Atonal is the proneural gene for Drosophila photoreceptors.

The Drosophila peripheral nervous system comprises four major types of sensory element: external sense organs (such as mechano-sensory bristles), chordotonal organs (internal stretch receptors), multiple dendritic neurons, and photoreceptors. During development, the selection of neural precursors for external sense organs requires the proneural genes of the achaete-scute complex, which encode basic-helix-loop-helix transcription factors. These genes do not, however, control precursor selection for chordotonal organs or photoreceptors, raising the question of whether other proneural genes exist or a different mechanism of neurogenesis operates. Here we show that atonal (ato), originally isolated as a proneural gene for chordotonal organs, is also the proneural gene for photoreceptors. Pattern formation in the Drosophila eye involves a succession of cell fate specifications. Of the eight photoreceptors within each ommatidium of the compound eye, the photoreceptor R8 is the first to appear in the eye imaginal disc, right behind the morphogenetic furrow. The appearance of other photoreceptors (R1-7) follows in a defined sequence that is thought to arise by induction from R8 (refs 8, 9, 11, 12). We find that photoreceptor formation requires the function of atonal at the morphogenetic furrow and that atonal is specifically required for R8 selection. Formation of other photoreceptors does not directly require atonal function, but does depend on R8 selection by atonal. Thus, photoreceptors are selected by two mechanisms: R8 by a proneural mechanism, and R1-7 by local recruitment.

The maternal sex determination gene daughterless has zygotic activity necessary for the formation of peripheral neurons in Drosophila.

The daughterless (da) gene is known to have separate maternal and zygotic functions: Maternally supplied daughterless activity is required for proper sex determination and dosage compensation in female embryos, whereas loss of zygotically supplied da+ activity causes embryonic lethality in both male and female embryos. We have found that the zygotic da+ activity is necessary for neural development: The use of neuron-specific antibodies and beta-galactosidase-marked X chromosomes has revealed that in both male and female embryos deletions or strong mutations of the da gene remove all peripheral neurons and associated sensory structures without disrupting the epithelium from which they derive. Partial da+ function causes partial removal of peripheral neurons. Our results indicate that da+ is required for the formation of peripheral neurons and their associated sensory structures.

Mechanism of suppression in Drosophila: regulation of tryptophan oxygenase by the su(s)+ allele.

The suppressor gene, su(s)2, in Drosophila melanogaster restores the production of red and brown eye pigments for some purple and vermilion mutant alleles, respectively. We showed previously that the product of the su(s)+ allele caused inhibition of the sepiapterin synthase A produced by the purple mutant but did not affect the wild-type enzyme. Suppression was accomplished by removing su(s)+ from the genome. We now report that the tryptophan oxygenase, produced by suppressible vermilion alleles, is also inhibited by extracts from su(s)+ flies. The inhibition of the vermilion enzyme can be reduced or eliminated, respectively, by prior storage of the extract at 4 or -20 degrees C or by boiling, whereas the wild-type enzyme is not affected by extracts of su(s)+ flies. Also, when the suppressible vermilion strain is raised on certain diets, brown eye pigment production occurs. This epigenetic suppression was reduced by the presence of an extra copy of su(s)+ in the genome. These data support a posttranslational mechanism for regulation of enzyme activity in which the activity of the mutant enzyme is reduced by the product of the su(s)+ allele. How the su(s)+ gene product can distinguish between the normal and the mutant forms of these two enzymes is discussed, along with other mechanisms for suppression that are currently under investigation.

Deletion polymorphism in a Drosophila melanogaster heat shock gene.

We have continued the transcriptional analysis of the region of cytological locus 67B that contains the four small heat shock genes and other genes. Transcription from one of the heat shock genes in the region, hsp 26, takes place during high temperature treatment and at certain developmental stages, without heat shock, in several tissues, such as imaginal discs and adult ovaries. Observations of unexpected products after nuclease protection experiments provided the first indication of what genomic blot experiments showed to be small deletions. The alleles containing the deletion are expressed at the same level as the wild type allele. The deletion shortens the protein product, implying that it is in the coding region. Furthermore, flies homozygous for one of the deletion alleles are viable.

Mechanism of suppression in Drosophila: evidence for a macromolecule produced by the su(s)+ locus that inhibits sepiapterin synthase.

Genetic suppression was studied in the purple mutant of Drosophila melanogaster and in suppressed purple by measurement of sepiapterin synthase activity. The addition of ammonium sulfate fractions from adult Drosophila that contain one, two, three or four doses of su(s)+ to the suppressed purple sepiapterin synthase resulted in an inhibition that increased progressively as the dosage of su(s)+ increased; the wild-type sepiapterin synthase was not inhibited. This inhibition is caused by a heat-labile macromolecule. We suggest that the mechanism of suppression is neither transcriptional nor translational but is the result of decreased amounts, or altered properties, of the normal product of the su(s)+ locus when su(s)+ is replaced by su(s)2 or su(s)e6.

A mutant affecting the crystal cells inDrosophila melanogaster.

Black cells (Bc, 2-80.6±) mutant larvae ofDrosophila melanogaster have pigmented cells in the hemolymph and lymph glands. In this report we present evidence that these melanized cells are a mutant form of the crystal cells, a type of larval hemocyte with characteristic paracrystalline inclusions.Bc larvae lack crystal cells. Furthermore, the distribution pattern of black cells inBc larvae parallels that of experimentally-blackened crystal cells in normal larvae (phenocopy).InBc/Bc zygotes black cells appear during mid embryonic development but inBc +/Bc zygotes pigmented cells are not found until late in the first larval instar.Crystal cells are present in the heterozygous larvae until this time, and paracrystalline inclusions can be seen in some of the cells undergoing melanization in these larvae.The rate of phenol oxidase activity inBc +/Bc larval cell-free extracts is less than half that ofBc +/Bc +extracts whereas enzyme activity is undetectable inBc/Bc larvae. We propose that theBc +gene product is required for maintaining the integrity of the paracrystalline inclusions; inBc/Bc larvae either the product is absent or nonfunctional so an effective contact between substrate and enzyme results in melanization of the cells.Phenol oxidase itself is either destroyed or consumed in the melanization process accounting for the absence of enzyme activity inBc/Bc larvae. These studies confirm that the crystal cells store phenolic substrates and are the source of the hemolymph phenol oxidase activity in the larva ofD. melanogaster.

Developmental changes of sepiapterin synthase activity associated with a variegated purple gene in Drosophila melanogaster.

A variegated position effect on the autonomous gene, purple, has been studid enzymologically in Drosophila melanogaster. Sepiapterin synthase, the enzyme system associated with pr+, was examined for activity in different developmental stages of the fly. The results indicate that T(y:22)prc5, cn/prc4 cn flies (flies in which pr+ has been translocated and which exhibit variegation) have a reduced amount of enzyme activity as compared with both Oregon-R and pr1 flies. This reduction in activity was not found in larval stages, which suggests that the inactivation process probably occurs in late larval or early pupal stages. The phenotype of the variegated adult has white eyes with red-colored spots and patches where drosopterins occur. The phenotype of the fly carrying the translocation is modified by the presence of additional Y chromosomes. This extends the observation from other systems that extra heterochromatin acts to suppress the variegated position effect. The advantages of studying the variegation by measuring enzyme activity, as well as the phenotypic expression, are several; for example, the developmental time at which variegation occurs may be estimated even though drosopterin synthesis is not occurring.

Genetics and physiological expression of beta-hydroxy acid dehydrogenase in Drosophila.

A mutant Hadnl was induced in Drosophila melanogaster and found to be deficient in beta-hydroxy acid dehydrogenase. This mutation was utilized to study the genetics and physiological expression of Had+ . Had+ was mapped to the X chromosome at 54.4 and seems to be the structural gene for the enzyme. Enzyme activity in male and female flies indicates that the gene shows both dosage compensation independent from dose effect and differential activity during ontogeny. Electrophoretic mobility data indicate that the enzyme is a dimer which forms by random association of subunits. The fact that the mutant shows no detrimental effect implies that the enzyme is dispensable, at least under laboratory conditions. The biological and technical implications of this gene--enzyme system are discussed.

Mechanism of suppression in Drosophila: control of sepiapterin synthase at the purple locus.

The amounts of sepiapterin and red pteridine eye pigments (drosopterins) in Drosophila melanogaster are known to be reduced in the purple mutant and restored to normal by a suppressor mutation. We show here that sepiapterin synthase activity is 30 percent of normal in pr and prbw, two naturally occurring alleles of purple, and is restored to nearly normal levels by the suppressor su(s)2. A heterozygote of two newly induced alleles of pr has even lower enzyme activity (less than 10 percent). The sepiapterin synthase activity is proportional to the number of wild-type pr alleles in flies when one and two copies of the allele are present and is higher in three-than in two-dose flies. We hypothesize that the purple locus may be a structural gene for sepiapterin synthase in Drosophila.

Genetic analysis of aspartate aminotransferase isozymes from hybrids between Drosophila melanogaster and Drosophila simulans and mutagen-induced isozyme variants.

The aspartate aminotransferases (designated GOT1 and GOT2) are two enzymes of Drosophila melanogaster for which naturally occurring electrophoretic variants were not found. There is an electrophoretic difference between D. melanogaster and D. simulans. Since the F1 hybrid offspring of these species are sterile, a genetic analysis of the ordinary type cannot be done on differences between the two species. A method was devised to make "partial hybrids" in which one chromosome arm is homozygous for melanogaster genes in an otherwise hybrid background. By using this method, Got1 was localized to 2R and Got2 to 2L. Once a gene can be assigned to a chromosome, it may be followed in crossing schemes and mutations from mutagen treatments may be looked for. At the locus of Got1 a mutation with low activity was recovered and designated Got1lo. It was located at a genetic map position of 75 on 2R. A Got2 mutant with a greater migration to the anode was recovered and designated Got2J. It was located at a genetic map position of 3.0, and in the salivary chromosome was between 22B1 and 22B4 inclusive.

Mechanism of suppression in Drosophila. III. Phenol oxidase activity and the speck locus.

A marked decrease in the amount of the A2 component of phenol oxidase occurs in the speck locus of Drosophila melanogaster. The amount of A2 in speck is restored to a normal amount in the presence of the suppressor mutant, su(s)2.

Electrophoretic variants of alpha-glycerophosphate dehydrogenase in Drosophila melanogaster.

Two alleles of Gdh, the locus specifying the electrophoretic mobility of alpha-glycerophosphate dehydrogenase, are found in Drosophila melanogaster. The gene is located on the second chromosome at a map position of 17.8. Hybrid enzyme molecules are found in heterozygotes.