FISH-based Karyotype of Gossypium herbaceum Generated with 45S rDNA and gDNA of Gossypium raimondii as Probes

The formula for FISH-based karyotype of Gossypium herbaceum was as 2n = 2x = 26 = 16 m + 10 sm(6 sat),with the range in relative length of short and long arms from 1.43 to 4.14,3.34 to 5.18,respectively.The ratio between the largest chromosomes and the smallest one was 1.63.Six satellite loci were mapped on short arms of the last three chromosomes.The satellites on chromosomes 12 and 13 were clearly visable in both DAPI and rhodamine/DAPI images,however the satellite on chromosome 11 was not detected in DAPI images.Six,in three pairs,of NORs were observed adjacent to the satellite sites,with one pair on telomere and two pairs near centromeres.When gDNA from G.raimondii was used as probe,GISH-NORs were scored in mitotic chromosomes of G.herbaceum with the same numbers,locations and sizes as 45S rDNA NORs.It could be therefore concluded that FISH-based karyotype analyses were more detailed than previous karyotype(non-FISH).Based on this study in conjunction with our other FISH results,there might be great amplifications or pericentric inversions of rDNA in modern A genome species after its contribution to allotetraploid originations,or deamplifications/deletions of tandem repeats like rDNA in extant allotetraploids following their polyploidization.An explanation to D genome specific GISH-NORs is that rDNA contents in D genome species may be much more than those in A genome species.The NORs or GISH-NORs herein may facilitate future locus-specific studies on rRNA gene evolution and function,and also may be useful in developing physical map specific to chromosome order in Gossypium.