Genetic Analysis of the Hairy Locus in DROSOPHILA MELANOGASTER

Mutations of the hairy locus in Drosophila may affect both adult chaeta differentiation and embryonic segmentation. In an effort to understand this phenotypic complexity, we have analyzed 30 mutant alleles of the locus. We find that the alleles fall into four groups according to their complementation properties, suggesting a structurally complex locus in which two distinct functions share a common coding region.     

Apparent Genetic Complexity Generated by Developmental Thresholds: The Apterous Locus in DROSOPHILA MELANOGASTER

Mutations at the apterous (ap) locus in Drosophila melanogaster give rise to three distinct phenotypes: aberrant wings, female sterility and precocious adult death. The wing phenotype includes five types of abnormality: blistering, deficiencies, duplications, high-order repetitions and transformation of structures. The mildest phenotype is seen with homozygous apblt animals which have either normal or slightly blistered wings. Most alleles produce, in the homozygote, a deficient wing in which part or all of the wing margin and wing blade is missing, but wing hinge and notum regions are normal. Animals hemizygous for each of 20 ap alleles, as well as apID/apXa heterozygotes, show duplication of parts of the notum associated with complete wing deficiency. Animals heterozygous for apc and the other tested ap alleles show repetitions of parts of the anterior wing margin, an engrailed-like transformation of posterior wing margin into anterior margin or both. Both apblt and apc show similar phenotypes in homozygotes and hemizygotes, yet both produce a less extreme phenotype than that of the other hemizygotes, suggesting that neither mutation causes loss of the entire ap+ function. The 15 alleles that cause precocious death and female sterility occur in six complementation groups based on complementation for these phenotypes. This supports the previous conclusion that the effects of apterous mutations on the wing do not correlate with their effects on viability and fertility. We propose an explanation for the effects of apterous mutations on the wing in which quantitative reductions in the activity of gene product give rise to qualitatively different phenotypes because of different threshold requirements of the ap+ function for critical events in wing disc development.     

Analysis of the Cut Locus of DROSOPHILA MELANOGASTER

Mutants of the cut (ct) locus can be divided into two classes: viable and lethal. Most of the viable alleles are characterized by varying degrees of scalloping and notching of the wings. One mutant, kinked femur, exhibits kinking of the femurs and failure of wing expansion, but no other changes in wing structure. In heterozygous combination with the other viable alleles, it exhibits complete complementation, but it fails to complement with lethal ct alleles with respect to its viable phenotype. Similarly, all of the other viable ct alleles express a mutant wing phenotype when heterozygous with lethal ct alleles.-Mapping experiments indicate that the lethal alleles, which comprise the majority of all ct mutations recovered, are confined to a small region at the right end of the locus. That this restriction is real and not an artifact imposed by the limited number of lethal mutations mapped in the locus is supported by an examination of the mutant ct(JC20), a presumptive deficiency for the left-most third of the locus. Despite its behavior as a deletion, ct(JC20) is viable, though mutant, in combination with the lethal alleles. The restriction of the noncomplementary lethals to a small part of the locus, distinct from the other ct mutants, suggests a polarity that may define a segment that functions only in cis within the complex.-Based on the comparison of the data with the prediction of several models, we suggest that the left portion of the locus, which contains the viable alleles, defines a regulatory region controlling the expression of the locus, while the segment encoding a polypeptide product is at the right end and only it is capable of mutating to a lethal state.     

Genetic and Developmental Analysis of a Temperature-Sensitive Minute Mutation of DROSOPHILA MELANOGASTER

A temperature-sensitive (ts) third chromosome Minute (M) mutation, designated Q-III, has been recovered and characterized. Q-III heterozygotes raised at 29° exhibit all of the dominant traits of M mutants including small bristles, rough eyes, prolonged development, reduced viability and interactions with several unrelated mutations. Q-III homozygotes raised at 29° are lethal; death occurs primarily during the first larval instar. When raised at 22°, Q-III heterozygotes are phenotypically normal and Q-III homozygotes display moderate M traits. In addition, Q-III elicits ts sterility and maternal-effect lethality. As it true of M lesions, the dominant traits of Q-III are not expressed in triploid females raised at 29°. Complementation tests suggest that Q-III is a ts allele of M(3)LS4, which is located in 3L near the centromere.—Reciprocal temperature-shift experiments revealed that the temperature-sensitive period (TSP) of Q-III lethality is polyphasic, extending from the first instar to the latter half of pupation. Heat-pulse experiments further resolved this into two post-embryonic TSPs: one occurring during the latter half of the second larval instar, and the other extending from the larval/pupal boundary to the second half of pupation. In addition, heat pulses elicited a large number of striking adult phenotypes in Q-III individuals. These included pattern alterations such as deficiencies and duplications and other morphological defects in structures produced by the eye-antennal, leg, wing and genital imaginal discs and the abdominal histoblasts. Each defect or pattern alteration is associated with a specific TSP during development.—We favor the interpretation that most of the major Q-III defects, particularly the structural duplications and deficiencies, result from temperature-induced cell death in mitotically active imaginal anlagen, while the small macrochaete phene probably results from the direct effects of Q-III on bristle synthesis. The hypothesis that the Q-III locus specifices a component required for protein synthesis is discussed, and it is concluded that this hypothesis can account for the pleiotropy of Q-III, and that perhaps it can be extended to M loci in general.     

Developmental and Genetic Studies on Kynurenine Hydroxylase from DROSOPHILA MELANOGASTER

The level of kynurenine hydroxylase was measured throughout the development of wild type and the eye color mutants v, cn, st, ltd, cd, kar, w, ca, bri and p(P) of Drosophila melanogaster. In all cases except cn a bimodal distribution of enzyme activity during development was observed. Activity is initially detectable in second instar. A maximum is reached in early third instar. Activity declines prior to puparium formation. Shortly after pupation, activity rises dramatically to reach a maximum about five times the peak larval level. Maximum activity persists for a short time, and then falls sharply prior to emergence. No activity is detectable in cn, cn(3), or cn(35K). In pupae which have zero, one, two or three doses of the cn(+) allele, activity is proportional to the number of the + alleles. This provides further evidence that the cn locus contains the structural gene for kynurenine hydroxylase. Kynurenine hydroxylase is a useful gene product for studying the events of imaginal disc differentiation.     

Developmental Analysis of the Achaete-Scute System of DROSOPHILA MELANOGASTER

The interpretation of the wild-type function of a gene depends on our knowledge of the phenotype caused by its absence. We have first defined the genetic extent of the achaete-scute system by studying the phenotype of different terminal and intercalary deficiencies including these genes. When these deficiencies were lethal, we have defined the phenoeffective phase of lethality and studied their phenotype in genetic mosaics (gynandromorphs and mitotic recombination clones). The achaete-scute system affects two functions, one necessary for the differentiation of the embryonic (central?) nervous system and the other necessary for the differentiation of peripheral nervous elements of the chaetes and sensillae of the adult cuticle. The possibility that these functions correspond to differential expression of a single mechanism is discussed.     

Genetic Analysis of the Dumpy Complex Locus in DROSOPHILA MELANOGASTER: Complementation, Fine Structure and Function

An extensive genetic analysis of the dumpy locus is presented. This study includes complementation, fine structure mapping and allelic interaction. A number of complementing recessive lethals of the dp complex have been genetically mapped. Two alleles of the ol(v) type that complement l alleles map to the left portion of the locus. A number of olv alleles that complement both l and lv lethals map within the right portion of the locus.——Fine-structure analysis demonstrated that both olv and o alleles are distributed among various subloci. Evidence for spacer regions between subloci is presented.——An extensive discussion of the data considers whether the locus is unicistronic or multicistronic. The conclusion reached is that the locus is not a single functional cistron. The possibility of a single cistron encoding a multifunctional polypeptide is discussed.——The hypothesis is proposed that the left portion of the map and the l mutations function as regulatory sequences and that the right portion of the map encodes structural sequences.     

Molecular Mapping of the ROSY Locus in DROSOPHILA MELANOGASTER

The DNA from the chromosomal region of the Drosophila rosy locus has been examined in 83 rosy mutant strains. Several spontaneous and radiation-induced alleles were associated with insertions and deletions, respectively. The lesions are clustered in a 4-kb region. Some of the alleles identified on the DNA map have been located on the genetic map by fine-structure recombination experiments. The genetic and molecular maps are collinear, and the alignment identifies the DNA location of the rosy control region. A rosy RNA of 4.5 kb has been identified; its 5' end lies in or near the control region.     

Temperature-Sensitive Mutations of the Notch Locus in DROSOPHILA MELANOGASTER

Temperature-conditional mutations of the Notch locus were characterized in an attempt to understand the organization of a "complex locus" and the control of its function in development. Among 21 newly induced Notch alleles, about one-half are temperature-conditional for some effects, and three are temperature-sensitive for viability. One temperature-sensitive lethal, l(1)N^ts1, is functionally non-complementing for all known effects of Notch locus mutations and maps at a single site within the locus. Among the existing alleles involved in complex patterns of interallelic complementation, Ax^59d5 is found to be temperature-sensitive, while fa ^g, spl, and l(1)N are temperature-independent. Whereas temperature-sensitive alleles map predominantly to the right-most fifth of the locus, fa^g, spl, and l(1)N are known to map to the left of this region. Temperature-shift experiments demonstrate that fa^g, spl, and l(1)N cause defects at specific, non-overlapping times in development.—We conclude (1) that the Notch locus is a single cistron (responsible for a single functional molecule, presumably a polypeptide); (2) that the right-most fifth of the locus is, at least in part, the region involved in coding for the Notch product; (3) that the complexity of interallelic complementation is a developmental effect of mutations that cause defects at selected times and spaces, and that complementation occurs because the mutant defects are temporally and spatially non-overlapping; and (4) that mutants express selected defects due to critical temporal and spatial differences in the chemical conditions controlling the synthesis or function of the Notch product. The complexity of the locus appears to reside in controlling the expression (synthesis or function) of the Notch product in development.     

Frequency-Dependent Selection at the LAP Locus in DROSOPHILA MELANOGASTER

Results of fitness estimates for the Lap locus in Drosophila melanogaster revealed that under crowded media conditions gene frequency equilibrium was maintained by frequency-dependent selection. Evidence was obtained that indicated that mating and egg-to-adult viability were frequency dependent.     

Excess Polymorphism at the Adh Locus in DROSOPHILA MELANOGASTER

The evolutionary history of a region of DNA encompassing the Adh locus is studied by comparing patterns of variation in Drosophila melanogaster and its sibling species, D. simulans. An unexpectedly high level of silent polymorphism in the Adh coding region relative to the 5' and 3' flanking regions in D. melanogaster is revealed by a populational survey of restriction polymorphism using a four-cutter filter hybridization technique as well as by direct sequence comparisons. In both of these studies, a region of the Adh gene encompassing the three coding exons exhibits a frequency of polymorphism equal to that of a 4-kb 5' flanking region. In contrast, an interspecific sequence comparison shows a two-fold higher level of divergence in the 5' flanking sequence compared to the structural locus. Analysis of the patterns of variation suggest an excess of polymorphism within the D. melanogaster Adh locus, rather than lack of polymorphism in the 5' flanking region. An approach is outlined for testing neutral theory predictions about patterns of variation within and between species. This approach indicates that the observed patterns of variation are incompatible with an infinite site neutral model.     

Position Effects at the Hairy Locus in DROSOPHILA MELANOGASTER

Radiation-induced chromosomal rearrangements of h(+) have given rise to several Drosophila stocks that exhibit apparent position-effect inactivation; i.e., flies carrying the rearranged chromosomes heterozygously with h show varying degrees of hairiness. The numbers of hairy chaetae produce a quantifiable index of position effect. Six such "position-allele" stocks are here discussed, both as to their basic expressions and in all possible pair-wise combinations with each other. Such crosses reveal complex interactions between the respective position alleles; little evidence is seen for clear-cut dominance or recessiveness. The stocks appear not to conform unequivocally to classical distinctions between variegated and stable types of position effects, nor to usual dicta relating the degree of inactivity to the proximity to heterochromatin. Indeed, these stocks appear to suggest additional dimensions to several of the principles to which position effects usually subscribe. The evidence additionally suggests that the hairy locus itself is associated with a tissue-specific suppressor effect on an otherwise polygenic system that produces the chaetae associated with the hairy phenotype.     

Genetic Limits of the Xanthine Dehydrogenase Structural Element within the Rosy Locus in DROSOPHILA MELANOGASTER

Experiments are described that provide an opportunity to estimate the genetic limits of the structural (amino acid coding) portion of the rosy locus (3:52.0) in Drosophila melanogaster, which controls the enzyme, xanthine dehydrogenase (XDH). This is accomplished by mapping experiments which localize sites responsible for electrophoretic variation in the enzyme on the known genetic map of null-XDH rosy mutants. Electrophoretic sites are distributed along a large portion of the null mutant map. A cis-trans test involving electrophoretic variants in the left- and right-hand portions of the map leads to the conclusion that the entire region between these variants is also structural. Hence most, if not all, of the null mutant map of the rosy locus contains structural information for the amino acid sequence of the XDH polypeptide. Consideration is given to the significance of the present results for the general problem of gene organization in higher eukaryotes.     

Genetic Analysis of the Achaete-Scute System of DROSOPHILA MELANOGASTER

Several mutations in the achaete-scute region of Drosophila have been analyzed phenotypically and cytologically. One group of them corresponds to point mutations, another to rearrangements with one breakpoint in this region. Trans heterozygotes of the different point mutations or of the different rearrangements show poor complementation or fail to complement; therefore, they could be interpreted as mutations affecting the same gene product. However, left-right inversion recombinants and duplication-deficiency combinations between rearrangements with different cytological breakpoints uncover a complex organization of the achaete-scute region. This region seems to contain several independent achaete and scute functions, as well as a lethal function, arranged as a tandem reverse repeat at both sides of a lethal locus. Since all of the mutants show the same phenotype qualitatively, though different quantitatively, we suggest that these functions are of a reiterative nature. The achaete-scute wild-type condition may well be dependent on a multimeric gene product made of several evolutionary related monomers.     

Resolution of an Equivocal Genetic Element in DROSOPHILA MELANOGASTER: Organization of the Maroon-like Locus

Despite extensive prior study, an understanding of the genetic organization of the maroon-like locus (ma-l:1–64.8) has been elusive. A large-scale, three-point fine structure recombination experiment is described, whose results provide documentation for an inescapable argument that maroon-like is a single element rather than a polycistronic system.