Integration of chromosome synapsis and recombination by AtZYP1 during Arabidopsis meiosis

Meiosis occupies a central role in the life cycles of all sexually reproducing eukaryotes. An understanding of this process is critical to furthering research on reproduction, fertility, genetics and breeding. Meiosis is a specialized form of cell-division during which a single round of DNA replication is followed by two cell-divisions thereby reducing the chromosome content from diploid to haploid. Accurate segregation of homologous chromosomes at the first meiotic division is dependent on the formation of physical connections, known as chiasmata, between homologous chromosome pairs (homologues). Chiasmata arise from homologous recombination during prophase I of meiosis and are the physical manifestation of genetic crossovers. In their absence the homologues segregate at random leading to the formation of aneuploid gametes following the separation of the sister chromatids at the second meiotic division. In the past few years it has emerged that the key events of meiosis, chromosome pairing and synapsis and homologous recombination that occur during prophase I of meiosis are highly integrated. Also, it is increasingly clear that many of the key decisions in the control of meiotic recombination occur in early prophase I at the time of the transition from leptotene to zygotene when homologous chromosome synapsis is initiated. Synapsis is mediated by the synaptonemal complex (SC), a structurally conserved, tripartite proteinaceous complex. The SC was discovered in 1956 and has subsequently been found in the majority of eukaryotes. It is comprised of two lateral elements running in parallel the length of the homologous chromosome pairs. These are separated by a distance of 100nm and are linked by transverse filaments that interdigitate to span the central region in a 'zipper-like' arrangement. Remarkably, the function of the SC remains a mystery. However, recent studies in budding yeast and our own in Arabidopsis point to a strong link between initiation of chromosome synapsis and the control of the recombination pathway. In this proposal it is our objective to elucidate the nature of this relationship in Arabidopsis. We have recently isolated ZYP1 the first SC protein to be identified in higher plants. It encodes the transverse filament of the SC. We have discovered that in the absence of ZYP1 normal recombination is perturbed such that inappropriate recombination events occur between non-homologous chromosomes. Hence accurate chromosome segregation at the first meiotic division is impossible. In this project we aim to establish how the ZYP1 protein prevents non-homologous recombination. Does it directly interact with the early recombination machinery to ensure the fidelity of recombination interactions? Or perhaps it affects chromosome dynamics in early prophase I, since is increasingly clear that there is a close relationship between the recombination pathway and chromosome organization throughout meiosis. We anticipate that these studies will establish for the first time a SC function beyond a simple structural role and provide further evidence that the biochemical processes that drive recombination are closely co-ordinated by chromosome dynamics

We have identified a ZYP1 a gene that encodes the transverse filaments of the Arabidopsis synaptonemal complex (SC). This is the first SC protein to be identified in higher plants. Analysis of ZYP1 deficient lines have revealed a novel link between the recombination pathway and chromosome synapsis. In the absence of the protein prophase I progression is substantially delayed and that crossover (CO) formation is reduced to ~70-80% of wild type. Analysis reveals that a majority of cells contain univalent chromosomes, reflecting the overall reduction on CO frequency or loss of the ability to specify the first (obligatory) CO. Some CO formation can occur as normal between homologous chromosomes, but recombination also occurs between non-homologous chromosomes. Extensive multivalent formation also occurs and there is evidence of chromosome interlocks. These observations are of considerable significance as they reveal that in addition to its function in SC biogenesis ZYP1 has an important role in ensuring the fidelity of meiotic recombination. The aim of this proposal is to explore how the recombination pathway and chromosome synapsis are integrated. We will investigate the interaction of ZYP1 and the recombination machinery to determine the chronology of ZYP1 expression and establish the DNA processing events that are required for localization of the protein. We will characterize interactions of ZYP1 with the recombination machinery and/or components of the chromosome axes. We will determine if the regulation of meiosis in ZYP1 deficient plants is subject to an effect of temperature such that the crossover/non-crossover decision is disrupted. The role of ZYP1 in ensuring the fidelity recombination will be investigated. We will determine the nature of these inappropriate interactions and establish this due to an effect on chromosome dynamics at the leptotene/zygotene transition or if ZYP1 actively dissociates inappropriate chromosomal associations.