Differential Expression of Two γ-Tubulin Isoforms during Gametogenesis and Development inDrosophila

Previous work identified a γ-tubulin gene, γTub23C,inDrosophila(Zhenget al.,1991). We now report identification of a second γ-tubulin gene, γTub37CD.Immunoblot analysis and immunolocalization show that γTub37CD and γTub23C are differentially expressed during gametogenesis and development. During oogenesis, γTub23C was detected at centrosomes and in the cytoplasm of mitotic germ cells, but was not detected in germ cells following completion of mitosis. Conversely, γTub37CD was not detected in proliferating germ cells, but appeared to accumulate in germ cells during egg chamber development. Neither γ-tubulin isoform was detected at the anterior or posterior poles of developing oocytes. During spermatogenesis, only γTub23C was detected at centrosomes, where it showed cell cycle- and differentiation-dependent organization. During the transition into the first meiotic division, γTub23C became organized as a corpuscular focus at centrioles until completion of meiosis II. During postmeiotic spermatid differentiation, γTub23C was detected first as a rod and then as a collar-like structure near the juncture of the nucleus and the elongating flagellum, but was not detected in bundles of mature sperm. The germline-specific CDC25 encoded bytwineis required for organization of γTub23C into corpuscular focus in spermatocytes, but not for separation of centriole pairs in M-phase or postmeiotic organization of γTub23C at centrioles. Following reconstitution of a canonical centrosome at fertilization, only γTub37CD was detected at centrosomes in syncytial embryos, but both γTub37CD and γTub23C were detected at centrosomes in cellularized embryos. Colocalization of these two isoforms suggests that γTub23C and γTub37CD both contain structural features of γ-tubulins essential for localization to centrosomes.