A DEFECT IN SYNAPSIS CAUSES MALE-STERILITY IN A T-DNA-TAGGED ARABIDOPSIS-THALIANA MUTANT

Fluorescence microscopy was used to study meiosis in microsporocytes f rom wild-type Arabidopsis thaliana and a T-DNA-tagged meiotic mutant. Techniques for visualizing chromosomes and beta-tubulin in other plant species were evaluated and modified in order to develop a method for analyzing meiosis in A. thaliana anthers. Like most dicots, A. thalian a microsporocytes undergo simultaneous cytokinesis in which both meiot ic divisions are completed prior to cytokinesis. However, two unique e vents were observed in wild-type A. thaliana that have not been report ed in other angiosperms: (1) polarization of the microsporocyte cytosk eleton during prophase I prior to nuclear envelope breakdown, and (2) extensive depolymerization of microtubules just prior to metaphase II. The first observation could have implications regarding a previously uncharacterized mechanism for determining the axis of the metaphase I spindle during microsporogenesis. The second observation is peculiar s ince microtubules are known to be involved in chromosome alignment in other species; possible explanations will be discussed. A T-DNA-tagged meiotic mutant of A. thaliana (syn1), which had previously been shown to produce abnormal microspores with variable DNA content, was also c ytologically characterized. The first observable defect occurs in micr osporocytes at telophase I, where some chromosomes are scattered throu ghout the cytoplasm, usually attached to stray microtubules. Subsequen t developmental stages are affected, leading to complete male sterilit y. Based on similarities to synaptic mutants that have been described in other species, it is suggested that this mutant is defective in syn aptonemal complex formation and/or cohesion between sister chromatids.