Nonautonomous sex determination controls sexually dimorphic development of the Drosophila gonad

Abstract

Sex determination in Drosophila is commonly thought to be a cell-autonomous process, where each cell decides its own sexual fate based on its sex chromosome constitution (XX versus XY). This is in contrast to sex determination in mammals, which largely acts nonautonomously through cell-cell signaling. Here we examine how sexual dimorphism is created in the Drosophila gonad by investigating the formation of the pigment cell precursors, a male-specific cell type in the embryonic gonad. Surprisingly, we find that sex determination in the pigment cell precursors, as well as the male-specific somatic gonadal precursors, is non-cell autonomous. Male-specific expression of Wnt2 within the somatic gonad triggers pigment cell precursor formation from surrounding cells. Our results indicate that nonautonomous sex determination is important for creating sexual dimorphism in the Drosophila gonad, similar to the manner in which sex-specific gonad formation is controlled in mammals.

Figure 1. SOX100B identifies two distinct sexually dimorphic cell types in the embryonic gonad

Embryonic gonads immunostained as indicated in the figure. Anterior is to the left in each panel. (A,B) Wild-type st. 17 male gonads. msSGPs express ABD-B (A) and EYA (B) (arrows), in addition to SOX100B, whereas the ensheathing cells express only SOX100B. In this genetic background, a few posterior SGPs also express both SOX100B and ABD-B, as has been observed previously (DeFalco et al., 2003). (C,D) St. 17 XX (C) and XY (D) gonads from embryos expressing Kr-GAL4, UAS-GFP constructs on a TM3 balancer chromosome. GFP is expressed in cells ensheathing male gonads (arrowhead), but is not expressed in msSGPs (arrow). (E,F) Embryos homozygous for the HCJ199 allele of Abd-B. Male gonads lack msSGPs at both st. 15 (E) and st. 17 (F), but still exhibit SOX100B-positive ensheathing cells at st. 17. (G,H) Embryos expressing evestr3+7-GAL4 in combination with UAS-GFP and UAS-GAL4 for permanent lineage tracing. msSGPs (arrows) express GFP at st. 13 (G) and st. 17 (H) but SOX100B-positive ensheathing cells do not (arrowheads).

Figure 2. SOX100B-expressing cells give rise to testis PCs

St. 17 gonads and adult testes immunostained as indicated in the figure. Anterior is to the left in each panel. (A,A’) Kr-GAL4, UAS-GFP adult testis examined by either light microscopy (A) or immunofluorescence (B). Inset: Slice of three-dimensional reconstruction of testis. Note that GFP expression is coincident with the yellow pigmentation of the testis and is only in the outer layer of cells. (B,C) The B-57 enhancer trap labels PCs in the adult testis (B) and st. 17 embryonic male gonad (C), and co-localizes with SOX100B (arrowheads). (D,E) Adult testes showing SOX100B-positive PCs in both wild-type (D) and AbdB[HCJ199] mutants (E). The AbdB[HCJ199] mutant testes do not have the normal, elongated appearance of wild-type testes, likely due to defects in interaction with the genital disc. (F,G) St. 17 embryonic male gonads immunostained to reveal msSGPs (co-expressing ABD-B and SOX100B) and PCs (SOX100B only, arrowheads). In addition, a few posterior SGPs also co-express ABD-B and SOX100B (DeFalco et al., 2003). Note that PCs are observed in wild-type (F) but not Wnt2 mutants (G).

Figure 3. PC precursors are specified from fat body cells surrounding the male gonad

St. 17 gonads immunostained as indicated in the figure. Anterior is to the left in each panel. (A) Expression of the svp-lacZ fat body enhancer trap overlaps with SOX100B-positive PC precursors. (B) Cells that express the PC precursor marker Kr-GAL4, UAS-GFP also express SRP. (C) A srp mutant gonad that lacks PC precursors but has msSGPs (co-expressing SOX100B and EYA, arrow). (D,E) EMS is expressed in cells that ensheath the male (D), but not female (E) embryonic gonad. (F,G) SOX100B-positive PC precursors surround the wild-type male gonad (F, arrowhead) but are not observed in ems mutant gonads (G), which only exhibit SOX100B expression in msSGPs (arrow).

Figure 4. tra and dsx control sexual dimorphism in the PC precursors

St. 17 gonads immunostained with anti-VASA (germ cells, red) and anti-SOX100B (green). Anterior is to the left in each panel. (A,B) SOX100B-positive PC precursors are observed in both XX (A) and XY (B) tra mutant embryos. (C,D) PC precursors are not observed in either XX (C) or XY (D) gonads when the female form of tra is ectopically expressed using UAS-tra^F; tubulin-GAL4. (E,F) PC precursors are observed in both XX (A) and XY (B) dsx mutant embryos. (G,H) Df (H99) mutants defective for apoptosis. Note that PC precursors (arrowheads) are still observed in XY (H) but not XX (G) embryos, similar to wild type. In contrast, msSGPs (arrows) are observed in both XY and XX embryos.

Figure 5. Non-cell autonomous control of sex determination in the Drosophila gonad

(A,B) Diagram of prd-GAL4 expression (green) in the embryonic gonad at st. 15 (A) and st. 17 (B). Anterior is to the left in each panel. Approximate PS numbers are indicated in (A) based on the known pattern of prd gene expression. The black line in (B) outlines the hub. (C-F, H-J) Immunostainings of embryonic gonads as indicated. (C) GFP expression from st. 15 male embryo expressing prd-GAL4 and UAS-GFP. Traffic Jam (TJ) staining labels PS10-12 SGP nuclei. Lines separate GFP-positive and GFP-negative regions of the gonad. Note that GFP is expressed in PS 11 SGPs and strongly in the msSGPs from PS 13. GFP is also expressed in subsets of cells surrounding the gonad. (D) GFP expression from st. 17 male embryo expressing prd-GAL4 and UAS-GFP. Note that the hub (outline, labeled with DN-cadherin) is normally composed of GFP-positive (PS11) and GFP-negative (PS10) cells. (E) A similar embryo as in (C) but expressing TRA instead of GFP. Note that the msSGPs are still present in the XY gonad even though they express TRA. (F) A similar embryo as in (D) but now expressing TRA in addition to GFP. Note that GFP/ TRA expressing cells do not contribute to the hub (outline), in contrast to GFP-alone expressing cells in (D). (G) St. 15 wild type embryo labeled by in situ hybridization for dsx mRNA. (H) St. 17 embryo expressing prd-GAL4, UAS-GFP and UAS-tra^F, immunostained for SOX100B and GFP. Note that GFP/ TRA expressing cells (arrowheads) can still become PC precursors based on their expression of SOX100B and their position and morphology. (I,J) tin zfh-1 double-mutant embryos. Note that msSGPs (arrows, co-labeled with EYA and SOX100B) are present at st. 12 (I), but not at st. 15 (J).

Figure 6. Wnt2 acts non-autonomously to regulate sex-specific PC precursor specification

(A-C) St. 17 embryos labeled by fluorescent in situ hybridization for Wnt2 mRNA (red) and anti-VASA immunostaining (green). Anterior is to the left in each panel. (A’-C’) Wnt2 mRNA channel alone. (A,B) Wild type embryos. Wnt2 expression is robust in male embryonic gonads at st. 17 (B,B’) but is not detectable above background in females (A,A’) at this stage. (C) dsx¹/dsx²³ mutant gonad. Wnt2 expression is now observed in 100% of embryos. (D-G) St. 17 embryos immunostained as indicated in the figure. (D) Female embryo exhibiting SOX100B-positive PC precursors when Wnt2 is ectopically expressed in the germ cells (UAS-Wnt2, nos-GAL4). (E-G) Embryos expressing prd-GAL4 and UAS-GFP either alone (E), or in combination with UAS-dTCFΔN (F) or UAS-sgg^act (G). Note that some GFP-positive cells become SOX100B-positive PC precursors when expressing GFP alone (E, arrowheads), but not when expressing UAS-dTCFΔN (F) or UAS-sgg^act (G).

Figure 7. Model for non-autonomous control of sexual dimorphism in the Drosophila gonad

Diagram of a male gonad. Anterior is to the left. Sexual dimorphism of three cell types is controlled non-autonomously via different types of male-specific cell-cell signaling from the SGPs: Wnt2 expression induces PC precursor formation, the JAK/STAT pathway regulates male germ cell development, and an unknown signal promotes msSGP survival in male gonads. Inset shows a model for PC specification, in which Wnt2 is regulated by dsx to induce male PC fate in surrounding fat body cells. Although Wnt2 is genetically downstream of dsx as indicated, our data indicate that dsx acts in the female gonad to directly or indirectly repress Wnt2 expression.