. * For correspondence (fax +34 91 394 4844; e-mail pradillo@bio.ucm.es). † E. Sá nchez-Morá n and J.L. Santos acted as joint senior authors. SUMMARYThe eukaryotic recombinases RAD51 and DMC1 are essential for DNA strand-exchange between homologous chromosomes during meiosis. RAD51 is also expressed during mitosis, and mediates homologous recombination (HR) between sister chromatids. It has been suggested that DMC1 might be involved in the switch from intersister chromatid recombination in somatic cells to interhomolog meiotic recombination. At meiosis, the Arabidopsis Atrad51 null mutant fails to synapse and has extensive chromosome fragmentation. The Atdmc1 null mutant is also asynaptic, but in this case chromosome fragmentation is absent. Thus in plants, AtDMC1 appears to be indispensable for interhomolog homologous recombination, whereas AtRAD51 seems to be more involved in intersister recombination. In this work, we have studied a new AtRAD51 knock-down mutant, Atrad51-2, which expresses only a small quantity of RAD51 protein. Atrad51-2 mutant plants are sterile and hypersensitive to DNA double-strand break induction, but their vegetative development is apparently normal. The meiotic phenotype of the mutant consists of partial synapsis, an elevated frequency of univalents, a low incidence of chromosome fragmentation and multivalent chromosome associations. Surprisingly, nonhomologous chromosomes are involved in 51% of bivalents. The depletion of AtDMC1 in the Atrad51-2 background results in the loss of bivalents and in an increase of chromosome fragmentation. Our results suggest that a critical level of AtRAD51 is required to ensure the fidelity of HR during interchromosomal exchanges. Assuming the existence of asymmetrical DNA strand invasion during the initial steps of recombination, we have developed a working model in which the initial step of strand invasion is mediated by AtDMC1, with AtRAD51 required to check the fidelity of this process.

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Triticum aestivum is an allohexaploid wheat (AABBDD) that shows diploid-like behaviour at metaphase-I. This behaviour is influenced by the action of several loci, Ph1 and Ph2 being the main loci involved.To study the effect of these two loci on chromosome pairing in T. aestivum we have analysed the synaptic pattern in fully traced spread nuclei at mid-and late-zygotene, and at pachytene, of three different genotypes of cv Chinese Spring: standard line, ph1b and ph2b mutants. The analysis of the synaptic progression showed that only a few nuclei accomplish synapsis in the ph2b genotype, whereas most nuclei completed synapsis in the standard and ph1b genotypes. This result indicates that the Ph2 locus affects synaptic progression. The number of synaptonemal complex (SC) bivalents and of the different SC multivalent associations were determined in each nucleus. The mean number of lateral elements involved in SC multivalent associations (LEm) at midzygotene was relatively high and showed similar values in the three genotypes. These values decreased progressively between mid-zygotene and pachytene in the genotypes with the Ph1 locus because of the transformation of multivalents into bivalents. In the ph1b genotype, this value only decreased between late-zygotene and pachytene. Therefore, multivalent correction was more efficient in the presence than in the absence of the Ph1 locus. It is concluded that the Ph1 and Ph2 loci bring about diploidization of allohexaploid wheat via a different mechanism.

RFLP analysis of 105 doubled haploid lines from a cross between the barley varieties 'Magnum' and 'Goldmarker' located the denso dwarfing gene on the long arm of chromosome 3(3H), approximately 8 cM distal to the RFLP locus XpsrUO. Lines with the denso gene showed a distinctive prostrate juvenile growth habit and tended to have later ear emergence times and lower plant grain weights, ear grain weights and 50 grain weights.

A spreading technique was used to perform a structural analysis of prophase I nuclei in pollen mother cells (PMCs) of wild-type Arabidopsis thaliana. In leptotene, all chromosomes developed fully axial elements before a presynaptic alignment was observed. Pairing and synapsis start in regions close to the telomeres at early zygotene. Interstitial synaptonemal complex (SC) stretches were found to occur at several sites per bivalent at mid zygotene. Within individual bivalents, extensive regions of SC formation often existed at the same time as other extensive regions that were unsynapsed. Also in the same nucleus, one bivalent might have several SC segments, while other bivalents have only a few. The classical bouquet was not so evident as in other plant species. Length measurements of the five pachytene bivalents have allowed the elaboration of a pachytene karyotype. Pachytene chromatin compaction in Arabidopsis was significantly less than that observed in the other species analysed and this is paralleled with a higher recombination rate (centimorgans per megabase).