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Sorghum has been improved by public and private breeding programs utilizing germplasm mostly from within the species Sorghum bicolor. Recently, hybridization with an Australian species, S. macrospermum ([AAB.sub.1][B.sub.1] YYZZ), has been demonstrated and the genomic relationship to S. bicolor ([AAB.sub.1][B.sub.1]) shown to be partially compatible. For this species to be potentially useful to sorghum improvement programs, there must be documented introgression into an S. bicolor background. Fifteen [BC.sub.1][F.sub.1] progeny were recovered using the interspecific hybrid as a female and embryo rescue. In these progeny, chromosome numbers ranged from 35 to 70 and all were essentially male-sterile. Repeated backcrossing with 5. bicolor pollen produced [BC.sub.2][F.sub.1] seed on 3 of the 15 [BC.sub.1][F.sub.1] plants. [BC.sub.2][F.sub.1] progeny had varying levels of male fertility; selfed seed set ranged from 0% to 95%, with only 2 individuals being completely male-sterile. Using AFLP and SSR markers, genomic introgression of $. macrospermum ranged from 0% to 18.6%. Cytogenetic analysis of 19 individuals revealed that chromosome numbers were 20, except for a single backcross that had 21 chromosomes. Molecular cytogenetic analysis confirmed the presence of recombinant introgression chromosomes as well as alien addition and alien substitution chromosomes within the [BC.sub.2][F.sub.1s]. Key words: Sorghum bicolor, introgression, fluorescent in situ hybridization, genomic in situ hybridization. Sorghum macrospermum. Le sorgho a fait l'objet de travaux d'amelioration genetique tant publics que prives en utilisant prineipalement les ressources genetiques disponibles au sein de l'espece Sorghum bicolor. Recemment, l'hybridation avec une espece australienne, S. macrospermum ([AAB.sub.1][B.sub.1] YYZZ), a ete demontree et la relation genomique au S. bicolor ([AAB.sub.1][B.sub.1]) s'est averee partiellement compatible. Pour que cette espece soil potentiellement utile dans des programmes d'amelioration du sorgho, il doit y avoir introgression documentee au sein du genome du S. bicolor. Quinze progenitures [BC.sub.1][F.sub.1] ont ete obtenues en employ ant l'hybride interspecifique comme femelle et le sauvetage d'embryons. Au sein de cette descendance, le nombre de chromosomes variait entre 35 et 70 et tous les individus etaient essentiellement androsteriles. Des retrocroisements repetes avec le pollen du S. bicolor ont permis d'obtenir des grains BC2F1 sur 3 des 15 plants [BC.sub.1][F.sub.1]. La progeniture [BC.sub.2][F.sub.1] presentait divers degres d'androsterilite ; le succes en autofecondation variait entre 0 et 95 %, avec deux individus completement androsteriles. A l'aide de marqueurs AFLP et SSR, I'introgression genomique du S. macrospermum variait entre U et 18,6 %. Une analyse cytogenetique sur 19 individus a revele la presence de 20 chromosomes, sauf pour un retrocroisement oil il y avait 21 chromosomes. Une analyse cytogenetique moleculaire a confirme la presence de chromosomes recombinants dotes d'introgressions lout comme des lignees d'addilion el de substitution ehromosomique au sein des [BC.sub.2][F.sub.1]. Mors-cies : Sorghum bicolor, introgression. hybridation in situ en fluorescence, hybridation genomique in situ, Sorghum macrospermum. [Traduit par la Redaction], Introduction Sorghum (Sorghum bicolor (L.) Moench) is an important food and feed crop around the world. The 2006 US grain sorghum crop was valued at approximately $715 million (USDA, NASS [...]

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. Heterochromafin 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.

We used structural genomic resources for Sorghum bicolor (L.) Moench to target and develop multiple molecular cytogenetic probes that would provide extensive coverage for a specific chromosome of sorghum. Bacterial artificial chromosome (BAC) clones containing molecular markers mapped across sorghum linkage group A were labeled as probes for fluorescence in situ hybridizauon (FISH). Signals from single-, dual-, and multiprobe BAC-FISH to spreads of mitotic chromosomes and pachytene bivalents were associated with the largest sorghum chromosome, which bears the nucleolus organizing region (NOR). The order of individual BAC-FISH loci along the chromosome was fully concordant to that of marker loci along the linkage map. In addition, the order of several tightly linked molecular markers was clarified by FISH analysis. The FISH results indicate that markers from the linkage map positions 0.0-81.8 cM reside in the short arm of chromosome 1 whereas markers from 81.8-242.9 cM are located in the long arm of chromosome 1. The centromere and NOR were located in a large heterochromatic region that spans ~60% of chromosome 1. In contrast, this region represents only 0.7% of the total genetic map distance of this chromosome. Variation in recombination frequency among euchromatic chromosomal regions also was apparent. The integrated data underscore the value of cytological data, because minor errors and uncertainties in linkage maps can involve huge physical regions. The successful development of multiprobe FISH cocktails suggests that it is feasible to develop chromosome-specific 'paints' from genomic resources rather than flow sorting or microdissection and that when applied to pachytene chromatin, such cocktails provide an especially powerful framework for mapping. Such a molecular cytogenetic infrastructure would be inherently cross-linked with other genomic tools and thereby establish a cytogenomics system with extensive utility in development and application of genomic resources, cloning, transgene localization, development of plant 'chromonomics,' germplasm introgression, and marker-assisted breeding. In combination with previously reported work, the results indicate that a sorghum cytogenomics system would be partially applicable to other gramineous genera.

Genomic in situ hybridization was performed to localize nucleolar organizer regions (NORs) bearing the 18S-26S ribosomal DNA repeat in a set of samples composed of five rose species and one tetraploid cultivar. Among the diploid species, one NOR per genome seen as a pair of strong subterminal signal was detected. The tetraploid rose cultivar, on the other hand, was shown to have four strong subterminal signals, thus, also representing one NOR per genome.

Bacterial artificial chromosomes (BACs) and high quality techniques for fluorescent in situ hybridization (FISH) to plant chromosome, provide unique tools for plant genome analysis. A 45 kb sorghum BAC (22B2) was found to differentially hybridize to centromere regions of 10 of the 20 sorghum chromosomes. Plants trisomic for chromosomes E, H, and I exhibited 11 signals, suggesting that they are in the subgenome marked by the BAC-FISH signal, providing evidence that sorghum is of tetraploid origin.

Anatomical and physiological responses to drought stress were compared in two Microseris species differing in DNA content and originating from contrasting habitats relative to water availability (M. bigelovii, DNA = 2.6 pg [nucleus.sup.-1], more xeric; M. laciniata, DNA = 6.8 pg [nucleus.sup.-1], mesic). Leaf mesophyll cell volume was positively correlated with DNA content and negatively correlated with tissue elasticity, i.e., low [epsilon] and thin cell walls. Drought stress increased leaf tissue elasticity (lower [epsilon], thinner cell walls). Cell volume, cell wall thickness, cell number, and leaf area were decreased most by drought stress in M. laciniata. Osmotic adjustment with a 20% increase in total solutes (mostly amino acids) after stress was observed in both species, but their estimated contribution to the change in osmotic potential was larger in M. bigelovii. These findings indicate that the Microseris species studied respond to low water availability by maintaining turgor with 1) small cell volumes, 2) elastic tissues (low [epsilon], thin cell walls), and 3) osmotic adjustment. Both enhanced tissue elasticity and small cell volume appear to be inherent characteristics in M. bigelovii and drought-induced responses in M. laciniata. These data are compatible with the hypothesis that natural selection may influence DNA content through differential sensitivity of cell growth to environmental stress.