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Sex Dev. 2009 April; 3(1): 26–37.
PMCID: PMC2836946

Sox100B, a Drosophila Group E Sox-domain Gene, Is Required for Somatic Testis Differentiation


Sex determination mechanisms are thought to evolve rapidly and show little conservation among different animal species. For example, the critical gene on the Y chromosome, SRY, that determines sex in most mammals, is not found in other animals. However, a related Sox domain transcription factor, SOX9, is also required for testis development in mammals and exhibits male-specific gonad expression in other vertebrate species. Previously, we found that the Drosophila orthologue of SOX9, Sox100B, is expressed male-specifically during gonad development. We now investigate the function of Sox100B and find, strikingly, that Sox100B is essential for testis development in Drosophila. In Sox100B mutants, the adult testis is severely reduced and fails to interact with other parts of the reproductive tract, which are themselves unaffected. While a testis initially forms in Sox100B mutants, it fails to undergo proper morphogenesis during pupal stages, likely due to defects in the pigment cells. In contrast, no substantive defects are observed in ovary development in Sox100B mutant females. Thus, as is observed in mammals, a Sox9 homolog is essential for sex-specific gonad development in Drosophila, suggesting that the molecular mechanisms regulating sexually dimorphic gonad development may be more conserved than previously suspected.

Key Words: Drosophila, Group E Sox, Sox100B, Testis


The development of sexually dimorphic gonads is a key outcome of sex determination programs in metazoans. In vertebrates, the formation of a testis or ovary subsequently directs the development of all other sexually dimorphic characteristics through hormonal signals (Capel, 2000). While the morphological development of the gonads is very similar in mammals, birds, and reptiles, the primary genetic mechanisms that determine sex are apparently not conserved. Vertebrates employ a wide range of methods for determining sex, ranging from a dominant male-determining gene to temperature-dependent mechanisms (Sinclair et al., 2002). Outside of the vertebrates, with the exception of the key model species Drosophilamelanogaster and Caenorhabditis elegans, comparatively little is known about the molecular mechanisms that determine sex or underlie the development of the gonads. Until recently, sex determination in Drosophila was thought to be an entirely cell-autonomous process controlled by the balance of sets of autosomes to X chromosomes and apparently unrelated to any mechanism used by vertebrates (Cline and Meyer, 1996). Contrary to this view a number of tissues in Drosophila have been shown to exhibit non-autonomous sex determination, including the gonad (DeFalco et al., 2008), the genital disk (Ahmad and Baker, 2002), and the germline (Nothiger et al., 1989). In the case of the gonad, the sexual phenotype of some male-specific somatic cell types is independent of the internal state of the somatic sex-determination hierarchy but is influenced by sex-specific signals from other somatic gonadal cells (DeFalco et al., 2008). These observations raise the possibility that aspects of sexual differentiation may be more similar between species than hitherto suspected. Indeed, many striking parallels can be drawn between flies and vertebrates in terms of the morphological development of the gonad (Capel, 2000). Therefore, despite the apparent disparate genetic methods used to determine sex, there are tantalizing hints that the molecular switches that are responsible for the development or differentiation of the testis may be conserved.

In eutherian mammals, where sexual differentiation has been extensively studied, a picture of the molecular hierarchy controlling the sexual specification of the gonad is beginning to emerge (Bowles and Koopman, 2001; Ronfani and Bianchi, 2004). In XY individuals the presence of the Y-linked transcription factor SRY (Sinclair et al., 1990), in conjunction with the orphan nuclear receptor Steroidogenic Factor 1 (SF-1), directly results in the up-regulation of the SOX9 transcription factor in somatic cells of the bipotential gonad primordium (Sekido and Lovell-Badge, 2008). Together with the FGF9 growth factor, SOX9 directs somatic cells to differentiate as Sertoli cells and hence the male pathway is initiated (Sekido et al., 2004; Kim et al., 2006). In the absence of SRY, Wnt4 signaling represses SOX9 and FGF9, resulting in female development (Kim et al., 2006). The fact that SOX9 is necessary for normal male development and is sufficient to direct testis development in XX individuals suggests that it is the key sex determining factor downstream of SRY (Vidal et al., 2001; Qin and Bishop, 2005; Barrionuevo et al., 2006). While the gene network controlled by SOX9 is not yet known, there is good evidence that SOX9 collaborates with SF-1 and Wilms tumor 1 (WT1) to activate the expression of anti-Müllerian hormone (AMH). Expression of AMH results in the regression of the female ductal system, and the differentiation of a male somatic gonad ensues (De Santa Barbara et al., 1998; Nachtigal et al., 1998; Arango et al., 1999).

While the Drosophila genome does not contain an orthologue of SRY, and none of the fly Sox genes have expression patterns or mutant phenotypes that suggest they are involved in the canonical primary sex determination pathway (Cremazy et al., 2001), there is a single orthologue of the vertebrate group E genes (Sox8, 9, and 10), Sox100B (Loh and Russell, 2000). Intriguingly, we previously demonstrated that Sox100B expression is sexually dimorphic in embryonic gonads, marking a testis-specific group of somatic cells, the male-specific somatic gonadal precursors (msSGPs) (DeFalco et al., 2003). More recently, we reported that Sox100B is also expressed in another male-specific somatic gonad cell type, the pigment cells. In the embryonic precursors of the pigment cells, Sox100B expression and the male phenotype is induced by Wnt2 signaling from male somatic gonad precursors (SGPs) independently of the internal state of the sex-determining hierarchy in the pigment cell precursors (DeFalco et al., 2008). While we have some understanding of how the expression of Sox100B is controlled in the embryonic gonad, our previous studies did not allow us to assess the requirements for Sox100B in testis development due to the lack of suitable mutations; here we report the isolation and analysis of Sox100B null mutations. Strikingly, we find that in the absence of Sox100B, testis development is severely affected while ovarian development is comparatively normal. We find that mutations in Sox100B lead to the loss of a sex-specific population of somatic cells in the testis, the pigment cells, which is associated with a complete failure in gonad morphogenesis during metamorphosis. Our observations suggest that, as in vertebrates, a group E Sox protein is involved in facilitating the development of somatic cell lineages in the testis.

Materials and Methods


Stocks were maintained on standard cornmeal-yeast-agar at 25°C. The following stocks were used (all nomenclature is according to FlyBase) (Drysdale, 2008): dcoj3B9 (Kloss et al., 1998); Df(3R)tll-e/TM6B and Df(3R)tll-g/TM6B (Lindsley and Zimm, 1992); y w; P[w+ dco+] (Zilian et al., 1999); eve stripe 3 + 7-Gal4 (gift of S. Small); TM3, P[Gal4-Kr.C]DC2, P[UAS-GFP.S65T]DC10, and CyO, P[Gal4-Kr.C]DC3, P[UAS-GFP.S65T]DC7 (Casso et al., 2000); LacZ[B-57] and LacZ[842] (Gift of E. Matunis) (Gonczy, 1995).


To isolate deletions of Sox100B, the dcoj3B9 line was crossed with a source of transposase (Dr P[ry+Δ2-3]99B) (Robertson et al., 1988). Resulting w males were tested for viability over Df(3R)tll-e and 84 lethal lines analysed by Southern blotting (Loh, 1999). The 4 Sox100B deletion lines recovered are illustrated in figure figure3A.3A. EMS-induced alleles were isolated as follows: w1118 males were fed EMS and crossed to dcoj3B9/TM3 females, single w; */dcoj3B9 males were crossed with w; Df(3R)j3B9-rvB females and chromosomes lethal with this Sox100B deletion were tested forlethality over Df(3R)j3B9-rv12,Df(3R)tll-e and Df(3R)tll-e, P[dco+] but viability over Df(3R)tll-g, P{lacW}l(3)03670S020514b (a lethal insertion in the gene distal to dco) and P{SUPor-P}Gycβ100BKG09937 (the next most proximal mutation).

Fig. 3
Isolation of Sox100B mutants.A Diagram of part of the 100B region on chromosome arm 3R with simplified gene models derived from FlyBase; arrowheads on the last exon indicate the direction of transcription. The insertion site of the j3B9P-element is indicated ...


Embryos, larval, pupal, and adult tissues were dissected and fixed according to standard procedures (Michaud et al., 1997; DeFalco et al., 2003). Throughout this report anti-Vasa staining is used to label the germline. The following antibodies were used: rabbit anti-Sox100B at 1:1,000 (Loh and Russell, 2000); chicken anti-Vasa at 1:5,000 (gift of K. Howard); rat anti-Vasa at 1:50 (Developmental Studies Hybridoma Bank, DSHB); rat anti-Dri at 1:200 (Gregory et al., 1996); rabbit anti-muscle myosin at 1:500 (Kiehart and Feghali, 1986); mouse anti-Abd-B[1A2E9] at 1:50, mouse anti-Eya[10H6] at 1:25; mouse anti-FasIII [7G10] at 1:30, mouse anti-Cut at 1:50 (DSHB); rabbit anti-βGal at 1:10,000 (Cappel); mouse anti-βGal at 1:10,000 (Promega); mouse anti-Orb[4H8] at 1:30 (DSHB); rabbit anti-Ems (U. Walldorf) at 1:500; mouse anti-GFP B-2 (Santa Cruz) at 1:50. Oligreen (Molecular Probes) was used to label DNA, at a 1:10,000 dilution in PBTx for 12–15 min after secondary antibody incubation and washes. Fluorescent images were collected with a Zeiss 510 Meta confocal microscope; all other images were collected with a Zeiss Axiophot microscope.


Sox100B Is Dynamically Expressed throughout Testis Development

Sox100B is dynamically expressed in several tissues during embryonic development, including the gut, malpighian tubules, and gonad (Loh and Russell, 2000). With respect to the developing gonad, Sox100B is first detected in a group of mesodermal cells that originate from parasegment 13 (PS13) during late stage 11 (DeFalco et al., 2003). After the gonads coalesce at stage 15, Sox100B is restricted to a specific group of somatic cells in the male gonad, the male-specific somatic gonadal precursors (msSGPs), which are eliminated in female embryos by apoptosis (fig. (fig.1A)1A) (DeFalco et al., 2003). There is no expression in the germline as indicated with the germ cell marker Vasa. Tracing the lineage of the msSGPs by marking the derivatives of PS13 with a GFP marker expressed under the control of the eve stripe 3 + 7 enhancer (a gift from S. Small) we find that these cells give rise to the terminal epithelium of the adult testis (fig. (fig.1B,1B, C). The seminal vesicle, which also expresses GFP in this genetic background, is derived from the genital imaginal disc not the gonad and does not express Sox100B. We notice that one or two Sox100B positives cells in stage 15 embryonic gonads do not express the terminal epithelium marker (fig. (fig.1B).1B). Towards the end of embryogenesis, at stage 17, Sox100B expression has declined in the msSGPs and is now evident as specific nuclear staining in a group of large cells surrounding the testis (fig. (fig.1D)1D) (Hempel and Oliver, 2007; DeFalco et al., 2008). We have recently shown that these superficial cells are independent of the msSGPs and are the precursors of the pigment cells of the adult testis. Co-staining adult testes for Sox100B and a pigment cell marker (LacZ B-57) confirms that Sox100B is expressed in these cells (DeFalco et al., 2008). Staining larval, pupal, and adult testes for Sox100B confirms that Sox100B is maintained in the pigment cells throughout testis development (fig. (fig.1E–J).1E–J). In addition to the pigment cells, Sox100B is also expressed in the region of the terminal epithelium in a superficial layer of cells (fig. (fig.1K),1K), and we also observe weak, predominantly cytoplasmic, staining in spermatogenic cysts (not shown). Sox100B is not expressed in the somatic cyst cells by co-staining with the cyst marker, Eya (fig. (fig.1L)1L) (Fabrizio et al., 2003). Thus, in the testis, Sox100B is expressed predominantly in the somatic pigment cells throughout testis development.

Fig. 1
Expression of Sox100B in the testis. In this and subsequent figures, the colors for each antibody staining are indicated by the colored text in each panel. A Wild type stage 15 embryonic testis stained with anti-Vasa to mark the germline and anti-Sox100B ...

In contrast to the expression in the male, in females we see little staining in the embryonic gonad, though a few posterior SGPs show weak, often cytoplasmic staining in some genetic backgrounds (not shown). During larval and pupal stages we observe Sox100B staining in cells that will give rise to the terminal filament of the adult ovary and this expression persists in the adult (fig. (fig.2A,2A, B). Somatic cells of the epithelial sheath surrounding the ovary express Sox100B and, as with the testis, these are distinct from Eya-expressing somatic cells (fig. (fig.2B).2B). Although we have not explored this expression further, these nuclei resemble epithelial sheath cells (Eya) and tracheolar cells (Sox100B) described by King et al. (1968). In the adult ovarioles we observe cytoplasmic staining in the terminal filament and the interfollicular stalk cells (fig. (fig.2C,2C, D). Similar to the situation in the testis, we notice transient nuclear accumulation in stage 12–13 egg chambers that becomes cytoplasmic again by stage 14 and predominantly cytoplasmic expression in follicle cells (not shown).

Fig. 2
Sox100B is expressed in the developing ovary. A Ovary from 3rd instar larva stained for Sox100B, the germline, and DNA. Sox100B expression is strongest in the anterior region, and is also present in posterior cells that may derive from adjacent fat (arrow). ...

Isolation of Sox100B Null Mutations

Since the orthologues of Sox100B play a key role in vertebrate testis development, we sought to recover mutations in Sox100B to assess any role it may play in Drosophila gonad development. Starting from a pupal lethal P-element insertion in the discs overgrown (dco) gene (Kloss et al., 1998), we carried out an imprecise excision screen assaying for lethality over Df(3R)tll-e, a deletion uncovering the 100B region and deficient for Sox100B (Strecker et al., 1986). P{lacW}dcoj3B9 is located within the first intron of the dco gene but does not affect Sox100B expression (fig. (fig.3A)3A) (Loh, 1999). We identified 4 lines carrying deletions of the Sox100B coding region, one of which (Df(3R)j3B9-rv12) removes the entire coding sequence (fig. (fig.3A).3A). All 4 lines are embryonic lethal as homozygotes or in combination with Df(3R)tll-e and show no staining with Sox100B antisera, indicating that they are molecular null alleles of Sox100B (Loh, 1999). None of the deletions extend as far as the next transcription unit (CG11317). Since the starting j3B9 line is pupal lethal and the excisions are embryonic lethal, either Sox100B is required for aspects of embryonic development or we have generated stronger alleles of dco. To separate the effects of Sox100B from dco we utilized a previously described genomic dco rescue construct, recombining this onto the Df(3R)tll-e chromosome (Zilian et al., 1999). Null alleles of dco are viable in combination with Df(3R)tll-e, P[dco+], indicating effective rescue of dco mutations. When hemizygous with the dco rescue chromosome, flies carrying all 4 of the Sox100B deletions survive to the pharate adult stage or die shortly after eclosing from the pupal case, indicating that Sox100B is not required for embryonic viability. Sox100B mutant pharate adults show no discernible external morphological phenotype. More recently, we isolated a series of EMS-induced alleles that are lethal or semi-lethal in combination with Df(3R)j3B9-rv12 and Df(3R)tll-e, P[dco+] but viable over Df(3R)tll-g, which, by in situ hybridization to polytene chromosomes and Southern blotting, does not uncover Sox100B (supplementary fig. 1; for online supplementary figures, see These alleles have similar phenotypes to the P-element excision lines, and 2 of the lines we tested exhibit no staining with anti-Sox100B antisera. For the subsequent analysis reported here we focused on the excision line Df(3R)j3B9-rv12, however, we emphasize that all of the mutations exhibit very similar phenotypes.

Sox100B Mutants Fail to Develop Normal Testes but Have Normal Ovaries

We dissected Sox100B mutantpharate adults from the pupal case and found severe defects in the male gonad while in females the ovaries appear comparatively normal (supplementary fig. 2). In males, the portion of the reproductive tract derived from the genital imaginal disc appears normal, with fully formed seminal vesicles, accessory glands, and external wild type cuticular structures. The testes, however, are severely reduced or absent (fig. (fig.3C,3C, D). As with other Sox gene mutations (Russell et al., 1996), we noticed variability in the null allele phenotypes, with occasional individuals developing partial testis lobes (fig. (fig.3C),3C), however, in no cases do we observe a normal coiled testis. When partially developed testes could be isolated from pharate adults or late-stage pupae, all stages of spermatogenesis, including cysts containing apparently normal elongating sperm bundles, were observed, indicating that at least some germ cells can progress through spermatogenesis in these mutants (supplementary fig. 3). Consistent with the expression of Sox100B in the pigment cells through development, we find that normally pigmented regions of the reproductive tract such as the seminal vesicle are unpigmented in Sox100B mutants (fig. (fig.3E,3E, F). Staining mutant testes for the germ cell marker Vasa indicates that some germ cells are still present although they are frequently aberrantly located (fig. (fig.3G,3G, H). We examined markers for the terminal epithelium (Eya) and testis sheath (Abd-B and Myosin) and found that, although the sheath and terminal epithelium is somewhat disorganized, markers appear to be normally expressed. With individuals that develop a partial testis lobe, we find that the muscle sheath, revealed by muscle myosin staining, is comparatively normal. However, these same specimens show no signs of the large nuclei characteristic of pigment cells (fig. (fig.3I,3I, J). Taken together, these observations indicate that Sox100B plays a role in somatic testis morphogenesis but is not absolutely required for germline survival or correct spermatogenesis.

Although we have not carried out a detailed analysis of other tissues in Sox100B mutant pharate adults, internal organs such as the alimentary canal and malpighian tubules appear normal. We do note, however, that the fat body is abnormal in both male and female animals and it appears as though the larval fat has not undergone its normal histolysis (Butterworth et al., 1988; Nelliot et al., 2006). While we have not explored this area further, it is interesting to note that the pigment cells and gonadal mesoderm are derived from the same lineage as the fat body (Moore et al., 1998; DeFalco et al., 2008). We speculate that this fat defect is responsible for the pharate lethality of the Sox100B mutants.

Sox100B Mutants Have No Discernable Embryonic or Larval Phenotypes

Since Sox100B is widely expressed in the embryo and exhibits sex-specific expression in the gonad, we examined Sox100B mutant embryos for morphological defects or changes in marker gene expression. We have previously shown that Sox100B expression marks a group of sexually dimorphic somatic cells in the embryonic gonad, the msSGPs, and that deletion of Sox100B does not affect the initial formation of these cells (DeFalco et al., 2003). As well as Sox100B, the msSGPs express other markers including Abd-B, Wnt2 and eya (DeFalco et al., 2004). In Sox100B null mutant embryos the expression of Abd-B is apparently normal (fig. (fig.4A,4A, B), confirming that msSGP formation is not dependent upon Sox100B function. Similarly, we find that expression of the LacZ-842 enhancer trap, which marks the precursors of the cyst and hub cells of the testis (Gonczy et al., 1992), is also unaffected in the mutant (fig. (fig.4C,4C, D). We also find that Ems expression, which marks the pigment cell precursors, is unaffected in Sox100B mutants (fig. (fig.4E,4E, F) as is the expression of Kr-Gal4, which marks the population of fat body cells that give rise to the pigment cell precursors (data not shown). These observations indicate that Sox100B is not required for the initial formation of the gonad or for the correct expression of a range of markers associated with the precursors of the 3 major somatic components of the gonad: the msSGPs, the cyst and hub cells, or the pigment cells. Despite a complex and dynamic zygotic expression pattern for Sox100B in the developing embryo, and a lack of maternally contributed transcript or protein (Loh and Russell, 2000), we detected no defects in Sox100B mutants in tissues that normally express the gene (e. g., the hindgut and malpighian tubules, supplementary fig. 3) (Loh, 1999). In addition, when we examined the normal function of the malpighian tubules by imaging uric acid production in early embryos via polarized light microscopy, we found no difference between wild type and mutant (supplementary fig. 3); (Loh, 1999). Taken together, these observations indicate that Sox100B is not essential for the specification or morphogenesis of the embryonic tissues in which it is expressed.

Fig. 4
Normal aspects of Sox100B mutant testes. A, B Stage 15 male embryonic gonads. Wild type (A) and Sox100B mutants (B) stained to reveal Abd-B expression in male-specific somatic gonadal precursors (msSGPs) and the germ cells. CF Stage 17 embryonic ...

We were surprised to find that the embryonic tissues expressing Sox100B, especially the testis, are apparently normal in the null mutant embryos and we therefore examined larval gonads. As with the embryo, we find no evidence of any defects in the mutant. Spermatogenesis is progressing normally, with wild type cysts observed in mutant testes (fig. (fig.4G,4G, H), which are of similar size to the wild type. Staining larval gonads for germ line (anti-Vasa) or the terminal epithelium (anti-Eya) also indicates that the mutant gonads are normal (fig. (fig.4I,4I, J). As in the embryo, we also find that Ems staining to mark the pigment cells is similar in wild type and mutant (fig. (fig.4K,4K, L). With female Sox100B pharate adults we find that dissected ovaries are also relatively normal (supplementary fig. 2). While we do find occasional abnormalities in the organization of the ovary, principally due to disorganization of the connective tissue and trachea covering the ovary, we emphasize that overall structure of the ovary is normal and oogenesis progresses apparently normally in most individuals. Thus, a Drosophila group E Sox factor has, as is the case in mammals, testis-specific mutant phenotypes.

Sox100B Mutant Testes Fail to Develop during Metamorphosis

Since the larval testis is apparently normal in Sox100B mutants, the defects leading to the adult phenotype must occur during metamorphosis. To investigate this, we dissected wild type and Sox100B mutant gonads from pre-pupae and pupae throughout the 4 days of metamorphosis. In the wild type, the first overt sign of differentiation is the caudal elongation of the testes as they grow to meet the somatic derivatives differentiating from the genital disc. Approximately 36 h after puparium formation, the elongating testes and genital disc derivatives meet and fuse to form the testicular duct (fig. (fig.5A).5A). Thereafter, the testes elongate and coil to form the mature adult organ (Bodenstein, 1950; Fuller, 1993). In contrast to the wild type, in Sox100B mutants we do not observe any signs of testis elongation, rather the gonads appear to remain in the larval condition and maintain the oval shape characteristic of the larval testis throughout the pupal stages (fig. (fig.5B–E).5B–E). We stained pupal testes for the pigment cell marker EMS and found that during the 1st day of post larval life the number of EMS-positive pigment cells were reduced (fig. (fig.5F,5F, G). Counts of pigment cell numbers indicate that, at this stage, there is approximately half the number of EMS positive cells in the mutant compared to wild type (wild type = 24 ± 5, n = 37; mutant = 10 ± 5, n = 30). In the remaining pigment cells EMS staining is reduced compared to the wild type. Similar conclusions were reached by staining pupal testes with a DNA dye to reveal the large pigment cell nuclei (not shown). Since larval testes appear normal and express pigment cell markers but adult testes fail to develop and lack pigment cells, we conclude that, in the absence of Sox100B, pigment cells degenerate or are eliminated during the pupal stage, which is associated with a failure in testis morphogenesis.

Fig. 5
Sox100B mutant testes fail to elongate during metamorphosis. A Diagram showing stages of male reproductive tract development during metamorphosis (after Bodenstein, 1950). Testis is shaded in dark grey and genital disc derivatives in light grey. At puparium ...


We show here that the Drosophila group E Sox-domain factor Sox100B is required for the correct differentiation of the testis. Sox100B mutations have relatively little effect on the development of the ovary since the ovaries are properly specified and normal oogenesis proceeds. Sox100B is the orthologue of the vertebrate Sox9 gene, a key component of the sex-determining machinery in mammals that is necessary and sufficient to specify the somatic Sertoli cell lineage, which is critical for testis development (Wagner et al., 1994; Vidal et al., 2001). The intriguing finding that orthologous transcription factors control the development of male-specific somatic gonadal cell lineages necessary for development of the testis in both mammals and an arthropod suggests that aspects of the molecular hierarchy underpinning sexual differentiation may be more conserved than hitherto suspected.

In Drosophila the sex of the gonad is apparent from when it is formed by the coalescence of germ cells and somatic mesodermal cells (Van Doren, 2006). We have previously shown that, under the control of the somatic sex determining hierarchy, a group of male-specific gonadal mesodermal cells expressing Sox100B (the msSGPs) are specifically eliminated from female embryos by programmed cell death (DeFalco et al., 2003). The msSGPs give rise to the terminal epithelium of the adult testis, a structure implicated in the late stages of spermatogenesis where it plays a role in sperm coiling and release of individual sperm from the somatic cyst that encapsulates the descendants of a single germ cell as it travels through the testis (Tokuyasu et al., 1972; Fuller, 1993). Surprisingly, we found that the specification of the terminal epithelium is apparently normal in the absence of Sox100B since several molecular markers for the embryonic msSGPs and adult terminal epithelium are expressed normally in the mutants. As well as the progenitors of the terminal epithelium, we found that there are no apparent consequences of loss of Sox100B in most other embryonic tissues where it is normally expressed. For example, Sox100B is expressed from the earliest stages of hindgut development and throughout malpighian tubule development (Loh and Russell, 2000) yet these tissues appear morphologically, and in the case of the tubules functionally, normal. Consistent with the lack of phenotype, in preliminary microarray-based gene expression studies we find no significant changes in gene expression comparing Sox100B mutant and wild type embryos (S.N. and S.R., unpublished). The reason for the apparent dispensability of Sox100B in the embryo is currently unclear. There is no maternal contribution of transcript or protein, and redundant functions with another Sox factor are unlikely since there is little relevant overlap in Sox gene expression in the embryo (Cremazy et al., 2001) and Sox100B is the only group E Sox gene in the fly genome. Intriguingly, when we over-express Sox100B via the GAL4 system we find that it is lethal in many tissues in the embryo or larva (Loh, 1999), however, when we over-express Sox100B with nos-GAL4 (germ cells in embryonic and larval stages) and c587-GAL4 (in a subset of somatic cells in the ovary) (Kai and Spradling, 2003) we find mature egg chambers in the female (DeFalco, unpublished observations). This aspect of Sox100B function clearly requires further investigation by, for example, defining and analyzing Sox100B binding targets in the genome.

We recently discovered that, in addition to the msSGPs, Sox100B is expressed in another male-specific somatic gonad lineage, the pigment cell precursors, which are the embryonic progenitors of the pigment cells (DeFalco et al., 2008). The pigment cells form the outer layer of the testis sheath in the adult and are responsible for developing the yellow color of the testis sheath and seminal vesicle (Fuller, 1993). As we have shown, pigment cells are specified in the gonad during late embryogenesis: during the pupal stages they migrate to cover and pigment derivatives of the genital disc (Stern and Hadorn, 1939; Fung and Gowen, 1957). Wnt2 signaling is necessary for the correct development of pigment cells since they are absent in Wnt2 mutants although Wnt2 is not expressed in the pigment cells (Kozopas et al., 1998). In the embryo, Wnt2 is expressed in somatic gonad precursors under the control of the sex-determining switch doublesex and non-autonomously specifies the pigment cell precursors from a population of cells derived from the fat body. Reception of the Wnt signal in fat body cells induces the expression of homeodomain transcription factor Empty spiracles (ems) that in turn activates Sox100B expression. In Wnt2 or ems mutant embryos, pigment cell precursors are not specified and Sox100B is thus not expressed in pigment cell precursors but it is normally expressed in the msSGPs (DeFalco et al., 2008).

Wnt2 and Sox100B mutants show similar phenotypes since larval testes develop apparently normally and contain spermatocyte cysts, but adult testes are absent or rudimentary. At the onset of metamorphosis in the wild type, the testis elongates and contacts the seminal vesicles growing out from the genital disc. In Wnt2 or Sox100B mutants there is an apparent block in testis development with the gonad remaining in a larval-like state, degenerating through the pupal stages to yield the severely atrophied gonads found in mutant adults. In both cases, genital disc morphogenesis appears to be normal and markers for the terminal epithelium and the muscle layer of the testis sheath are expressed, suggesting these tissues are specified normally. It is known that the muscle sheath is specified in, and derives from, the genital disc and it has been proposed that migration of muscle precursors from the genital disc to the gonad provides a signal for testis morphogenesis (Kozopas et al., 1998). In Wnt2 mutants, pigment cells are never formed and we show that in Sox100B mutants the pigment cells are lost during metamorphosis. During the first day of pupal life we find less than half the normal number of pigment cells in Sox100B mutants, and in adults that form partial testis lobes we find no pigment cells. We therefore suggest that the failure in testis morphogenesis associated with Wnt2 and Sox100B mutants is due to the absence of pigment cells. If this is the case it suggests that perhaps pigment cells are necessary for receiving or mediating the response to the muscle signal. Since in both Wnt2 and Sox100B mutants the larval gonad does not elongate at all, there must either be a signal intrinsic to the gonad or it must receive a long-range signal to initiate outgrowth towards the genital disc. In this respect, the accumulation of Sox100B expressing cells at the basal end of the wild type testis is suggestive.

It has been known for some time now that the gonadal mesoderm and fat body share a common developmental origin (Moore et al., 1998). The finding that the pigment cell precursors are recruited from the fat body reinforces this relationship and may provide a possible avenue for exploring the lethality of Sox100B mutants that is common to both sexes. As we note, pharate adult Sox100B mutants are for the most part morphologically normal but their abdomens are full of what appears to be larval fat that has failed to undergo normal autophagy (Butterworth et al., 1988). Since we observe Sox100B expression in L3 larval fat body nuclei (DeFalco, unpublished observations) it suggests that Sox100B may have a role in fat metamorphosis. It will be critical to characterize the relationship between Sox100B and larval fat body in the future.

Proteins of the Sox family are defined by their homology with the HMG-box DNA-binding domain of the mammalian sex determining protein, SRY (Sinclair et al., 1990). The Sox family is a metazoan-specific class of transcription factors and in mammals there are at least 20 members that are divided into 8 subgroups based on sequence identity within and outwith the HMG-box (Soullier et al., 1999; Bowles et al., 2000; Schepers et al., 2002). In Drosophila, the situation is considerably simpler, with 8 Sox genes representing 5 subgroups (B to F) (Cremazy et al., 2001). In all animals studied to date, Sox proteins participate in a wide variety of developmental processes (Wegner, 1999) and some of these, particularly the function of group B Sox genes in CNS development, appear to have been conserved from flies to man (Blanco et al., 2005; Sanchez-Soriano and Russell, 1998). Sequence evidence suggests that Sox100B is indeed the orthologue of the vertebrate group E genes: not only is the HMG domain well-conserved but an intron splitting the HMG domain of the fly and vertebrate genes is located in exactly the same place (Bowles et al., 2000). Thus it is tempting to speculate that in a common ancestor, a group E Sox protein may have been involved in an ancient metazoan-specific process; our results clearly suggest this may have been an aspect of male gonad development.

Along with the Sox family, another group of conserved genes have been implicated in aspects of sexual dimorphism in the metazoans. Members of the DMRT (doublesex and Mab-3 related transcript) family are key factors involved the sex determination hierarchies of flies and worms (Raymond et al., 1998). In many vertebrates, DMRT homologues are expressed in the gonads (Zarkower, 2001; Hong et al., 2007) and in the mouse it is clear that Dmrt1 plays a critical role in male gonad differentiation (Kim et al., 2007). It is therefore intriguing that, as is the case with group E Sox factors, conserved DNA-binding proteins are playing sex-specific roles in organisms as diverged as flies and mammals. Since aspects of Sox100B expression in the testis are regulated by dsx in flies (DeFalco et al., 2003) and at least one dsx-related fly gene, Dmrt93B, is reported to be expressed in the testis according to the FlyAtlas expression database (Chintapalli et al., 2007), it will be interesting to explore whether group E Sox and Dmrt factors represent elements of a conserved testis differentiation pathway.

Our analysis shows that in Drosophila Sox100B has a relatively late role in testis development, but in vertebrates Sox9 acts early in the testis development pathway. While Sox9 is required for sexual differentiation in eutherians by controlling the development of a testis-specific somatic lineage, in other vertebrates the situation is less clear. Although Sox9 expression is sexually dimorphic in chicken, alligator and turtles, male-specific expression is only evident after the expression of anti-Müllerian hormone has been initiated (Oreal et al., 1998; Western et al., 1999; Shoemaker et al., 2007), suggesting that activation of Sox9 is not a primary sex-determining gene in these species. In these non-eutherian vertebrates it is suggested that Sox9 acts to control the correct differentiation of the somatic Sertoli cell lineage rather than its specification (e.g., Smith et al., 1999). While it is still possible that in these species there is differential activity of Sox9 protein that mediates specification in the gonad, there is clearly not a specific transcriptional up-regulation of Sox9 associated with male specification as in eutherians. It is therefore possible that the ancestral function of a Sox9-like gene was in specific aspects of testis differentiation, a role that has been co-opted in mammals to specify sex. If this is the case, an exploration of the genes regulated by Sox100B in the fly may shed light on aspects of sex determination in man.


This work was supported by research grants from the UK Medical Research Council (S.R. and Michael Ashburner), BBSRC (S.R.) and National Institutes of Health (HD46619, M.V.D.). We are grateful to the many colleagues who provided reagents and fly stocks, our colleagues in Cambridge and Baltimore for advice and discussion. S.R. is particularly indebted to John Roote for invaluable genetics advice and to Paul Overton for assistance with early stages of this work.


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