Growth, root aerenchyma, and profiles of radial O2 loss (ROL) along adventitious roots were evaluated in 35 'wild' Hordeum accessions and cultivated barley (H. vulgare L. ssp. vulgare) when grown in stagnant nutrient solution (deoxygenated and containing 0.1% agar). When grown in stagnant solution, accessions from wetland and 'intermediate' habitats were superior, compared with accessions from non-wetland habitats, in maintaining relative growth rate, tillering, and adventitious root mass. Constitutive aerenchyma formation in adventitious roots was ≥10% in 22 accessions (cf. H. vulgare at 2%). When grown in stagnant solution, aerenchyma was ≥ 20% in the adventitious roots of 14 accessions (cf. H. vulgare at 12%). Variation among the accessions in the volume of aerenchyma formed when grown in aerated or stagnant solution was not determined by the waterlogging regime of the species' natural habitat. However, the genus Hordeum comprises four genomes and when grown in stagnant solution accessions with the X genome formed, on average, 22% aerenchyma in adventitious roots (50 mm behind apex), whereas those with the H genome averaged 19%, and those with the Y or I genomes averaged 16 and 15%, respectively. Sixteen accessions formed a barrier to ROL in the basal region of adventitious roots when grown in stagnant solution. The formation of a barrier to radial O2 loss was predominant in accessions from wet habitats, and absent in accessions from non-wetland habitats. In addition, this trait was only present in accessions with the X or H genomes. The combination of aerenchyma and a barrier to ROL enhances the longitudinal diffusion of O2 within roots towards the apex. The possibility of a link between having a barrier to ROL and the X or H genomes in Hordeum species might, in future studies, enable a genetic analysis of this important trait.
Intergeneric crosses were made between the four species of Eremopyrum (2n = 14, 28) and 16 species (2n = 14, 28, 42) from 8 genera of the Triticeae. Seed set was obtained in 22 of 42 different combinations. Only 9 resulted in progeny and only one was diploid. The hybrids were morphologically intermediate between the parents and it has been confirmed that perenniality dominates over annuality. The very low pollen fertility of the hybrids was caused by very low chromosome pairing in meiosis and supports the uniqueness of the Eremopyrum genome. Autosyndesis between the chromosomes of E. orientale is proposed and for that reason reevaluation of the relationships within Eremopyrum is needed.
One dihaploid (n=2x=14) and one true hybrid (2n=5x=35) were produced from the intergeneric cross Elymus shandongensis X Triticum aestivum. The dihaploid was a result of selective elimination of T. aestivum chromosomes early in embryo development. Meiotic chromosome behaviour was studied in the parents, dihaploid, and hybrid. E. shandongensis (2n=4x=28) and the hexaploid wheat formed only bivalents at metaphase I. Some homoeologous chromosome pairing with an average of 0.68 bivalents, ranging from zero to four, and a very low frequency of trivalents was observed in the dihaploid. This reveals a low affinity between S and Y genomes of E. shandongensis. A haplo-insufficient (hemizygous ineffective) genetic control of homoeologous chromosome pairing may also exist in the species. Homologous chromosome association in the hybrid (0.25 chiasmata/cell) was distinctly lower than in the dihaploid (0.73 chiasmata/cell). This indicates that chromosome pairing suppressors, e.g., the Ph gene system, in T. aestivum could affect homoeologous chromosome pairing between the genomes of wild species.
In the hybrid Hordeum vulgare × Psathyrostachys fragilis the two genomes were differentiated (i) by length, the P. fragilis chromosomes being 31% longer than the H. vulgare chromosomes; (ii) by a difference in staining intensity of C-banded chromosomes (of possible use for exact localization of breakpoints), the H. vulgare chromosomes being the more heavily stained; (iii) by widely different C-banding patterns; and (iv) by the difference between N-banded H. vulgare and non-N-banded P. fragilis chromosomes. Only C-banding patterns identified each chromosome. Aneuploid cells had lost between one and five P. fragilis chromosomes. Loss of H. vulgare chromosomes is ascribed to squashing. No haploid H. vulgare cell was observed. The P. fragilis chromosomes were characterized by diminished centric constrictions, suppression of nucleolar constrictions, and nucleolus activity, i.e., differential amphiplasty, and generally a peripheral location on the metaphase plate. The same characteristics are normally observed in hybrids producing haploids H. vulgare, suggesting a common mechanism of chromosome elimination. Some cells had a side-by-side arrangement of genomes. The only effect of the hybrid condition on H. vulgare chromosomes was the formation of wider nucleolar constrictions and larger nucleolus organizer regions (NORs) than in parental H. vulgare, suggesting a compensational mechanism for nucleolus activity. The passage of H. vulgare chromosomes through the hybrid to the dihaploid did not influence chromosomal characteristics.Key words: Hordeum, Psathyrostachys, hybrids, elimination of chromosomes, banding.
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