Phf7 controls male sex determination in the Drosophila germline

Abstract

Establishment of germline sexual identity is critical for production of male and female germline stem cells, as well as sperm versus eggs. Here we identify PHD Finger Protein 7 (PHF7) as an important factor for male germline sexual identity in Drosophila. PHF7 exhibits male-specific expression in early germ cells, germline stem cells, and spermatogonia. It is important for germline stem cell maintenance and gametogenesis in males, whereas ectopic expression in female germ cells ablates the germline. Strikingly, expression of PHF7 promotes spermatogenesis in XX germ cells when they are present in a male soma. PHF7 homologs are also specifically expressed in the mammalian testis, and human PHF7 rescues Drosophila Phf7 mutants. PHF7 associates with chromatin, and both the human and fly proteins bind histone H3 N-terminal tails with a preference for dimethyl lysine 4 (H3K4me2). We propose that PHF7 acts as a conserved epigenetic "reader" that activates the male germline sexual program.

Figure 1

Expression profiles of Phf7. A, B) in situ hybridization of stage 17 male and female embryos for Phf7. The signal at the anterior (left) of the female embryo comes from hybridization of the Dfd-lacZ transgene used for sexing. C) RT-PCR of Phf7 in adult tissues. Carcass refers to whole adult flies with gonads removed. Ribosomal protein RpLP0 was used as internal control. D–F) Immunofluorescence of the C-terminally HA-tagged PHF7 BAC transgene, co-labeled as indicated on the panels. The ' panels display the PHF7 channel alone. Anti-VASA labels the germline and DAPI labels the DNA. D, D') Adult testis. E, E') Adult ovary. F, F', F") Colocalization of BAM with HA-tagged PHF7 in adult testis. The hub is indicated with a white asterisk. F" displays only the BAM channel. G) Fractions of stage 17 embryonic gonads expressing Phf7 by in situ hybridization of stage 17 mutant embryonic gonads. Genotypes are as indicated. Note that Phf7 is induced in XX germ cells in a male soma (tra-) and repressed in XY germ cells in a female soma (traF). See also Figure S1.

Figure 2

Loss of Phf7 causes male specific developmental defects in the germline. A) Average fecundity of single female flies mutant for Phf7 compared to heterozygous controls. B) Average fecundity of single male flies mutant for Phf7 compared to mutant males rescued with either the BAC, UAS-Phf7 transgene, or UAS-human PHF7 (hPhf7) construct. Error bars in A and B represent standard deviations. C) Representative images of control or Phf7-mutant genetic mosaic clones at 10 days post clone induction. Clones are indicated by loss of GFP (green); VASA (germline, red); N-CADHERIN (hub, blue). Genotypes are as indicated in Experimental Procedures. D) Average numbers of GSC control (green diamonds) and mutant (blue squares) clones plotted for 2, 5, and 10 days after clonal induction. E) Average numbers of control (green diamonds) or mutant (blue squares) spermatocyte clones at 2, 5, and 10 days after clonal induction. See also Table S1.

Figure 3

Profiles of GSCs and spermatogonia in hemizygous Phf7-mutant testes compared to controls. A) Average numbers of GSCs in mutant and BAC-rescued testes. Error bars represent standard deviations. B) Percentages of centrosome pairs in GSCs that did not exhibit hub-proximal orientation in mutant and BAC-rescued testes. C) FLP-mediated lineage analysis of GSCs and developing spermatogonia in hemizygous Phf7-mutant (blue squares) and control testes (green diamonds). Genotypes are as in Experimental Procedures. The number of spermatogonial clones at 1-, 2-, 4-, 8-, and 16-cell stage per GSC clone is plotted at 2 days post clonal induction. D–G) Adult testes are immunolabeled as indicated. D' and E' show the BAM staining alone. Hubs are marked with white asterisks. Note the reduced population of BAM-positive spermatogonial cysts in Phf7-mutant (D, Phf7^ΔN2/Y) compared to BAC-rescued testes (E, Phf7^ΔN2/Y; BAC/+). F, G) Expanded population of esg^M5-4-expressing germline in Phf7-mutant (F, Phf7^ΔN2/Y) testes compared to controls (G, Phf7^ΔN2/Y; BAC/+). Arrow points to a branched fusome connecting a 4-cell spermatogonial cyst. H) Localization of spectrosomes is altered in Phf7-mutant GSCs compared to BAC-rescued counterparts. Cartoons on the left depict the typical spectrosome localization patterns that are different in male and female GSCs. The percentages of GSCs exhibiting hub-proximal spectrosomes are plotted on the graph to the right for both Phf7-mutants and BAC-rescued samples. For all bar graphs, the numbers at the base of the bars indicate the sample size for each genotype. See also Figure S3.

Figure 4

Overexpression of Phf7 induces apoptosis only in the undifferentiated female germline. A–E) Ovaries from freshly eclosed females are immunolabeled as indicated; VASA (green, germline), N-CADHERIN (red, terminal filaments in female, hub in male), HTS (blue, fusome). A, control ovary (nos-Gal4). B, EY03023/w; nos-Gal4/+. C, EY03023/w; UAS-Gal4/+; Bam-Gal4/+. D, EY03023/c587-Gal4. E, UAS-Phf7/nos-Gal4. F, G) Testes from newly eclosed males are immunolabeled as in A-E. F, control testis (nos-gal4). G, EY03023/Y; nos-Gal4/+. H) Average numbers of activated Capase-3 positive germ cells in control (nos-Gal4) and UASPhf7/nos-Gal4 3^rd instar ovaries. I–K) 3^rd instar larval ovaries are immunolabeled as indicated. I) UAS-Phf7/nos-Gal4 3^rd instar ovaries highlighting germline undergoing Capase-3-dependent apoptosis. I' shows the activated Caspase-3 signal alone. J, K) Loss of germline observed in 3^rd instar ovaries of control (J, nos-Gal4) and UAS-Phf7/nos-Gal4 (K).

Figure 5

Phf7 controls the male germline sexual program. A–C) Adult ovaries are immunolabeled as indicated. A, control XX ovary (+/+; U2AF-traF/+). B, Feminized XY gonad overexpressing traF (+/Y; U2AF-traF/+). C, Phf7-mutant XY gonad overexpressing traF (Phf7^ΔN2/Y; U2AF-traF/+). Note the significant rescue of germ cell number and organization in C compared to B. D–G) Adult testes are immunolabeled as indicated. D, control XY testis (nos-Gal4). E, masculinized (tra-mutant) XX pseudo-testis (+/+; nos-Gal4, tra¹/Df(tra)). F and G, Phf7-overexpressing, tra mutant XX pseudo-testis (EY03023/+; nos-Gal4, tra1/Df(tra)). F is an example of an unrescued pseudo-testis, while G is an example of a fully-rescued, wild-type looking pseudo-testis. H–J) Phase contrast images of adult testes. H, XY control (Oregon-R, +/Y). I, XO males (yw/O; st¹ncd^D/+). J, Phf7-overexpressing, tra mutant XX pseudo-testis (EY03023/+; nos-Gal4, tra¹/Df(tra)). K–M) Nuclear condensation of maturing sperm at the base of adult testis coils revealed by DAPI stains. K, XY control (Oregon-R, +/Y). L, XO males (yw/O; ncd^D/+). M, Phf7-overexpressing, tra mutant XX pseudo-testis (EY03023/+; nos-Gal4, tra¹/Df(tra)). J and M are from examples of Phf7-rescued pseudo-testes. See also Figure S4.

Figure 6

PHF7 is a histone code reader. A–C) Polytene chromosomes isolated from salivary glands expressing UAS-Phf7-HA with fkh-Gal4 are co-labeled with antibodies for HA (PHF7) and various modifications on histone H3 tails as indicated. DAPI labels the DNA. A, H3K4me3. B, H3K4me2. C, H3K27me3. Smaller panels represent larger magnification of regions of the chromosomes (white rectangles) showing either the H3 mark (green) or HA-PHF7 (red) alone, or with the single channels split and aligned on the same image for direct comparison. D) α-MBP western blot analyses of MBP-tagged proteins precipitated with H3 N-terminal tail peptides modified as indicated. dPHF7-N is the N-terminal PHD finger domain of Drosophila PHF7, hPHF7 is full length human PHF7 and hING2 is the PHD finger of human ING2. Inputs are 1% for dPHF7 and hING2 and 10% for hPHF7. Note that the low specific activity of dPHF7-N in the binding assay was due to a tendency to aggregate in solution that was remarkably consistent with different protein fragments and purification conditions. See also Figure S5.