Rolf Nöthiger

The sex-determining gene tra-2 of Drosophila encodes a putative RNA binding protein

The gene transformer-2 (tra-2) of Drosophila is necessary not only for female sexual differentiation but also for normal spermatogenesis in males. We have cloned and characterized the tra-2 gene. Its putative protein has a domain that is homologous to RNA binding proteins. This suggests that the tra-2 protein might achieve the female-specific splicing of the transcript of dsx, a sex-determining gene whose mode of expression depends on tra-2. The protein coding region of the tra-2 transcript is identical in males and females. In both sexes, a low level of transcript is present in the soma and a high level in the germ line. This indicates that tra-2 is regulated in a way different from other sex-determining genes.

Cell-autonomous and inductive signals can determine the sex of the germ line of Drosophila by regulating the gene Sxl

To investigate the mechanism of sex determination in the germ line, we analyzed the fate of XY germ cells in ovaries, and the fate of XX germ cells in testes. In ovaries, germ cells developed according to their X:A ratio, i.e., XX cells underwent oogenesis, XY cells formed spermatocytes. In testes, however, XY and XX germ cells entered the spermatogenic pathway. Thus, to determine their sex, the germ cells of Drosophila have cell-autonomous genetic information, and XX cells respond to inductive signals of the soma. Results obtained with amorphic and constitutive mutations of Sxl show that both the genetic and the somatic signals act through Sxl to achieve sex determination in germ cells.

The Larval Development of Imaginal Disks

The life of holometabolous insects, such as Drosophila, is characterized by two separate and completely different phases of development. Out of the egg hatches a larva which then becomes transformed during metamorphosis into the adult insect, the imago. The future adult is “hidden” within the larva in the form of the so-called imaginal cells. After fertilization a series of syncytial divisions produces a homogeneous population of nuclei (energids) which later move into the cortical periplasm at the egg’s periphery where they form the cellular blastoderm (review by Anderson, 1972). This is probably the stage at which the presumptive imaginal cells are segregated from the larval cells (seep. 8). These two distinct populations of cells perform their specific vital functions at entirely different periods of development. During metamorphosis, the larval organization breaks down, and the adult insect is formed anew from the imaginal cells. (There are a few exceptional organ systems, e.g. the Malpighian tubules, that persist through metamorphosis.)

Gynandromorphs reveal two separate primordia for male and female genitalia inDrosophila melanogaster

SummaryThe derivatives of 110 mosaic genital discs of gynandromorphs have been analysed microscopically. It has been found that theanalia of both sexes are homologous and derive from a single primordium (see Fig. 1a). Whether male or female anal plates are formed depends on the genetic constitution of the cells. This is analogous to the development of male sex combs versus female transversal rows on the forelegs of gynandromorphs. In contrast, the data for thegenitalia (see Fig. 1 b) are best explained if it is assumed that there are two genital primordia in everyDrosophila embryo: a male primordium that will only develop into genitalia if populated by XY (or XO) nuclei, and a female primordium that will only do so if populated by XX nuclei. This model, as depicted in Figure 2, is compatible with all our gynandromorph data and also with observations onMusca andCalliphora where in fact two separate genital primordia are found.

Sex determining genes and vitellogenin synthesis in Drosophila melanogaster

SummaryThis study investigates the relationship between sexual phenotype and ability to synthesize vitellogenin (yolk proteins, YPs) in Drosophila. Various mutations were used to transform XX and XY animals into intersexes or pseudomales (Table 1). The presence or absence of YPs in the haemolymph and in the fat body was determined by SDS gel electrophoresis, fluorography, and precipitation of YPs with anit-YP antibody (see Fig. 1). YPs were synthesized whenever the flies displayed at least some female morphological characteristics, regardless of their sex chromosome constitution (Table 1; Fig. 2). Pseudomales (definition see p. 1) did not produce detectable amounts of YPs despite their female XX-karyotype. Immature ovaries, transplanted into adult males or pseudomales, developed normally and synthesized YPs, but the fat bodies of the host males or pseudomales were not induced to synthesize YPs. Vitellogenesis was, however, induced in the fat bodies of males and pseudomales by injection of 20-hydroxyecdysone (ecdysterone) (Fig. 3). The results are interpreted to mean that the sexual pathways are controlled by a small number of key genes that regulate the synthetic activities of many sex-specific genes. However, the female-specific YP genes can be activated with ecdysterone although the genetic signals are set for male differentiation.

The embryonic organization of the genital disc studied in genetic mosaics ofDrosophila melanogaster

SummaryThe embryonic organization of the sexually dimorphic genital disc was studied in genetic mosaics resulting (a) from early loss of a chromosome or (b) from mitotic recombination.(a)Early Loss of a Chromosome. Three types of mosaics were produced — purely female mosaics, purely male mosaics, and gynandromorphs. They show that the genital disc arises from a group of cells in the ventral region of the embryo somewhat larger than that giving rise to a single foreleg (Table 2). Within this group of cells three regions can be distinguished that are present in both sexes: an anterior, a medial, and a posterior one, with distances of only 3–4 sturts between adjacent regions. The anterior region gives rise to the female genitalia, the medial region to the male genitalia, and the posterior region forms the analia of both sexes and the parovaria of the female (Figs. 2 and 3). The relative positions of the three regions were deduced from sturt distances (Tables 1 and 5), and from frequencies of mosaicism (Table 2).(b)Mitotic recombination was induced at the blastoderm stage in order to produce twin spots in the external genitalia and analia of purely male and female flies. Clone sizes indicate that these structures arise from a small number of precursor cells (Table 4). Clones overlapped right and left sides, but no clones were found extending over analia and genitalia. However, within either the analia or the genitalia of each sex, no clonal restrictions could be observed, and the clones comprised structures that were up to 12 sturts apart. A comparison of clone sizes and sturt distances in the foreleg and in the genital disc indicates that equal gynandromorph distances involve equal numbers of cells in different regions on the ellipsoid egg (Fig. 5). The results obtained from all mosaics provide a consistent picture of the embryonic organization of the genital disc. This becomes apparent in the summarized fate maps (Fig. 4), where the map derived from normal gynandromorphs can be produced by a simple superposition of the male and the female maps. The data are also discussed with respect to mechanisms of sexual differentiation in the genital disc.

A female nervous system is necessary for normal sperm storage in Drosophila melanogaster: a masculinized nervous system is as good as none

A male Drosophila melanogaster deposits many more sperm in a females bursa copulatrix than are stored in her ventral receptacle or paired spermathecae soon after copula has ended. The remaining sperm are expelled by the female. These observations suggest a sexual conflict over the processes involved in sperm storage. We used genetically manipulated flies to study the role of the central nervous system in sperm storage. Flies with female bodies but masculinized nervous systems, or isolated female abdomens, stored significantly fewer sperm than did control females. Furthermore, compared with control flies, there were relatively more sperm in the ventral receptacle and relatively fewer in the spermathecae. These results suggest that the female nervous input counteracts the males attempts to force sperm into the ventral receptacle during copula and promotes active transport of sperm to the spermathecae during and after copula. The female is clearly a very active partner in influencing processes involved in sperm competition, especially as only stored sperm can be used later to fertilize eggs. To our knowledge, this is the first study to show directly the involvement of the female nervous system in sperm storage.

Alternatively spliced transcripts of the sex-determining gene tra-2 of Drosophila encode functional proteins of different size.

The Drosophila transformer‐2 gene (tra‐2) is required for female sex determination in somatic cells and for spermatogenesis in male germ cells. We studied the organization of the tra‐2 gene and characterized the transcripts in wild type and mutant animals. Two transcripts are detected in males and females; they differ in their abundance and in the presence (minor transcript Tmin) or absence (major transcript Tmaj) of one exon. Two other transcripts are present only in male germ cells. One of these is rare (msTmin) and represents a spliced form of the other, more abundant transcript (msTmaj). The transcript Tmaj encodes a protein of 264 amino acids, whereas transcripts Tmin and msTmaj encode proteins that are truncated at the N‐terminus. All three putative proteins contain a stretch of approximately 90 amino acids, the ribonucleoprotein motif (RNP motif), which shows similarity to a variety of different ribonucleoproteins. Transformation studies reveal that a cDNA corresponding to the transcript Tmaj can provide all the functions for female sex determination and male fertility. Surprisingly, a cDNA corresponding to the transcript msTmaj could only supply some female sex‐determining function, but was unable to restore fertility in mutant males. Sequence analysis of two temperature‐sensitive mutations provides evidence that the RNP motif represents an important functional domain of the tra‐2 protein.

Genetic and developmental evidence for a repressed genital primordium in Drosophila melanogaster

Abstract Studies with chromosomal mosaics have indicated that the blastodermal anlage of the genital disc of Drosophila consists of three primordia: one for the female genitalia, one for the male genitalia, and one for the analia. In normal development, only one of the two genital primordia forms adult derivatives whereas the other becomes repressed. The present report investigates the fate of the repressed primordia and analyzes the function of the transformer (tra) gene in the development of the genital primordia (the mutation tra transforms chromosomal females into males). The repressed male genital primordium (RMP) in the female disc was localized by determining the site from which male genital clones were growing. For this purpose, clones homozygous for tra were generated late in the third instar by irradiating female larvae heterozygous for tra + . Their genital discs were cultured in vivo to allow for additional cell proliferation before metamorphosis. After culture, 19 out of 224 discs showed local outgrowths, presumably representing tra tra clones. These outgrowths were cut out of the discs and transplanted into larvae, where they metamorphosed into male genital structures. All clones were found in the anterior third of the dorsal disc epithelium. If isolated from a normal female disc, this region does not form any adult structures. We therefore assume that it harbors the repressed male genital primordium (RMP). The clones also demonstrate that the action of tra+ is continuously required to maintain the female sexual pathway. The intersexual genital disc of the genotype X X ; dsx D + exhibits well-developed male and female genital primordia. The site of the male primordium coincides with the site of the RMP as determined above. The female primordium is located in the ventral epithelium where the normal male genital disc has a slighly enlarged region which does not form any known imaginal structures. We propose that this part of the male disc contains the repressed female genital primoridum (RFP).

Sex determination in Drosophila

Abstract Sex determination and differentiation are processes that create morphological, physiological and behavioral differences between the sexes by initiating and completing one of two alternative developmental programs. In Drosophila , as in other organisms whose genetics of sex determination has been studied, this is achieved by a hierarchical control system in which a chromosomal signal affects a small number of regulatory genes whose state of activity then controls the many differentiation genes needed to materialize all the differences between the sexes. The genetic pathway that specifies sexual differentiation can serve as a paradigm of how developmental processes in general are controlled.