Presynaptic events in meiocytes of Lilium longiflorum and their relation to crossing-over: a preselection hypothesis.

We are proposing a "Preselection Hypothesis" to account for the regulation of crossing-over in eukaryotic organisms. The hypothesis characterized meiosis in terms of three major physiological stages: (1) a presynaptic stage when pairs of homologous DNA stretches are selected so as to become trapped within the synaptinemal complex during synapsis, (2) an alignment of homologous chromosomes and stabilization of paired bivalents via the synaptinemal complex, and (3) a scission and rejoining of DNA stretches leading to the formation of chiasmata and crossovers. The hypothesis centers on the first stage and is based on evidence for the occurrence of significant cytological and biochemical changes prior to synapsis. The major feature of the hypothesis is that crossing-over occurs only in trapped DNA stretches. Thus, potential crossing-over sites, though not crossing-over itself, are determined well before chromosomes pair. Since, to a large degree, crossovers are distributed randomly along the length of each chromosome, the preselection process must result in a random assortment of trapped DNA stretches, the assortment differing from one meiocyte to another.