Live imaging of rapid chromosome movements in meiotic prophase I in maize

Abstract

The ability of chromosomes to move across the nuclear space is essential for the reorganization of the nucleus that takes place in early meiotic prophase. Chromosome dynamics of prophase I have been studied in budding and fission yeasts, but little is known about this process in higher eukaryotes, where genomes and chromosomes are much larger and meiosis takes a longer time to complete. This knowledge gap has been mainly caused by difficulties in culturing isolated live meiocytes of multicellular eukaryotes. To study the nuclear dynamics during meiotic prophase in maize, we established a system to observe live meiocytes inside intact anthers. We found that maize chromosomes exhibited extremely dynamic and complex motility in zygonema and pachynema. The movement patterns differed dramatically between the two stages. Chromosome movements included rotations of the entire chromatin and movements of individual chromosome segments, which were mostly telomere-led. Chromosome motility was coincident with dynamic deformations of the nuclear envelope. Both, chromosome and nuclear envelope motility depended on actin microfilaments as well as tubulin. The complexity of the nuclear movements implies that several different mechanisms affect chromosome motility in early meiotic prophase in maize. We propose that the vigorous nuclear motility provides a mechanism for homologous loci to find each other during zygonema.chromosome dynamics ͉ cytogenetics ͉ meiosis ͉ cell biology I n early meiotic prophase, the nucleus undergoes a major spatial reorganization, which includes a general repositioning of chromatin and juxtaposition of homologous chromosomes (1). In many species, including maize, the nucleolus is located in the center of the nucleus during leptonema, and at the onset of zygonema, moves to a peripheral position (1, 2). Concurrently with the nucleus migration, all chromosome ends attach to the nuclear envelope (NE) and cluster on a single site forming the ''telomere bouquet'' (3, 4), which has been observed in most plants, animals, and fungi, including budding and fission yeasts, mouse, and maize. The telomeres remain clustered throughout zygonema. When the telomeres are clustered, centromeres are oriented in the opposite direction than the telomeres, resulting in a telomere-centromere polarization of the meiocyte nucleus. The presence of the bouquet coincides with pairing of homologous chromosomes (3, 5). In plants, mammals, and fungi, chromosome pairing depends upon the progression of meiotic recombination (5-7). However, a recombination-driven homology recognition mechanism can only operate across a relatively short distance, probably Ϸ1.2 m (6). In large-genome species, such as maize, where the zygotene nucleus is Ϸ20 m in diameter, this mechanism may not be sufficient to reach across the chromatin mass in the nucleus, even when the chromosomes are brought together by the bouquet (6). These constraints suggest that homologous chromosome segments must first be positioned close to each other, before the homology search...

Chromosome Organization and Dynamics during Interphase, Mitosis, and Meiosis in Plants

“…Prophase chromosome movements in maize appear more complex in comparison to other species, as they include both coordinated chromatin rotations as well as movements of individual chromosome segments. In contrast to maize, only uncoordinated movements of individual chromosomes or chromosome segments are seen in budding yeast, while only coordinated movements of the entire chromatin have been reported in fission yeast (Schizosaccharomyces pombe) and rat (Rattus norvegicus) spermatocytes (Sheehan and Pawlowski, 2009).…”

“…In plants, studies using intact live anthers of maize showed that meiotic chromosomes exhibit complex and stage-specific motility patterns in zygotene and pachytene (Sheehan and Pawlowski, 2009). During zygotene, short chromosome segments adjacent to chromosome ends exhibit robust shortrange movements, while movements of interstitial chromosome segments are more restrained.…”

“…Analyses of meiotic prophase chromosome dynamics in a number of species, including maize, indicate that the physical forces responsible for chromosome movements are generated in the cellular cytoskeleton (Koszul and Kleckner, 2009;Sheehan and Pawlowski, 2009). From there, they are conveyed onto the nuclear envelope and then, by the virtue of telomere attachment to the nuclear envelope, further onto chromosome ends.…”

“…Live-imaging observations in a number of species of plants, animals, and fungi, most of them conducted during the past few years, have demonstrated that early stages of meiotic prophase are a period of extremely dynamic chromosome movements (Koszul and Kleckner, 2009;Sheehan and Pawlowski, 2009;Baudrimont et al, 2010). In plants, studies using intact live anthers of maize showed that meiotic chromosomes exhibit complex and stage-specific motility patterns in zygotene and pachytene (Sheehan and Pawlowski, 2009).…”

“…It has been suggested that zygotene chromosome movements may aid homologous chromosome pairing by facilitating interchromosome interactions and disrupting associations of nonhomologous chromosomes (Koszul and Kleckner, 2009;Sheehan and Pawlowski, 2009). The pachytene movements, on the other hand, may help resolve chromosome entanglements (known as interlocks) that form during chromosome pairing in zygotene.…”