The desynaptic mutant of maize as a combined defect of synaptonemal complex and chiasma maintenance

Abstract

The phenotype of the desynaptic (dy) mutant of maize in microsporocytes at meiotic prophase was compared with normal microsporocytes of a closely related strain and with microsporocytes of a maize inbred line (KYS) assumed to be normal. Strikingly more univalents and open arms of bivalents were found in the mutant cells than in normal cells at diakinesis, and where there was heterozygosity for a distal knob (heterochromatic region), separation was usually equational, indicating the occurrence of normal crossing-over followed by failure of chiasma maintenance in the mutant. Differences found in the mutant by electron microscopy were a statistically significant wider dimension of the synaptonemal complex central region and also less twisting of synapsed configurations at pachytene. It is suggested that these are side-effect symptoms of a defect in the synaptonemal complex (or associated substance), which is expressed later as sporadic loss of chiasma maintenance.

The maize (Zea mays) desynaptic (dy) mutation defines a pathway for meiotic chromosome segregation, linking nuclear morphology, telomere distribution and synapsis

“…2B,E) but not for dy, where persistent ASY1 staining is believed to indicate incomplete synapsis leading to asynapsis. This view differs slightly from previously published descriptions of the mutant, in which the homologs synapse and exhibit a normal pachytene but fail to maintain chiasmata (Maguire et al, 1991), unless complete synapsis in dy is followed by partial desynapsis and ASY1 reloading.…”

“…First described in 1952, dy is a mutant characterized by a partial male semi-sterility phenotype in which bivalent desynapsis leads to univalents by late prophase. Subsequently, others have shown dy lines of maize have a defect in chiasma maintenance (Maguire et al, 1991), recombination (Ji et al, 1999), and are prone to telomere-NE interaction defects at mid-prophase . Given these prior observations, we reasoned that a detailed 3D cytological analysis of dy would provide insights into the complex mechanisms that link telomere dynamics to chromosome behavior during meiosis, especially at midprophase when synapsis and recombination go to completion.…”

“…Disruption of interlock resolution could leave physical barriers to the formation of complete synaptonemal complexes, such as those described by (Maguire et al, 1991). Exactly how the completion of synapsis is monitored by the cell in maize meiosis remains unknown, but incomplete synapsis could trigger checkpoints or affect feedback loops.…”

“…The chromosomes in dy appear normal at the pachytene stage of male meiotic prophase, but many of the bivalents separate into univalents by late prophase. In previous studies, the chromosomes at the pachytene stage of prophase I appear fully synapsed at first, but they exhibit extensive precocious desynapsis and chiasma-maintenance failure (Maguire et al, 1991;Maguire et al, 1993). Others have shown dy to affect the control of meiotic recombination (Ji et al, 1999).…”

Induced Desynaptic Male Sterile Lines in Soybean

“…The latter seems to rely on the activity of the synaptonemal complex (SC) central region. Recent studies demonstrating that desynaptic plants have a defective SC which led Maguire et al (1991Maguire et al ( , 1993 to suggest that the substance binding sister chromatids is derived from the SC. A multisubunit complex of conserved protein called cohesions is thought to provide longitudinal links along the chromatids and maintaining the chiasma in place (Bhatt et al 1999, Nasmyth et al 2000.…”