Homolog pairing and segregation in Drosophila meiosis

Pairing of homologous chromosomes is fundamental to their reliable segregation during meiosis I and thus underlies sexual reproduction. In most eukaryotes homolog pairing is confined to prophase of meiosis I and is accompanied by frequent exchanges, known as crossovers, between homologous chromatids. Crossovers give rise to chiasmata, stable interhomolog connectors that are required for bipolar orientation (orientation to opposite poles) of homologs during meiosis I. Drosophila is unique among model eukaryotes in exhibiting regular homolog pairing in mitotic as well as meiotic cells. I review the results of recent molecular studies of pairing in both mitosis and meiosis in Drosophila. These studies show that homolog pairing is continuous between pre-meiotic mitosis and meiosis but that pairing frequencies and patterns are altered during the mitotic-meiotic transition. They also show that, with the exception of X-Y pairing in male meiosis, which is mediated specifically by the 240-bp rDNA spacer repeats, chromosome pairing is not restricted to specific sites in either mitosis or meiosis. Instead, virtually all chromosome regions, both heterochromatic and euchromatic, exhibit autonomous pairing capacity. Mutations that reduce the frequencies of both mitotic and meiotic pairing have been recently described, but no mutations that abolish pairing completely have been discovered, and the genetic control of pairing in Drosophila remains to be elucidated.