HOMOLOG PAIRING - INITIATION SITES AND EF FECTS ON CROSSING-OVER AND CHROMOSOME SEGREGATION IN DROSOPHILA-MELANOGASTER

The role of homologue pairing and chromocentral association of chromos omes in recombination and segregation during cell division is discusse d. Peculiarities of mitotic and meiotic chromosome pairing in Drosophi la males and females are considered. On the basis of our own and publi shed data, the presence and localization of sites of homologue pairing initiation in euchromatin are substantiated. The effects of transfer of initiation sites along a chromosome (exemplified by inversions) on chromosome pairing (asynapsis), crossing over (intrachromosomal, inter chromosomal, and centromeric effects), and segregation are discussed. To record the effects of pairing sites on crossing over, a method of c omparing crossing-over frequencies in an inverted region with those in a region of the same size and position with regard to the centromere on cytological maps was proposed. Chromosomes orient toward opposite d ivision poles during paracentromeric heterochromatin pairing, This occ urs after successful euchromatin pairing, during which the chromocentr al circular structure is reorganized. If heterochromatin pairing is di srupted because of structural or locus mutations, nonexchange bivalent s segregate randomly. In this case, chromosome coordination may occur due to proximal chiasmata or chromocentral associations between homolo gues.