The bond between sisters

Abstract

The accurate segregation of chromosomes in mitosis is ensured by the association of sister chromatids from S phase until their stable orientation toward opposite poles of the metaphase spindle. Only at the onset of anaphase is the cohesion between sister chromatids released, allowing them to segregate from one another. Sister chromatid cohesion is likewise essential during meiosis, but in a more complicated manner. In meiosis, cohesion must be maintained in pericentromeric regions throughout the first (reductional) division, when homologous chromosomes segregate from one another, until anaphase of the second (equational) division, when sister chromatids segregate from one another. Moreover, if crossing over has occurred between homologous chromosomes, as it does in most meioses, cohesion must be maintained along the chromosome arms to stabilize chiasma position (Darlington, 1932; Maguire, 1974). This cohesion along the chromosome arms must be released (at least distal to chiasmata) to allow chiasma release during anaphase of the first meiotic division. Recently, the molecular analysis of a Drosophila sister chromatid cohesion gene, meLS332, previously studied genetically and cytologically, has shed light on the process of meiotic sister chromatid cohesion (Kerrebrock et al., 1995 [this issue of Celfj). mei-S332 The original mei-S332 mutation was discovered by Sandler et al. (1968) in a screen of natural populations for meiotic mutations. The S in the name refers to the location at which the fly carrying this mutation was captured: a winery on the Via Saleria outside of Rome (L. Sandler and D. Lindsley had the rather inspired idea of doing their screen in Italy; see Hawley, 1993). The mei-S332 mutation had two properties that made it unique among known Drosophila meiotic mutations. First, it caused high levels of nondisjunction that appeared to occur primarily at the second meiotic division. Second, it was the first mutant that affected the segregation of all chromosomes in both male and female meiosis and thus defined a function shared between the two sexes. This was somewhat surprising, since meiosis proceeds through fundamentally different pathways in Drosophila melanogaster males and females, and these pathways had been previously thought to be under separate genetic control. In Drosophila females, meiosis follows a typical pathway in which synapsis and exchange between homologous chromosomes allow their stable orientation toward opposite poles of the spindle. In males, however, recombination, synaptonemal complex, and chiasmata are normally absent, and chromosome pairing is based on the cohesion of specific pairing sites (McKee and Karpen, 1990). Minireview