Cell lineage restrictions in the genital disc ofDrosophila revealed byMinute gynandromorphs

Genetic and developmental analysis of the sex-determining gene ‘double sex’ (dsx) of Drosophila melanogaster

Sex determination in Drosophila depends on the ratio of X chromosomes to sets of autosomes (X:A). This chromosomal signal is used to regulate a few control genes whose state of activity selects either the male or the female sexual pathway. We have studied the structure and function of dsx (double sex) which appears to be the last regulatory gene on whose function the sexual pathway eventually depends. We have mutagenized the locus, varied the doses of dominant dsx-mutations and wildtype alleles, and combined different dsx-alleles with recessive mutations in other sex-determining genes, such as ix, tra-2 and tra.

The locus dsx harbours two genetic functions, dsxm to implement the male program, dsxf to implement the female program. We found that dsxm and dsxf can mutate independently although most mutations abolish both functions. We conclude that dsxm and dsxf each have their specific domain, but also share a large region of DNA that is essential for both functions. We present evidence that the dominant mutations correspond to a constitutive expression of the male-determining function dsxm, with the simultaneous abolishment of the female-determining function dsxf. This effect can be counteracted by two doses of expressed dsxf so that a female phenotype results. The products of one dose of expressed dsxm and one dose of expressed dsxf in the same cell appear to neutralize each other which leads to a null phenotype. The mutant combinations suggest that the product of dsxf requires the products of ix+, tra-2+ and tra+ to become functional.

Allocation and specification of the genital disc precursor cells in Drosophila

The adult structures of Drosophila melanogaster are derived from larval imaginal discs, which originate as clusters of cells within the embryonic ectoderm. The genital imaginal disc is composed of three primordia (female genital, male genital, and anal primordia) that originate from the embryonic tail segments A8, A9, and A10, respectively, and produce the sexually dimorphic genitalia and analia. We show that the genital disc precursor cells (GDPCs) are first detectable during mid-embryogenesis as a 22-cell cluster in the ventral epidermis. Analysis of mutant and double mutant phenotypes of embryonic patterning genes in the GDPCs, together with their expression patterns in these cells, revealed the following with respect to the origins and specification of the GDPCs. The allocation of the GDPCs from the ventral epidermis requires the function of ventral patterning genes, including the EGF receptor and the spitz group of genes. The ventral localization of the GDPCs is further restricted by the action of dorsal patterning genes. Along the anterior-posterior axis, several segment polarity genes (wingless, engrailed, hedgehog, and patched) are required for the proper allocation of the GDPCs. These segment polarity genes are expressed in some, but not all of the GDPCs, indicating that anterior and posterior compartments are not fully established in the GDPCs. In addition, we found that the three primordia of the larval genital disc have already been specified in the GDPCs by the coordinated actions of the homeotic (Hox) genes, abdominal-A, Abdominal-B, and caudal. By identifying how these different patterning networks regulate the allocation and primordial organization of the 22 embryonic precursors of the compound genital disc, we demonstrate that at least some of the organization of the larval disc originates as positional information in the embryo, thus providing a context for further studies on the development of the genital disc.

Sexually dimorphic neurons in the terminalia of Drosophila melanogaster: I. Development of sensory neurons in the genital disc during metamorphosis

The adult terminalia of Drosophila melanogaster males and females are sexually dimorphic. The sensory neurons that innervate the sex-specific bristles of the analia and genitalia arise within the genital disc during metamorphosis. Using antibodies that recognize peripheral neurons, I have followed the development of neurons in the female genital, the male genital and the shared anal primordium. Neurons form only in the expressed genital and anal primordium at the onset of the white prepupal stage. Two basic types of neurons differentiate in the genital disc; exteroreceptors that innervate the external bristles and enteroreceptors that innervate the internal reproductive organs. Within the genital disc, each structure has a sex-specific time during pupation when its neurons differentiate even when neurons are derived from homologous primordia. The data suggest that the sexually dimorphic arrangement of sensory neurons is a product of sex-specific neurogenesis.

Sexually dimorphic neurons of the terminalia of Drosophila melanogaster: II. Sex-specific axonal arborizations in the central nervous system

In order to understand how the peripheral nervous system in male and female flies differ, I have determined the projection pattern of sensory neurons that innervate the sex-specific adult terminalia, the genitalia and analia, of Drosophila melanogaster. In the adult male and female fly, mechanosensory bristles arranged on the external terminalia are innervated by sensory neurons that arborize in the abdominal ganglion. The distribution of axonal arbors differs between males and females. In males, sensory neurons terminate in a greater variety of patterns over a larger area of neuropil than those in females. Mutations in a sex-determining gene (transformer-2, tra-2,) which regulates the development of secondary sexual characteristics in somatic tissues, cause female flies to be transformed into phenotypic males. The sensory neurons of the terminalia are also transformed leading them to arborize appropriately for male neurons.

The segmentation gene Krüppel of Drosophila melanogaster has homeotic properties

In Drosophila hindgut, Malpighian tubules and posterior midgut develop from the most posterior region of the blastoderm. One of the genes that influences the differentiation of the Malpighian tubules is Krüppel (Kr), a segmentation gene of the gap class. Kr homozygous embryos lack thoracic and abdominal segments and, depending on the allele, develop nearly normal Malpighian tubules or do not differentiate them at all. In the wild type, injection of horseradish peroxidase (HRP) into cells of the early gastrula at various posterior positions results in labeling of hindgut (93%), Malpighian tubules (46%), and posterior midgut (20%). Malpighian tubules were labeled only in combination with hindgut. In Kr1 homozygous embryos that lack Malpighian tubules, the label was restricted to hindgut (84%) and posterior midgut (24%). Because we could not find significant cell death in the posterior region of Kr1 embryos, we counted the cell nuclei in the hindguts of wild-type and mutant embryos. The results show that the hindgut in Kr1 embryos contains those cells that would differentiate into Malpighian tubules in wild type. Therefore, we conclude that the Krüppel gene exhibits a homeotic function in addition to its role as a segmentation gene and is involved in separating hindgut and Malpighian tubule cells and in the elongation process as well.