The lethal(1)TW-6cs mutation of Drosophila melanogaster is a dominant antimorphic allele of nod and is associated with a single base change in the putative ATP-binding domain

Meiotic chromosome distribution in Drosophila oocytes: roles of two kinesin-related proteins

Recent new information regarding the proteins required for proper distribution of chromosomes in meiosis has come from studies of Drosophila mutants. These studies reveal that proteins related to the microtubule motor protein, kinesin, function in meiotic chromosome segregation in Drosophila females. The two proteins identified thus far are likely to play very different roles in the process. The ncd protein is a spindle motor in meiosis but may perform a different role in the early mitotic divisions of the embryo. nod functions earlier in meiosis than ncd, prior to the meiotic divisions, and may be either chromosome or spindle associated. The identification of nod as a kinesin protein raises new questions regarding the distributive model of meiotic chromosome segregation.

Kinesins in the spindle: an update

Eukaryotes contain a superfamily of microtubule-based motor proteins comprising kinesin and a number of related proteins that are thought to participate in various forms of intracellular motility, including cell division and organelle transport. The role of various members of the kinesin superfamily in chromosome segregation and spindle morphogenesis was described in TCB last year in parts of a series on cytoplasmic motor proteins. In this brief update, Helen Epstein and Jon Scholey comment on new findings that have improved our understanding of the functions of kinesin-related proteins in mitosis and meiosis.

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.