Molecular cytogenetic characterisation of Elytrigia ×mucronata, a natural hybrid of E. intermedia and E. repens (Triticeae, Poaceae)

BACKGROUND

Interspecific hybridisation resulting in polyploidy is one of the major driving forces in plant evolution. Here, we present data from the molecular cytogenetic analysis of three cytotypes of Elytrigia ×mucronata using sequential fluorescence (5S rDNA, 18S rDNA and pSc119.2 probes) and genomic in situ hybridisation (four genomic probes of diploid taxa, i.e., Aegilops, Dasypyrum, Hordeum and Pseudoroegneria).

RESULTS

The concurrent presence of Hordeum (descended from E. repens) and Dasypyrum + Aegilops (descended from E. intermedia) chromosome sets in all cytotypes of E. ×mucronata confirmed the assumed hybrid origin of the analysed plants. The following different genomic constitutions were observed for E. ×mucronata. Hexaploid plants exhibited three chromosome sets from Pseudoroegneria and one chromosome set each from Aegilops, Hordeum and Dasypyrum. Heptaploid plants harboured the six chromosome sets of the hexaploid plants and an additional Pseudoroegneria chromosome set. Nonaploid cytotypes differed in their genomic constitutions, reflecting different origins through the fusion of reduced and unreduced gametes. The hybridisation patterns of repetitive sequences (5S rDNA, 18S rDNA, and pSc119.2) in E. ×mucronata varied between and within cytotypes. Chromosome alterations that were not identified in the parental species were found in both heptaploid and some nonaploid plants.

CONCLUSIONS

The results confirmed that both homoploid hybridisation and heteroploid hybridisation that lead to the coexistence of four different haplomes within single hybrid genomes occur in Elytrigia allopolyploids. The chromosomal alterations observed in both heptaploid and some nonaploid plants indicated that genome restructuring occurs during and/or after the hybrids arose. Moreover, a specific chromosomal translocation detected in one of the nonaploids indicated that it was not a primary hybrid. Therefore, at least some of the hybrids are fertile. Hybridisation in Triticeae allopolyploids clearly and significantly contributes to genomic diversity. Different combinations of parental haplomes coupled with chromosomal alterations may result in the establishment of unique lineages, thus providing raw material for selection.

Molecular cytogenetic analysis of wheat – Elymus repens introgression lines with resistance to Fusarium head blight

Elymus repens (L.) Gould (2n = 6x = 42, StStStStHH) is a hexaploid perennial wheatgrass species from the tribe Triticeae, distantly related to bread wheat Triticum aestivum L. (2n = 6x = 42, AABBDD). As a potential source of resistance to Fusarium head blight (FHB), E. repens was crossed to common wheat to transfer resistance genes. The progeny were advanced to homozygosity by single seed descent. A total of eight BC1F9 progeny lines were selected and characterized in this study. The chromosome numbers of these derived lines ranged from 42 to 56, including lines with 44, 52, and 54 chromosomes. All of the lines were cytologically stable in terms of meiotic chromosome behavior. The univalent frequency in the lines varied between 0.34 and 2.36 per cell. Similarly, the multivalent frequency did not exceed 1% in any of the lines. GISH analysis revealed that the number of intact wheat chromosomes in the various lines varied between 40 and 44. Numerous translocated chromosomes were detected in all lines. The translocations involved chromosomal segments from wheat, and the St and H genomes of E. repens. Furthermore, trigenomic translocated chromosomes were detected in some of the lines. The introgression into wheat chromosomes included not only terminal types but also interstitial segments. The Fusarium head blight resistance of the eight lines, following point inoculation, varied from 5.65% infected florets to 11.46% compared with the check cultivars T. aestivum ‘Roblin’ and T. aestivum ‘Crocus’ at 100% and 85%, respectively.

Chromosomal characterization in native populations of Elymus scabrifolius from Argentina through classical and molecular cytogenetics (FISH–GISH)

The karyotype of Elymus scabrifolius (Döll) J.H. Hunz. (2n = 4x = 28) was investigated by DAPI staining and in situ hybridization. All the accessions studied presented a symmetric and uniform karyotype constituted by 9m+2m–sm+3sm. DAPI stain showed 1–7 conspicuous bands in all the chromosomes and polymorphisms between accessions. FISH experiments carried out with 45S rDNA as probe (pTa71) showed strong hybridization signals on the metacentric SAT-chromosome pair 8; the submetacentric SAT-chromosome pair 13 presented weaker hybridization. FISH using pSc119.2 clone as probe identified five chromosome pairs. Then, the combination of chromosome morphology, DAPI-staining, and FISH enabled the accurate identification of each chromosome pair in E. scabrifolius. Genomic in situ hybridization (GISH) experiments using Hordeum DNA as probe on mitotic metaphases confirmed unequivocally the presence of the H genome in E. scabrifolius, allowing us to observe six uniformly labeled chromosome pairs and two chromosome pairs with only one arm labeled. The remaining six chromosome pairs were weakly labeled. The rehybridization of FISH slides with Hordeum DNA as probe allow us to assign the genomic provenance of most of the chromosomes in the studied accessions. Moreover, intergenomic rearrangement was detected between genome H and the still unknown progenitor genome.

Reticulate evolution, introgression, and intertribal gene capture in an allohexaploid grass

Recent molecular phylogenetic studies of polyploid plants have successfully clarified complex patterns of reticulate evolution. In this study of Elymus repens, an allohexaploid member of the wheat tribe Triticeae, chloroplast and nuclear DNA data reveal an extreme reticulate pattern, revealing at least five distinct gene lineages coexisting within the species, acquired through a possible combination of allohexaploidy and introgression from both within and beyond the Triticeae. Earlier cytogenetic studies of E. repens suggested that Hordeum (genome H) and Pseudoroegneria (St) were genome donors to E. repens. Chloroplast DNA data presented here (from the rpoA gene and from the region between trnT and trnF) identify three potential maternal genome donors (Pseudoroegneria, Thinopyrum, and Dasypyrum), and information from previous molecular work suggests that, of these, Pseudoroegneria is the most likely maternal donor. Nuclear starch synthase gene data indicate that both Hordeum and Pseudoroegneria have contributed to the nuclear genome of E. repens, in agreement with cytogenetic data. However, these data also show unexpected contributions from Taeniatherum, and from two additional donors of unknown identity. One of the sequences of unknown origin falls within the Triticeae, but is not closely associated with any of the sampled diploid genera. The second falls outside of the clade containing Triticeae and its outgroup Bromus, suggesting the acquisition of genetic material from a surprisingly divergent source. Bias toward the amplification of certain starch synthase variants has complicated attempts to thoroughly sample from within individuals, but the data clearly indicate a complex pattern of reticulate evolution, consistent not only with allohexaploidy, but also with introgression from unexpectedly divergent sources.

Isolation, characterization, and analysis of Leymus-specific DNA sequences

Genomic Southern hybridization using labeled total genomic DNA of Leymus mollis as probe showed intense hybridization signals on all restriction enzyme digested DNA from five species of Leymus Hochst., and four species of Psathyrostachys Nevski. Experiments using the same L. mollis probe, but with unlabeled blocking DNA from Psathyrostachys, showed no hybridization at all. These two genera evidently had the same genomic content. Southern hybridization without blocking allowed identification of DNA fragments abundant in Leymus and Psathyrostachys. Fragments potentially specific to Leymus were cloned. Five repetitive DNA clones from L. mollis and L. arenarius were characterized: pLmIs1, pLmIs44, pLmIs51, pLmIs53, and pLaIs56. These clones hybridized to both Leymus and Psathyrostachys on Southern blots - no clone hybridized to only one of these genera. Both Southern blot and fluorescence in situ hybridization (FISH) experiments showed that all the clones contained dispersed repetitive sequences. They painted all and whole chromosomes uniformly except at centromeres, telomeres, and nucleolar organiser regions. Three of these clones, i.e., pLmIs1, pLmIs44, and pLmIs53, were essentially specific to Leymus and Psathyrostachys - little or no hybridization was detected in other genera such as Triticum, Hordeum, Thinopyrum, or Elymus. Sequence analysis further revealed that the clones were part of retroelements. In particular, the clone pLmIs44 produced hybridization profiles suitable for analysis of genetic relatedness among species. The present study shows that Leymus and Psathyrostachys share the same basic genome, Ns, and therefore provides strong evidence for combining these two genera.

The genome composition of hexaploid Psammopyrum athericum and octoploid Psammopyrum pungens (Poaceae: Triticeae)

The genomic constitution of two species in the genus Psammopyrum, i.e., Ps. athericum (2n = 6x = 42) and Ps. pungens (2n = 8x = 56), was studied by genomic in situ hybridization (GISH). In Ps. athericum, one diploid chromosome set hybridized to a genomic probe from Pseudoroegneria ferganensis (St genome), one diploid set to a probe from Agropyron cristatum (P genome), and one diploid set to a probe from Thinopyrum junceiforme (EbEe genomes) or Th. bessarabicum (Eb genome). Substituting the St-genome probe with an L-genome probe from Festucopsis serpentinii resulted in exactly the same hybridization pattern, suggesting a genomic constitution of EStP or ELP for Ps. athericum. The same probes used on Ps. pungens showed two diploid sets of chromosomes hybridizing to the St-genome probe, one diploid set hybridizing to the P-genome probe, and one diploid set hybridizing to the EbEe-genome probe. The L-genome probe hybridized to approximately 14 of the chromosomes that were labeled by the St-genome probe. Hence the genomic constitution for Ps. pungens is proposed to be EStStP or EStLP.

Molecular cytogenetics of introgressive hybridization in plants

Introgressive hybridization (introgression) is genetic modification of one species by another through hybridization and repeated backcrossing. Introgression is important in the evolution of flowering plants. It is also important in plant breeding where a desirable trait can be transferred from wild to crop species. One of the most recent advances in molecular techniques for studying hybridization and introgression is in situ hybridization of genomic probes to cytological preparations (GISH, genomic in situ hybridization). The present paper describes a successful GISH protocol for detection of intergenomic introgression in breeding materials and in allopolyploid species. In addition, the paper introduces a new possibility of using dispersed repeats to detect introgression and to gain insights into its molecular basis. The approach is referred to as dFISH for dispersed fluorescence in situ hybridization, and the best candidate for this type of probes is probably a retroelement. Southern hybridization data are also presented to support the effectiveness of GISH and dFISH for introgression mapping.