The temporal sequence of synaptic initiation, crossing over and synaptic completion.

Questions are raised as to the validity of arguments that crossover positions have been demonstrated to be normally established only during pachytene (after synapsis is maximal). An alternative and testable hypothesis is that crossover commitment can occur at events of synaptic initiation.-Measurements are presented of extents of pachytene synapsis and failure in and around a region of maize chromosome heterozygous for a short paracentric inversion, and these are compared to conjectured expectations from observations of crossover frequencies within the inversion. Various hypotheses consistent with the results are considered. It is pointed out that the hypothesis that increases in crossover frequency in the synapsed region of the inversion are compensatory to crossover inhibitions elsewhere requires complex assumptions: that the adjustment must take place among, not within cells and that the enhancement is preferentially expressed within the inversion instead of elsewhere in the genome. The hypothesis that the fixing and squashing procedure forces apart non-crossover regions previously synapsed but lacking a crossover also requires complex assumptions. The simplest hypothesis proposes that crossover commitment may determine synaptic expression. A role of the synaptonemal complex in the establishment of crossover sites is questioned or minimized.-Evidence is also presented with respect to conceivable function of the telomere in synaptic initiation. Restrictions on such a function, if it exists, seem to be required to account for the observations.