The proper pairing, recombination, and segregation of chromosomes are central to meiosis and sexual reproduction. The syn1 mutation was previously identified as a synaptic mutant in a T-DNA-tagged population of plants. SYN1 has been isolated and found to exhibit similarity to Schizosaccharomyces pombe RAD21 and RAD21-like proteins, which are required for chromosome condensation and sister chromatid cohesion during mitosis. Plants homozygous for syn1 are male and female sterile and show defects in chromosome condensation and pairing beginning at leptonema of mei-osis I. Fragmentation of the chromosomes was observed at metaphase I. Alternative promoters produced two SYN1 transcripts. One transcript was expressed at low levels in most tissues, whereas the other was expressed only in pre-bolting buds. DNA blot analyses suggest that Arabidopsis contains a small RAD21 gene family. Consistent with the DNA blot data, a second Arabidopsis RAD21-like gene has been identified. These results suggest that different RAD21-like proteins play essential roles in chromosome condensation and pairing during both meiosis and mitosis.

The proper pairing, recombination, and segregation of chromosomes are central to meiosis and sexual reproduction. The syn1 mutation was previously identified as a synaptic mutant in a T-DNA-tagged population of plants. SYN1 has been isolated and found to exhibit similarity to Schizosaccharomyces pombe RAD21 and RAD21-like proteins, which are required for chromosome condensation and sister chromatid cohesion during mitosis. Plants homozygous for syn1 are male and female sterile and show defects in chromosome condensation and pairing beginning at leptonema of meiosis I. Fragmentation of the chromosomes was observed at metaphase I. Alternative promoters produced two SYN1 transcripts. One transcript was expressed at low levels in most tissues, whereas the other was expressed only in prebolting buds. DNA blot analyses suggest that Arabidopsis contains a small RAD21 gene family. Consistent with the DNA blot data, a second Arabidopsis RAD21 -like gene has been identified. These results suggest that different RAD21-like proteins play essential roles in chromosome condensation and pairing during both meiosis and mitosis. INTRODUCTIONThe normal segregation of chromosomes during meiosis in most eukaryotic organisms is dependent on the successful pairing of homologous chromosomes during the zygotene stage of prophase I. After DNA replication, each chromosome enters meiosis as two chromatids. During leptonema of meiotic prophase I, the dispersed chromosomes condense to form long thin threads. During zygonema, homologous chromosomes align, and the synaptonemal complex, an elaborate proteinaceous structure that holds homologs closely apposed along their lengths, is formed. At pachynema, nonsister chromatids of the paired chromosomes recombine, forming chiasmata, which become visible as the chromosomes start to desynapse during diplonema. During meiosis I, sister chromosomes must be attached so that they can orient properly and achieve reductive division. During anaphase I, sister chromosome cohesion is released in preparation for the second meiotic division. Therefore, the formation and proper maintenance of sister chromatid cohesion are essential for the proper alignment and segregation of chromosomes during meiosis (Miyazaki and Orr-Weaver, 1994).A considerable amount of information is available on the events associated with meiosis in a number of eukaryotic systems, including plants (Dawe, 1998). However, much less is known about the genes required during meiosis in plants than in several other systems, including yeast (Roeder, 1995), Drosophila (Orr-Weaver, 1995), and Caenorhabditis elegans (Zetka and Rose, 1995). Based on similarities to meiotic genes identified in other organisms, several genes expressed during meiosis have been identified in Arabidopsis (Sato et al., 1995;Klimyuk and Jones, 1997), lily (Kobayashi et al., 1994), and wheat (Ji and Langridge, 1994). In addition, meiotic mutants have been identified in numerous plants species, including Arabidopsis (Dawson et al., 1993;Chaudhury et al., 1994;He et al., 1996...

Fluorescence microscopy was used to study meiosis in microsporocytes from 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. thaliana microsporocytes undergo simultaneous cytokinesis in which both meiotic divisions are completed prior to cytokinesis. However, two unique events were observed in wild-type A. thaliana that have not been reported in other angiosperms: (1) polarization of the microsporocyte cytoskeleton 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 since 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 cytologically characterized. The first observable defect occurs in microsporocytes at telophase I, where some chromosomes are scattered throughout the cytoplasm, usually attached to stray microtubules. Subsequent development stages are affected, leading to complete male sterility. Based on similarities to synaptic mutants that have been described in other species, it is suggested that this mutant is defective in synaptonemal complex formation and/or cohesion between sister chromatids.

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