISH images with 45S and 5S on chromosomes D₉05, D₉07, D₉09 and D₉12 and that with 5S on chromosome D ₉09 for D₉, respectively. p–r: FISH images with 45S and 5S on chromosomes D₁₁05, D₁₁07 and D₁₁09 and that with 5S on the chromosome D ₁₁09 for D₁₁, respectively. s, t: FISH images with 45S and 5S on chromosomes D₆07 and D₆09 and that with 5S on chromosome D ₆09 for D₆, respectively. u–w: FISH images with 45S and 5S on chromosomes D₅09, D₅02 and D₅11 and that with 5S on chromosome D ₅09 for D₅, respectively.

Figure 3. Locations of 5S and 45S rDNA in two A-genome species by dual-FISH.

BAC DNA: green and weak fluorescence signals with green arrow; 5S rDNA: green fluorescence signals; 45S rDNA: red fluorescence signals. Marked chromosomes with green arrow were enlarged at the top-right corner with the short arm on the top, and the 45S or 5S signal were marked with white arrow. Bar =5µm. a–c: FISH images with 45S and 5S on chromosomes A ₁05, A ₁07 and A ₁09 and that with 5S on chromosome A ₁09 for A₁, respectively. d–f: FISH images with 45S and 5S on chromosomes A _1-a05, A _1-a07 and A _1-a09 and that with 5S on chromosome A _1-a09 for A_1-a, respectively.

Figure 4. Simplified phylogeny of Gossypium genus included in idiograms chromosomes bearing 5S and 45S rDNA signals.

The phylogeny is according to the reference of Wendel [24]. The words under the chromosomes indicated the chromosomes bearing the rDNA locus. 5S rDNA: green signals; 45S rDNA: red signals. * and ** indicate the differences according 45S rDNA pattern in the present study from previous report [24], respectively; *** indicates G. herbaceum var. africanum .