Three-dimensional light microscopy and the excellent cytological featu
res of maize meiotic chromosomes are used to analyze the early events
of chromosome synapsis. We demonstrate that the chromosomes undergo a
dramatic structural reorganization prior to synapsis in zygotene. The
unique features of prezygotene chromosomes are a partial separation of
sister chromatids, an elongation of knob heterochromatin, an increase
in surface complexity, a 50% increase in total chromosome volume, and
a peripheral localization and alignment of telomeric, but not proxima
l, loci. At zygotene, chromosome volume decreases and chromosomes appe
ar as single fibers. The specialized prezygotene chromosome morphology
may facilitate homology recognition once the homologs have been broug
ht together.