Distribution of 5S and 18S-28S rDNA loci in a tetraploid cotton (Gossypium hirsutum L.) and its putative diploid ancestors Robert E. Hanson 1 , M. Nurul Islam-Faridi 1 , Edward A. Percival 2 , Charles F. Crane 1 , Yuanfu Ji 1 , Thomas D. McKnight 3 , David M. Stelly 1 , H. James Price 1 1 Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843-2474, USA (e-mail: monosom@rigel.tamu.edu) 2 USDA Agricultural Research Service, College Station, TX 77845, USA 3 Department of Biology, Texas A&M University, College Station, TX 77843, USA &misc:Received: 5 November 1995; in revised form: 16 February 1996 / Accepted: 28 February 1996 &p.1: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-ge- nome species are most closely related to the diploid pro- genitors of the tetraploid. We used fluorescent in situ hy- bridization (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. hir- sutum. 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 ob- served loci to 11. Differences in the intensity of the hy- bridization signal at these loci allowed us to designate them as major, intermediate, or minor 18S-28S loci. Us- ing genomic painting with labeled A-genome DNA, five 18S-28S loci were localized to the G. hirsutum A-subge- nome 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-ge- nome 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 (A 1 ) and G. thurberi (D 1 ), the 5S locus is syntenic to a major 18S-28S locus, but in G. arboreum (A 2 ) and G. raimondii (D 5 ), the pro- posed D-genome progenitor of G. hirsutum, the 5S loci are syntenic to minor and intermediate 18S-28S loci, re- spectively. 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 previous- ly envisioned. The relative variability of 18S-28S rDNA loci versus 5S rDNA loci suggests that the behavior of tandem repeats can differ widely. Introduction There are approximately 50 species in the cotton genus Gossypium, including 45 diploids and at least 5 tetraplo- ids (Fryxell 1992). Collectively, they grow throughout tropical and subtropical parts of the world and are ge- nomically diversified in relation to their geographic ranges (Skovsted 1934). Seven major diploid genomic groups of Gossypium have been identified and are desig- nated by the letters A through G (Beasley 1940, 1942; Edwards and Mirza 1979). The species Gossypium hirsutum is an AD disomic tetraploid (2n=4x=52) that originated from an interspe- cific hybridization event(s) between diploid Gossypium species thought to be closely related to the A 1 -genome species G. herbaceum and the D 5 -genome species G. ra- imondii (Endrizzi et al. 1985).The G. hirsutum chromo- somes of A-genome origin are larger, as a group, and are numbered 1–13, whereas those of D-genome origin are numbered 14–26. The A-genome and D-genome diplo- ids are estimated to have diverged from a common an- cestor between 6 and 11 million years ago, and the hy- bridization event(s) leading to G. hirsutum is estimated to have occurred 1 to 2 million years ago (Wendel 1989; Wendel and Albert 1992). Previous efforts at interspecif- ic comparisons within Gossypium have employed mor- phological (Fryxell 1992), meiotic (Menzel 1954), karyotypic (Beasley 1940), genetic and molecular crite- ria (Wendel 1989). In this study, we have used fluorescent in situ hybrid- ization (FISH) to examine the distribution of 5S and 18S-28S rDNA sites in G. hirsutum and closely related diploid species in order to address three objectives. The first objective was to determine the respective homoeo- logous relationship of chromosomes that bear 5S and Edited by: R. Appels Correspondence to: R.E. Hanson&/fn-block: Chromosoma (1996) 105:55–61 © Springer-Verlag 1996