The β3-tubulin gene of Drosophila melanogaster is essential for viability and fertility

We have previously shown that the β3-tubulin gene of Drosophila melanogaster encodes a divergent isoform expressed in a complex developmental pattern. The β3 gene is transiently expressed in the embryo and again in the pupa at high levels in the developing musculature, and at lower levels in several different pupal tissues of ectodermal origin. Adult expression is confined to specific somatic cells in the gonads. In some of the cell types in which it is expressed, β3 is the sole or predominant β-tubulin, while in others the β3 protein is a minor component of the β-tubulin pool. The sites and timing of β3 expression demonstrated that β3-tubulin is utilized primarily in cytoplasmic microtubule arrays involved in changes in cell shape and tissue organization, and suggested to us that this isoform may be functionally specialized. To determine whether the expression of the β3 gene is essential for normal development, and to examine the specific functions of this divergent isoform, we have generated mutations within the gene. We determined that the small deficiency Df(2R)Px², which deletes the 60C5,6-60D9,10 region of chromosome 2, removes all of the β3 coding sequences, and that the distal breakpoint of the deficiency is approximately 2 kb upstream from the start of transcription of the β3 gene. We have generated a total of 31 ethyl methanesulfonate- or diepoxybutane-induced recessive lethal or visible mutations which map within the deficiency. These mutations define 12 new lethal complementation groups, which together with two previously identified visible mutations, altogether identify 14 genes in this interval of the second chromosome. A lethal complementation group comprising mutations in the β3-tubulin gene (βTub60D) was identified by rescue of their lethality by a wild-type copy of the gene introduced into the genome via P element-mediated germ line transformation. Analysis of the homozygous and transheterozygous phenotypes of the five β3 mutations recovered (alleles designated B3t¹-B3t⁵) demonstrates that β3-tubulin is essential for viability and fertility.