Comparative Transcriptomics Reveals 129 Transcripts That Are Temporally Regulated during Anther Development and Meiotic Progression in Both Bread Wheat (Triticum aestivum) and Rice (Oryza sativa)

Meiosis is a specialised type of cell division in sexually reproducing organisms that generates genetic diversity and prevents chromosome doubling in successive generations. The last decade has seen forward and reverse genetic approaches identifying many genes in the plant kingdom which highlight similarities and differences in the mechanics of meiosis between taxonomic kingdoms. We present here a high throughput in silico analysis, using bread wheat and rice, which has generated a list of 129 transcripts containing genes with meiotic roles and some which are currently unknown

Microarray expression analysis of meiosis and microsporogenesis in hexaploid bread wheat

BACKGROUND: Our understanding of the mechanisms that govern the cellular process of meiosis is limited in higher plants with polyploid genomes. Bread wheat is an allohexaploid that behaves as a diploid during meiosis. Chromosome pairing is restricted to homologous chromosomes despite the presence of homoeologues in the nucleus. The importance of wheat as a crop and the extensive use of wild wheat relatives in breeding programs has prompted many years of cytogenetic and genetic research to develop an understanding of the control of chromosome pairing and recombination. The rapid advance of biochemical and molecular information on meiosis in model organisms such as yeast provides new opportunities to investigate the molecular basis of chromosome pairing control in wheat. However, building the link between the model and wheat requires points of data contact. RESULTS: We report here a large-scale transcriptomics study using the Affymetrix wheat GeneChip(® )aimed at providing this link between wheat and model systems and at identifying early meiotic genes. Analysis of the microarray data identified 1,350 transcripts temporally-regulated during the early stages of meiosis. Expression profiles with annotated transcript functions including chromatin condensation, synaptonemal complex formation, recombination and fertility were identified. From the 1,350 transcripts, 30 displayed at least an eight-fold expression change between and including pre-meiosis and telophase II, with more than 50% of these having no similarities to known sequences in NCBI and TIGR databases. CONCLUSION: This resource is now available to support research into the molecular basis of pairing and recombination control in the complex polyploid, wheat

Whole genome approaches to identify genes involved in early meiosis.

Meiosis is a process which occurs in sexually reproducing organisms to halve the genetic complement prior to fertilisation. During meiosis a single round of DNA replication is followed by two successive rounds of chromosome segregation and cell division. The meiotic pathway in plants is complex from multiple perspectives. From a mechanical view; prior to the first meiotic division the chromosomes must replicate during meiotic interphase, then while retaining sister chromatid cohesion the homologous chromosomes must align, physically synapse and also concomitantly recombine (with the majority of sites being non-randomly positioned). Further complexities arise in allopolyploids such as bread wheat, which contains three very similar genomes from slightly diverged progenitors. Despite having homoeologous chromosomes present in the same nucleus, bread wheat displays diploid-like behaviour during meiosis I. Such an involved physical process as meiosis also has complexity reflected in the transcriptome and proteome, whether the organism be a simple eukaryote such as yeast, or a more complex eukaryote such as bread wheat. Initially, this study utilised whole genome approaches to identify novel genes that could be involved in early meiosis, focusing on bread wheat in particular. Analysis of the wheat meiotic transcriptome over seven stages of anther development identified at least 1,350 transcripts which displayed meiotic regulation. The expression profiles of a subset of selected transcripts were analysed with Q-PCR and found to correlate strongly to those obtained in the microarray. Available meiotic transcriptome data from rice was compared to the wheat data, which enabled the identification of similar sequences, many previously unidentified, which also displayed meiotic regulation. Selected candidate genes from the microarray study were also mapped in bread wheat. This data was combined with available literature and approximately 70% of candidate meiotic loci were located on chromosome group 3 or 5, which historically has been shown to contain multiple loci involved in chromosome pairing control. One of the candidates located on chromosome group 3, a plant-specific mismatch repair gene, Triticum aestivum MSH7 (TaMSH7), has previously been speculated to suppress homoeologous chromosome associations. Independent transgenic wheat plants produced using RNA interference (RNAi) were functionally characterised to ascertain a greater understanding of the role TaMSH7 has during early meiosis in bread wheat. Localisation of a synaptonemal complex-associated protein (TaASY1) displayed subtle abnormalities in these mutants when compared to wild-type. Feulgen staining of meiotic chromosomes at metaphase I in these mutants revealed some interlocking and multivalent associations. These results suggest that TaMSH7 may be linked to the mechanism underlying the phenotype that is observed in the ph2a/ph2b mutant, however further research still needs to be conducted to conclusively demonstrate that this is the case. A component of the research presented in this study was performed in the model plant Arabidopsis thaliana due to the limitations of bread wheat. Extensive mutant banks and a sequenced genome have aided a decade of meiotic research in Arabidopsis and the identification of close to 50 meiotic genes. One of these, AtMER3, has been shown to control the non-random location of well above half of the recombination events that occur in many species. AtMER3 was localised in meiotic nuclei in wild-type Arabidopsis and found to form foci on freshly synapsed regions of chromosomes in quantities far in excess of the average number of crossovers, indicating that AtMER3 does not localise exclusively to sites of crossovers. AtMER3 localisation was also analysed in several mutant backgrounds and found to act in an AtSPO11-dependent manner. However, AtMER3 loading onto meiotic chromosomes was not affected in Atrad51, Atdmc1 or Atmsh5 mutant backgrounds.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 200

What limits meiotic crossovers?

Comment on: Crismani W, et al. Science 2012; 336:1588-90; PMID:22723424; http://dx.doi.org/10.1126/science.1220381absen

Interference des crossovers: il n'y a qu'à ZYPer

International audienc

MCM8 is required for a pathway of meiotic double-strand break repair independent of DMC1 in Arabidopsis thaliana

Mini-chromosome maintenance (MCM) 2-9 proteins are related helicases. The first six, MCM2-7, are essential for DNA replication in all eukaryotes. In contrast, MCM8 is not always conserved in eukaryotes but is present in Arabidopsis thaliana. MCM8 is required for 95% of meiotic crossovers (COs) in Drosophila and is essential for meiosis completion in mouse, prompting us to study this gene in Arabidopsis meiosis. Three allelic Atmcm8 mutants showed a limited level of chromosome fragmentation at meiosis. This defect was dependent on programmed meiotic double-strand break (DSB) formation, revealing a role for AtMCM8 in meiotic DSB repair. In contrast, CO formation was not affected, as shown both genetically and cytologically. The Atmcm8 DSB repair defect was greatly amplified in the absence of the DMC1 recombinase or in mutants affected in DMC1 dynamics (sds, asy1). The Atmcm8 fragmentation defect was also amplified in plants heterozygous for a mutation in either recombinase, DMC1 or RAD51. Finally, in the context of absence of homologous chromosomes (i.e. haploid), mutation of AtMCM8 also provoked a low level of chromosome fragmentation. This fragmentation was amplified by the absence of DMC1 showing that both MCM8 and DMC1 can promote repair on the sister chromatid in Arabidopsis haploids. Altogether, this establishes a role for AtMCM8 in meiotic DSB repair, in parallel to DMC1. We propose that MCM8 is involved with RAD51 in a backup pathway that repairs meiotic DSB without giving CO when the major pathway, which relies on DMC1, fails