COOPERATIVE INTERACTIONS BETWEEN THE CENTRAL SPINDLE AND THE CONTRACTILE RING DURING DROSOPHILA CYTOKINESIS

We analyzed male meiosis in mutants of the chickadee (chic) locus, a D rosophila melanogaster gene that encodes profilin, a low molecular wei ght actin-binding protein that modulates F-actin polymerization. These mutants are severely defective in meiotic cytokinesis. During ana-tel ophase of both meiotic divisions, they exhibit a central spindle less dense than wild type; certain chic allelic combinations cause almost c omplete disappearance of the central spindle. Moreover, chic mutant sp ermatocytes fail to form an actomyosin contractile ring. To further in vestigate the relationships between the central spindle and the contra ctile ring, we examined meiosis in the cytokinesis-defective mutants K LP3A and diaphanous and in testes treated with cytochalasin B. In all cases, we found that the central spindle and the contractile ring in m eiotic ana-telophases were simultaneously absent. Together, these resu lts suggest a cooperative interaction between elements of the actin-ba sed contractile ring and the central spindle microtubules: When one of these structures is disrupted, the proper assembly of the other is al so affected. In addition to effects on the central spindle and the cyt okinetic apparatus, we observed another consequence of chic mutations: A large fraction of chic spermatocytes exhibit abnormal positioning a nd delayed migration of asters to the cell poles. A similar phenotype was seen in testes treated with cytochalasin B and has been noted prev iously in mutants at the twinstar locus, a gene that encodes a Drosoph ila member of the cofilin/ADF family of actin-severing proteins. These observations all indicate that proper actin assembly is necessary for centrosome separation and migration.