Nonrandom chromosome variation and morphogenic potential in cell lines of bread wheat (Triticum aestivum L.)

Identification, characterization, and rescue of CRISPR/Cas9 generated wheat SPO11-1 mutants

Increasing crop yields through plant breeding is time consuming and laborious, with the generation of novel combinations of alleles being limited by chromosomal linkage blocks and linkage-drag. Meiotic recombination is essential to create novel genetic variation via the reshuffling of parental alleles. The exchange of genetic information between homologous chromosomes occurs at crossover (CO) sites but CO frequency is often low and unevenly distributed. This bias creates the problem of linkage-drag in recombination 'cold' regions, where undesirable variation remains linked to useful traits. In plants, programmed meiosis-specific DNA double-strand breaks, catalysed by the SPO11 complex, initiate the recombination pathway, although only ~5% result in the formation of COs. To study the role of SPO11-1 in wheat meiosis, and as a prelude to manipulation, we used CRISPR/Cas9 to generate edits in all three SPO11-1 homoeologues of hexaploid wheat. Characterization of progeny lines shows plants deficient in all six SPO11-1 copies fail to undergo chromosome synapsis, lack COs and are sterile. In contrast, lines carrying a single copy of any one of the three wild-type homoeologues are phenotypically indistinguishable from unedited plants both in terms of vegetative growth and fertility. However, cytogenetic analysis of the edited plants suggests that homoeologues differ in their ability to generate COs and in the dynamics of synapsis. In addition, we show that the transformation of wheat mutants carrying six edited copies of SPO11-1 with the TaSPO11-1B gene, restores synapsis, CO formation, and fertility and hence opens a route to modifying recombination in this agronomically important crop.

45S rDNA regions are chromosome fragile sites expressed as gaps in vitro on metaphase chromosomes of root-tip meristematic cells in Lolium spp

BACKGROUND

In humans, chromosome fragile sites are regions that are especially prone to forming non-staining gaps, constrictions or breaks in one or both of the chromatids on metaphase chromosomes either spontaneously or following partial inhibition of DNA synthesis and have been well identified. So far, no plant chromosome fragile sites similar to those in human chromosomes have been reported.

METHODS AND RESULTS

During the course of cytological mapping of rDNA on ryegrass chromosomes, we found that the number of chromosomes plus chromosome fragments was often more than the expected 14 in most cells for Lolium perenne L. cv. Player by close cytological examination using a routine chromosome preparation procedure. Further fluorescent in situ hybridization (FISH) using 45S rDNA as a probe indicated that the root-tip cells having more than a 14-chromosome plus chromosome fragment count were a result of chromosome breakage or gap formation in vitro (referred to as chromosome lesions) at 45S rDNA sites, and 86% of the cells exhibited chromosome breaks or gaps and all occurred at the sites of 45S rDNA in Lolium perenne L. cv. Player, as well as in L. multiflorum Lam. cv. Top One. Chromatin depletion or decondensation occurred at various locations within the 45S rDNA regions, suggesting heterogeneity of lesions of 45S rDNA sites with respect to their position within the rDNA region.

CONCLUSIONS

The chromosome lesions observed in this study are very similar cytologically to that of fragile sites observed in human chromosomes, and thus we conclude that the high frequency of chromosome lesions in vitro in Lolium species is the result of the expression of 45S rDNA fragile sites. Possible causes for the spontaneous expression of fragile sites and their potential biological significance are discussed.

Gross chromosome rearrangements are occurring in an accession of the grass Lolium rigidum

Chromosome structure was analysed at mitosis in root tip meristems of eight genotypes of Lolium rigidum. FISH revealed changed positions in the rDNA sites indicating extensive chromosome rearrangements; indeed no two genotypes were the same. In one genotype, there were differences between cells within individual root tips. The changed positions of the rDNA sites appear to be reflections of chromosome translocations and this was confirmed by the presence of quadrivalents at metaphase I of meiosis. Possible mechanisms are discussed for this exceptional level of chromosome instability.

Molecular cytogenetic analysis of intergeneric chromosomal translocations between wheat (Triticum aestivum L.) and Dasypyrum villosum arising from tissue culture

Fluorescence in situ hybridization (FISH) was applied with total genomic DNA extracted from Dasypyrum villosum (L.) Candargy as a probe to characterize chromosome translocations arising from tissue culture in hybrids of Triticum aestivum x (T. durum - D. villosum, amphiploid). Chromosome translocations between wheat and D. villosum occurred in callus cells at an average frequency of 1.9%. Translocations existed not only in callus cells but also in regenerants. Three plants with translocation chromosomes were characterized among 66 regenerants of T. aestivum 'Chinese Spring' x 'TH1W' and 'NPFP' x 'TH1'. One of them proved to be a reciprocal translocation with an exchange of about one third of a wheat chromosome arm with about one half of a chromosome arm of D. villosum. The breakpoints of the other two translocations were located at, or near centromeres. The results are similar for both callus cells and regenerants and provide further evidence that translocations take place in tissue culture. Other structural chromosomal changes, for example, fragments, telocentrics, dicentromeres, and deletions, as well as numerical alterations including aneuploidy and polyploidy were recorded both in callus cells and regenerants.