Distribution of 5S and 18S-28S rDNA loci in a tetraploid cotton ( Gossypium hirsutum L.) and its putative diploid ancestors

Abstract

The most widely cultivated species of cotton, Gossypium hirsutum, is a disomic tetraploid (2n=4x=52). It has been proposed previously that extant A- and D-genome species are most closely related to 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, and simultaneous, dual-label FISH was used to determine the syntenic relationships of 5S rDNA loci relative to 18S-28S rDNA loci. These techniques provided greater sensitivity than our previous methods and permitted detection of six new G. hirsutum 18S-28S rDNA loci, bringing the total number of observed loci to 11. Differences in the intensity of the hybridization signal at these loci allowed us to designate them 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 three loci and both D-genome species had four. G. hirsutum has two 5S rDNA loci, both of which are syntenic to major 18S-28S rDNA loci. All four 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 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 that the behavior of rDNA loci in cotton is nondogmatic, and considerably more complex and dynamic than previously envisioned. The relative variability of 18S-28S rDNA loci versus 5S rDNA loci suggests that the behavior of tandem repeats can differ widely.

Completely Distinguishing Individual A-Genome Chromosomes and Their Karyotyping Analysis by Multiple Bacterial Artificial Chromosome–Fluorescence in Situ Hybridization

“…Its signal locus had some overlap with that of the 5S rDNA (also located on chromosome A9) and was too weak to be detected in multiple FISH when we adjusted the photo contrast. Hanson et al (1996) found two major NORs in the A-subgenome of G. hirsutum, then they found another major NOR and located all three NORs on chromosomes At5, At7, and At9 ( Ji et al 1999). All of these results support our findings and indicate that G. arboreum was one of the ancestors of G. hirsutum.…”

“…Two major NOR loci were found using rDNA FISH in different G. arboreum species (Hanson et al 1996;Wang et al 2001;Bie et al 2004). At the same time, two major NOR loci The genetic markers and their corresponding BAC clones were indicated on the left and right of the chromosome diagrams, respectively.…”

Chromosomal divergence and evolutionary inferences in Rhodniini based on the chromosomal location of ribosomal genes

“…This hypothesis would imply either a partial transfer of ribosomal genes from the X to the Y chromosome, as mediated through the transposition of mobile elements (Schubert & Wobus 1985) or chromosomal rearrangements promoted through ectopic recombination (homologous recombination between repetitive sequences of non-homologous chromosomes) (Hanson et al 1996). In almost all of the Rhodniini species described in this paper (Figs 1, 2), the Y chromosome ribosomal signal was less intense than that observed for the X chromosome, indicating that the ribosomal cluster copy number is significantly lower on the Y chromosome.…”

Classical and Molecular Cytogenetic Tools to Resolve the Bixa Karyotypes

“…Robertsonian interchange is characterized by centromeric fusion (Jones 1998, Zhang et al 2001, Oakey and Beechey 2002, Switonski et al 2003 between 2 acrocentric (Gupta and Gupta 1991) or telocentric (Zhang et al 2001) non-homologous chromosomes, resulting in one metacentric chromosome (Gupta and Gupta 1991, Jones 1998, Zhang et al 2001, Oakey and Beechey 2002. Considering that many species present the SC in the distal portion of the short arm (Leitch and HeslopHarrison 1992, Hanson et al 1996, Liu et al 1997) and the morphometric data of the chromosome 1, a break involving the long arm of 1 chromosome and the short arm of the other probably led to the formation of the metacentric chromosome 1 with the SC adjacent to the centromere, during the genomic evolution of these species.…”