ABSTRACT
Three α-tubulin proteins contribute to microtubules during oogenesis and early embryogenesis in Drosophila melanogaster: αTUB84B, αTUB84D, and αTUB67C. αTUB67C is unique in two respects. It is a structurally divergent α-tubulin, sharing only 67% amino acid identity with the generic isotypes αTUB84B and αTUB84D, and its expression is exclusively maternal. The genetic analysis of the Tub67C gene described here demon strates that αTUB67C is required for nuclear division in the oocyte and early embryo. Both meiosis and cleavage-stage mitoses are severely affected by mutations that result in a substantial decrease in the ratio of αTUB67C/ αTUB84B+ αTUB84D. A large increase in this ratio, achieved by increasing the gene dosage of Tub67C, has little or no effect on meiosis, but severely disrupts mitotic spindle function. Thus, both classes of α-tubulin isotype present in the mature oocyte, αTUB67C and αTUB84B/84D, are essential for normal spindle function in early Drosophila development. These α-tubulins provide the first example of tubulin isotypes known to be coexpressed in wild-type animals whose encoded variation is required for the normal function of a microtubule array.
