Chiasma formation in Arabidopsis thaliana accession Wassileskija and in two meiotic mutants

Meiotic chiasmata were analysed in metaphase I pollen mother cells (PMCs) o f wild-type Arabidopsis thaliana and in two meiotic mutants. Fluorescence i n situ hybridisation (FISH) with 45S rDNA and 5S rDNA as probes was used to identify the five chromosome pairs. A wild-type chiasma frequency of 9.24 per cell was found, consistent with estimated genetic recombination values. Individual bivalent chiasma frequencies varied according to chromosome siz e; chromosome 1 had the highest mean chiasma frequency (2.14) while the sho rt acrocentric chromosomes had the lowest frequencies (1.54 and 1.56). FISH analysis was extended to two meiotic mutants (asy1 and dsy1) having low re sidual bivalent and chiasma frequencies. Mutant dsy1 gave no indication of chromosome preference for residual bivalent formation; instead it showed a general reduction in bivalent and chiasma frequencies. In asy1, the longest chromosome (1) had the lowest bivalent frequency and chiasma frequency whi le the short acrocentric: chromosome 2 had the highest frequencies. This ch romosome pair may be preferentially involved in synapsis and chiasma format ion because of their association with the nucleolus. However, other factors may be operating since the other acrocentric chromosome (4), with similar size and structure to chromosome 2, did not share these chiasma properties.