Dissecting plant meiosis using Arabidopsis thaliana mutants.

Meiosis is a key stage in the life cycle of all sexually reproducing eukaryotes. In plants, specialized reproductive cells differentiate from somatic tissue. These cells then undergo a single round of DNA replication followed by two rounds of chromosome division to produce haploid cells that then undergo further rounds of mitotic division to produce the pollen grain and embryo sac. A detailed cytological description of meiosis has been built up over many years, based on studies in a wide range of plants. Until recently, comparable molecular studies have proved too challenging, however, a number of groups are beginning to use Arabidopsis thaliana to overcome this problem. A range of meiotic mutants affecting key stages in meiosis have been identified using a combination of screening for plants exhibiting reduced fertility and, more recently, using a reverse genetics approach. These are now providing the means to identify and characterize the activity of key meiotic genes in flowering plants.

Asy1, a protein required for meiotic chromosome synapsis, localizes to axis-associated chromatin in Arabidopsis and Brassica.

The Arabidopsis thaliana ASY1 gene is essential for homologous chromosome synapsis. Antibodies specific to Asy1 protein and its homologue BoAsy1 from the related crop species Brassica oleracea have been used to investigate the temporal expression and localization of the protein in both species. Asy1 is initially detected in pollen mother cells during meiotic interphase as numerous punctate foci distributed over the chromatin. As leptotene progresses the signal appears to be increasingly continuous and is closely associated with the axial elements but not to the extended chromatin loops associated with them. By the end of zygotene the signal extends almost the entire length of the synapsed homologues, although not to the telomeres. The protein begins to disappear as the homologues desynapse, until by late diplotene it is no longer associated with the chromosomes. Immunogold labelling in conjunction with electron microscopy established that Asy1 localizes to regions of chromatin that associate with the axial/lateral elements of meiotic chromosomes rather than being a component of the synaptonemal complex itself. These data together with the previously observed asynaptic phenotype of the asy1 mutant suggest that Asy1 is required for morphogenesis of the synaptonemal complex, possibly by defining regions of chromatin that associate with the developing synaptonemal complex structure.