H. J. Price

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

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 speciesG. herbaceum andG. arboreum, the D-genome speciesG. raimondii andG. thurberi, and the AD tetraploidG. 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 newG. hirsutum 18S–28S rDNA loci, bringing the total number of observed loci to 11. Differences in the intensity of the hybrizization signal at these loci allowed us to designate them as major, intermediate, or minor 18–28S loci. Using genomic painting with labeled A-genome DNA, five 18S–28S loci were localized to theG. hirsutum A-subgenome and six to the D-subgenome. Four of the 11 18S–28S rDNA loci inG. hirsutum could not be accounted for in its presumed diploid progenitors, as both A-genome species has 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 wer examined contained a single 5S locus. InG. herbaceum (A1) andG. thurberi (D1), the 5S locus is syntenic to a major 18S–28S locus, but inG. arboreum (A2) andG. raimondii (D5), the proposed D-genome progenitor ofG. hirsutum, the 5S loci are syntenic tominor and intermediate 18S–28S loci, respecitively. 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 repearts can differ widely.

Comprehensive Molecular Cytogenetic Analysis of Sorghum Genome Architecture: Distribution of Euchromatin, Heterochromatin, Genes and Recombination in Comparison to Rice

Cytogenetic maps of sorghum chromosomes 3–7, 9, and 10 were constructed on the basis of the fluorescence in situ hybridization (FISH) of ∼18–30 BAC probes mapped across each of these chromosomes. Distal regions of euchromatin and pericentromeric regions of heterochromatin were delimited for all 10 sorghum chromosomes and their DNA content quantified. Euchromatic DNA spans ∼50% of the sorghum genome, ranging from ∼60% of chromosome 1 (SBI-01) to ∼33% of chromosome 7 (SBI-07). This portion of the sorghum genome is predicted to encode ∼70% of the sorghum genes (∼1 gene model/12.3 kbp), assuming that rice and sorghum encode a similar number of genes. Heterochromatin spans ∼411 Mbp of the sorghum genome, a region characterized by a ∼34-fold lower rate of recombination and ∼3-fold lower gene density compared to euchromatic DNA. The sorghum and rice genomes exhibit a high degree of macrocolinearity; however, the sorghum genome is ∼2-fold larger than the rice genome. The distal euchromatic regions of sorghum chromosomes 3–7 and 10 are ∼1.8-fold larger overall and exhibit an ∼1.5-fold lower average rate of recombination than the colinear regions of the homeologous rice chromosomes. By contrast, the pericentromeric heterochromatic regions of these chromosomes are on average ∼3.6-fold larger in sorghum and recombination is suppressed ∼15-fold compared to the colinear regions of rice chromosomes.

Distribution and sequence analysis of the centromere-associated repetitive element CEN38 of Sorghum bicolor (Poaceae).

Fluorescence in situ hybridization (FISH) of a large-insert genomic clone, BAC 22B2, previously suggested that Sorghum bicolor (2n = 20) has the tetraploid architecture A(b)A(b)B(b)B(b). Here, we report on BAC 22B2 subclone pCEN38 (1047-bp insert) as related to sorghum and sugarcane. Mitotic FISH of six different subclones of BAC 22B2 showed that pCEN38 produced the strongest specificity to the A(b) subgenome and signal occurred primarily near centromeres. Southern blots of pCEN38 to 21 crop plants revealed a narrow taxonomic distribution. Meiotic metaphase I FISH positioned pCEN38 sequences near active centromeres. Pachytene FISH revealed that the distributions are trimodal in several B(b) and possibly all sorghum chromosomes. DNA sequencing revealed that the pCEN38 fragment contains three tandemly repeated dimers (<280 bp) of the same sequence family found in sorghum clone pSau3A10, and that each dimer consists of two divergent monomers (<140 bp). Sequence comparisons revealed homology between the pCEN38 monomers and the SCEN 140 bp tandem repeat family of sugarcane. FISH of pCEN38 yielded signal in centromere regions of most but not all sugarcane chromosomes. Results suggest that sugarcane and sorghum share at least one ancestor harboring elements similar to pCEN38 and SCEN and that each species had an ancestor in which the repetitive element was weakly present or lacking.

Aneuploidy, structural chromosome changes, and DNA amounts in the annual taxa of theHaplopappus spinulosus complex

Haplopappus gracilis (n=2),Haplopappus revenil (n=4), andHaplopappus wigginsii (n=4) are isolated by F1 hybrid sterility due mainly to translocation heterozygosity. There is no evidence that this can be overcome at the diploid level so that introgression can occur among them. They are also separated geographically, but occasional populations ofH. gracilis andH. ravenil may be brought together along roadways to form sterile hybrids. There were no statistically significant differences in nuclear DNA content among the same or structurally different aneuploidn=2 andn=3 chromosome races or ecotypes ofH. gracilis. Some of theH. gracilis races were not significantly different from one race of the ancestralH. ravenii, and these samples of both species were from plants growing on poor soils in contrast to accessions from normal habitats. How much and which classes of DNA in these species are subject to changes induced by environmental effects is not known. There were no correlations between DNA amounts and altitude, latitude, and longitude.H. wigginsii had a greater amount of DNA per nucleus than eitherH. ravenii orH. gracilis, and its increased DNA content may reflect a more rapid accumulation of noncoding sequences due to facultative self-compatibility not found in the other two species.