MUTATIONS IN TWINSTAR, A DROSOPHILA GENE ENCODING A COFILIN ADF HOMOLOG, RESULT IN DEFECTS IN CENTROSOME MIGRATION AND CYTOKINESIS

We describe the phenotypic and molecular characterization of twinstar (tsr), an essential gene in Drosophila melanogaster. Two P-element ind uced alleles of tsr (tsr(1) and tsr(2)) result in late larval or pupal lethality. Cytological examination of actively dividing tissues in th ese mutants reveals defects in cytokinesis in both mitotic (larval neu roblast) and meiotic (larval testis) cells. In addition, mutant sperma tocytes show defects in aster migration and separation during prophase /prometaphase of both meiotic divisions. We have cloned the gene affec ted by these mutations and shown that it codes for a 17-kD protein in the cofilin/ADF family of small actin severing proteins. A cDNA for th is gene has previously been described by Edwards et al. (1994). Northe rn analysis shows that the tsr gene is expressed throughout developmen t, and that the tsr(1) and tsr(2) alleles are hypomorphs that accumula te decreased levels of tsr mRNA. These findings prompted us to examine actin behavior during male meiosis to visualize the effects of decrea sed twinstar protein activity on actin dynamics in vivo. Strikingly, b oth mutants exhibit abnormal accumulations of F-actin. Large actin agg regates are seen in association with centrosomes in mature primary spe rmatocytes. Later, during ana/telophase of both meiotic divisions, abe rrantly large and misshaped structures appear at the site of contracti le ring formation and fail to disassemble at the end of telophase, in contrast with wild-type. We discuss these results in terms of possible roles of the actin-based cytoskeleton in centrosome movement and in c ytokinesis.