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In Drosophila melanogaster, a testis-specific β-tubulin (β2) is required for spermatogenesis. A sequence motif was identified in carboxyl termini of axonemal β-tubulins in diverse taxa. As a test of whether [...]

We have investigated the cellular basis for lethality of mutant alleles of the Drosophila melanogaster [Beta]3-tubulin gene, [Beta]Tub60D. Lethal [Beta]3 mutations can be grouped into two classes: the most severe mutations (Class I alleles) cause death during the first larval instar, while weaker alleles (Class II) cause death in later larval stages or in early pupal development. Since [Beta]3 is not expressed during larval development, lethality of the Class I mutations must reflect essential functions of [Beta]3 in embryogenesis. [Beta]3-tubulin is zygotically expressed during midembryogenesis in the developing mesoderm, and the major site of [Beta]3 accumulation is in the developing muscles during myogenesis. We show that the embryonic pattern of [Beta]3 expression, including accumulation in the developing musculature, is conserved in other Drosophila species. However, we found that loss of [Beta]3 function does not cause discernible defects in either the ultrastructure or function of the larval muscle. Thus [Beta]3-tubulin is dispensable in its highest site of accumulation. Rather, the essential site of function of [Beta]3 in embryos is in cells of the visceral mesoderm. Lethality of Class I alleles is caused by defects in midgut morphogenesis and failure of gut function. Although the folding pattern is irregular and the gut is smaller than normal, a complete folded gut forms in mutant larvae, and the visceral muscle functions normally to move food through the gut. However, mutant larvae cannot absorb nutrients across the gut wall. Thus loss of [Beta]3 function in the mesoderm results in defects in the underlying endodermally derived layer of the gut. Our data provide an assay for cellular interactions between mesoderm and endodermal tissues and reveal a role for the microtubule cytoskeleton of the visceral mesodermal cells in differentiation of the endodermal cell layer of the larval gut.

Genetic experiments enable the identification of lesions in various mutant alleles of beta Tub85D gene encoding the testis-specific beta2-tubulin isoform in Drosophila melanogaster. Structural analysis of mutant alleles of the beta2 gene reveals the interaction of two isotype-defining domains in a three-dimensional structure in wild-type beta-tubulins. Proper axoneme assembly requires the integrity of this structure in the Drosophila testis beta2-tubulin, while the structure is not essential for general microtubule functions. Construction of axoneme morphology needs the amino acid sequence of the beta2-tubulin internal variable region.

Researchers have identified the degree to which a beta-tubulin isoform without its carboxy terminus can integrate into operational suprastructures by producing two early-stop-codon variants of the gene for the testis-specific beta-tubulin (beta 2) in Drosophila melanogaster. The carboxy terminus of beta 2-tubulin is essential for the alignment of microtubule suprastructures in spermatogenesis.

We have sought to define the developmental and cellular roles played by differential expression of distinct [Beta]-tubulins. Drosophila [Beta]3-tubulin ([Beta]3) is a structurally divergent isoform transiently expressed during midembryogenesis. Severe [Beta] mutations cause larval lethality resulting from failed gut function and consequent starvation. However, mutant larvae also display behavioral abnormalities consistent with defective sensory perception. We identified embryonic [Beta]3 expression in several previously undefined sites, including different types of sensory organs. We conclude that abnormalities in foraging behavior and photoresponsiveness exhibited by prelethal mutant larvae reflect defective [Beta]3 function in the embryo during development of chordotonal and other mechanosensory organs and of Bolwig's organ and nerve. We show that microtubule organization in the cap cells of chordotonal organs is altered in mutant larvae. Thus transient zygotic [Beta]3 expression has permanent consequences for the architecture of the cap cell microtubule cytoskeleton in the larval sensilla, even when [Beta]3 is no longer present. Our data provide a link between the microtubule cytoskeleton in embryogenesis and. the behavioral phenotype manifested as defective proprioreception at the larval stage.

Phylogeny and Molecular Data Biologists with an interest in animal evolution have eagerly looked forward to the results of the new sequencing studies of genetic material, which many colleagues hope [...]