Spermiogenesis in Two Male-Sterile Third-Chromosome Mutants of \u3cem\u3eDrosophila melanogaster\u3c/em\u3e

An ultrastructural study of sperm differentiation was made on two male-sterile mutants of Drosophila melanogaster. The mutations were induced on the third chromosome with the monoalkylating agent ethyl methanesulfonate and were maintained in stock cultures over a balancer chromosome using a system of multiple inversions. The stages of sperm differentiation, a most dramatic form of cellular differentiation, were then studied at the electron microscope level to determine the effect of the mutations on sperm structure and to correlate those changes with normal sperm development. Studies of the genetic control of cellular differentiation have been hampered by difficulties in devising an in vitro system. Drosophila melanogaster provides an excellent system for these types of studies since the genome is well characterized, sperm development is a continual process, it is a higher organism with many complex interactions and normal sperm differentiation has been well characterized and can serve as the control over such genetic manipulations. Analysis of the first of the two mutants studied, ms(3)10R, presents some evidence for an inductive action of cytoplasmic microtubules on the condensation of chromatin in the nuclei of developing sperm which, in turn, influences the normal differentiation and shaping of the sperm head. Additionally, the study of this mutant appears to indicate that this locus exercises an organizational rather than a structural control. The second mutant studied, ms(3)3R, shows normal sperm differentiation until a very late stage of development. The sperm tails then proceed to degenerate in an ordered fashion that differs from the degenerative changes to the sperm tails described for Y-chromosome mutations and deficiencies. In addition to the ultrastructural changes seen, the male-sterile flies fail to copulate. It appears, therefore, that this genetic locus on the third chromosome functions in the maintenance of sperm structure and also the normal behavior of the flies. The behavioral abnormality observed suggests that the mutation affects the normal endocrine system of Drosophila melanogaster