[Modification of crossing over frequency and mechanism of chromosomal control of exchange distribution in Drosophila females]

Abstract

Features of main recombination stages in eukaryotes-homologous pairing, cleavage of specific DNA sites and their delayed repair, formation of heteroduplexes (X-structures), resolution of X-structures by crossing over or gene conversion--and various factors affecting these stages and changing the number of X-structures and frequency of crossing-over are reviewed. Formation of X-structures is thought to result from coinciding breaks of DNA that participate in homologous synapsis. Resolution of X-structures (realization of exchanges) occurs after termination of homologous pairing in pericentromeric regions and after reorganization of meiotic chromosomes and the chromocenter, which has a regular structure in Drosophila. Resolution of X-structures and chiasma distribution along a chromosome are controlled by intrinsic elasticity generated by deformation of axial chromosomal elements (lateral elements of the synaptonemal complex). The deformation is caused by the formation of links between chromosomes in pericentromeric regions of chromosome arms (the centromeric effect) and by random formation of the first exchange and chiasma (chromosomal interference). The deformational energetic barrier prevents reciprocal crossover exchanges, and the X-structure is realized as a gene conversion. Decompacted chromosome regions suppress both the centromeric effect and interference, resulting, e.g., in the absence o f interference across the centromere. Chiasmatic halfspirals act as primers for further spiralization of chromatids.