Homologue Pairing and Synaptic Behavior at Zygotene in Maize

A new fixation technique has permitted more detailed light microscope observations of the leptotene to pachytene meiotic progression of maize microsporocyte chromosomes than has been possible following use of traditional fixatives. With use of this new procedure, squash preparation produces excellent spreading of chromosomes at stages where the tightness of the synizetic knot has previously prohibited analysis. Using this technique, a novel finding was that at zygotene stage, extensive homologue alignment, which can be either loose or closely parallel, but at greater than synaptic distance, apparently precedes synapsis. Synaptic initiation is frequently intercalary and there may be a number of sites of initiation per bivalent. Telomeres and conspicuous heterochromatic regions are often among the last parts to synapse. The clear alignment of homologues in advance of synapsis is regarded as strongly suggestive of the existence of multiple chromosome sites which are active at homologue pairing and function in advance of synapsis.