Chromosome pairing in maize.

This report summarizes our observations at pachytene on opposite-arms intercrosses between stocks of interchanges that involve chromosomes 1 and 5 in maize.-Pairing does not begin at the centromeres in these intercrosses.-We propose a model which assumes different probability values along each chromosome arm for the initial or primary site of pairing. Observations on the frequencies of the different types of configurations at pachytene were used to estimate probability values which satisfactorily fit the data.-There is a relatively low probability (of the order of.1 to.3) for the initial pairing to be in a short terminal segment (about.1 of the arm length). Initial pairing in the one or two short segments adjacent to the tip segment is much higher. Initial pairing is much lower in segments successively closer to the middles of the chromosome arms, and then zero or nearly zero in the proximal half of the arm. This means that the initial pairing may fail occasionally even in a relatively long interchanged segment and produce a T-shaped (3-armed) configuration.-After the initial pairing has occurred, the average probability that a secondary site of pairing is adjacent to the centromere in a segment.3 to.4 the length of an arm is low (.13, ranging from.02 to.29).-We can predict that in an intercross in which both breakpoints in both parental interchanges are far out on the chromosomes, "pairs" will be formed with nonhomologous ends (homologous differential segments paired). In these pairing could have begun at any point in the interstitial segments, but not likely in segments close to the centromeres.-Multiple secondary sites which vary in time or in order of pairing will explain the variation in position of the cross-shaped pachytene configuration in interchange heterozygotes.-The observed configuration in any one cell is the result of a particular combination of pairing events at the various sites. This is a very different concept of pairing from previous interpretations which described it as a result of zipper-like action, and the variation in position of the pachytene cross-configuration as the result of "shifts" in position.-Our cytogenetic results and their interpretation are in close agreement with reports on chromosome ultrastructure and molecular events in the early stages of meiosis, i.e. the attachment of chromosome ends to the nuclear membrane, the manner in which synaptonemal complexes develop, and the regions of DNA whose replication is delayed until zygonema.