Gene name - Sex lethal
Cytological map position - 6F4-7B3
Function - splice factor
Keyword(s) - sex determination
Symbol - Sxl
Genetic map position - 1-19.2
Classification - RNA-binding protein
Cellular location - nuclear
Sex determination in the developing fly is like a one ring circus that occasionally complicates an already exciting scene by adding a second and a third ring, providing the audience a variety of different acts taking place at different times, and occasionally, a choice of viewing among simultaneous events (see Schematic of the sex determination hierarchy in Control of male sexual behavior in Drosophila by the sex determination pathway, Billeter, 2006). Whatever goes on, the circus master in charge is Sex lethal (Sxl).At least three acts get top billing in this circus under the direction of Sxl: the development of female somatic fate; the dosage compensation in males during embryonic development, and finally, the sex specific development of female and male germ lines.
Act 1: Immediately after fertilization there is an assessment of the ratio of X chromosomes to autosomes. Three genes are X linked: when there are two X chromosomes, as in females, the ratio of their gene products like Runt (Torres, 1994), Sisterless-A and Sisterless-B (also known as Scute) is higher than in males. These three genes bind the Sex-lethal promoter and induce activation. In the case of males, where there is only one X chromosome, the autosomal proteins Daughterless, Deadpan and Extramachrochaete, absent sufficient activator proteins, act as repressors of Sex lethal. Thus, despite Sex lethal's central position as ringmaster in this developmental circus, it is the crowd of at least a half dozen transcription factors that regulate Sex lethal's activities.
Sex lethal is an RNA splicing enzyme. Sex lethal's immediate target is Transformer mRNA. Since Sex lethal is not transcribed in males, its action on Transformer is restricted to females. Sex lethal acts positively in the functional splicing of Transformer mRNA. Transformer is another splice factor, acting in turn on downstream RNAs that require sex-specific splicing (Sosnowski, 1994). Transformer protein thus determines female developmental fate.
Act 2. In the spotlight here is dosage compensation, regulated by Sex lethal.The immediate target of SXL is male specific lethal-2 (MSL-2), a transcription factor. In the presence of SXL, MSL-2 is spliced into an inactive form, one that cannot function in dosage compensation. In the absence of SXL (in males), MSL-2 splicing is productive, and the active MSL-2 transcription factor effectively carries out dosage compensation.
A word about dosage compensation is in order. The ratio of sex chromosomes to autosomes in females (1:1) is different from the ratio in males (0.5:1). This is because males, by definition, have one X chromosome and not two. This presents a dosage problem. The ratio of gene products coded for by the sex chromosome will be different in males and females, unless some compensatory action is taken. The sex chromosome carries a lot of genes that are simply along for the ride, and the creation of a dosage imbalance spells catastrophe for development. What to do?
Two alternatives are possible. One of the X chromosomes could be shut off, inactivated in females. This is the solution humans and other higher vertebrates have employed. A second option would be to heighten the activity of the single X chromosome in males. This is the route flies take. MSL-2, spliced into a functional form in males, serves to heighten transcriptional activation of the solitary X chromosome. MSL-2 acts in concert with three partners in this task: Maleless, Male-specific lethal-1 and Male specific lethal-3. They bind to about one hundred sites on the male chromosome, modifying the chromatin structure to permit heightened gene activation (Bashaw, 1995). The action of these male specific transcription factors is very similar to proteins of the trithorax complex.
Act 3. In the center ring here is the regulation by Sex lethal of sex specific RNA splicing in ovaries. Ovaries represent a completely different tissue milieu from somatic cells, and consequently Sex-lethal splicing in these tissues will demand a different kind of regulation (Granadino, 1993). Germ line Sex-lethal function requires an XX karyotype, a female soma, and action of the genes ovp, otu and snf (sans fille). SNF is an RNA binding protein and an integral component of the machinery required for splice site recognition (Flickinger, 1994 and Salz, 1996). ovo, a zinc finger protein and presumably a transcription factor, has a higher level of transcription in females than in males, responding to the number of sex chromosomes in the cell (Oliver, 1994). otu (ovarian tumor) gene also acts upstream of Sex lethal (Pauli, 1993 and Bopp, 1993). Thus ovarian Sex lethal activation and splicing is regulated by a completely different set of proteins (with the exception of SNF, whose function is general) from those in the embryo.
Biological systems operate at a level of complexity that continually astounds the student. A tiny fly carries a whole circus at the core of its development, as far as sex determination is concerned.
Sex determination in the Drosophila germ line is regulated by both the sex of the surrounding soma and cell-autonomous cues. How primordial germ cells (PGCs) initiate sexual development via cell-autonomous mechanisms is unclear. This study demonstrates that, in Drosophila, the Sex lethal (Sxl) gene acts autonomously in PGCs to induce female development. Sxl is transiently expressed in PGCs during their migration to the gonads; this expression, which was detected only in XX PGCs, is necessary for PGCs to assume a female fate. Ectopic expression of Sxl in XY PGCs was sufficient to induce them to enter oogenesis and produce functional eggs when transplanted into an XX host. These data provide powerful evidence that Sxl initiates female germline fate during sexual development (Hashiyama, 2011).
Primordial germ cells (PGCs) are able to differentiate into eggs or sperm. It is thought that PGCs do not assume a sexual fate until they reach the gonads, where sexual dimorphism is imposed by both the sex of the surrounding soma and cell-autonomous cues. In Drosophila, pole cells or PGCs differentiate to a male fate in response to JAK/STAT signaling from the gonadal soma. The method by which female sexual development is initiated in pole cells, however, has not been elucidated. To clarify the mechanism that initiates a female fate in pole cells, a female-specific marker for this cell type was identified. Although several sex-specific markers, including mgm-1, disc proliferation abnormal, and minichromosome maintenance 5, have been reported, they are all expressed only in male pole cells after gonad formation (stage 15), based on signals from the male gonadal soma. lesswright (lwr), a gene that regulates posttranslational modification of proteins by small ubiquitin-related modifiers, is expressed in pole cells during embryogenesis. lwr is not characterized by sex-specific expression. When a dominant-negative form of lwr (lwrDN) was expressed in the pole cells of either sex, however, apoptosis was induced only in female (XX) pole cells during migration to the gonads. This effect caused a significant reduction in the number of XX pole cells in the gonads. Introduction of female-specific germline apoptosis induced by a dominant-negative form of lwr (f-gal) provides a previously uncharacterized marker of female sexual identity in migrating pole cells (Hashiyama, 2011).
Sex determination is controlled by the Sex lethal (Sxl) gene, which is first expressed at the blastodermal stage in the embryonic soma. Sxl encodes an RNA binding protein involved in alternative splicing and translation. In the soma of XX embryos, it functions through transformer (tra) and transformer-2 (tra-2), which in turn regulate alternative splicing of the doublesex (dsx) gene to produce a female-specific form of Dsx. In male (XY) embryos, this pathway is turned off, and a male-specific form of Dsx is produced by default. These Dsx proteins determine the sexual identity of somatic tissues. Previous reports, however, suggested that Sxl does not induce female sexual development in the germ line, as it does in the soma. Although Sxl is autonomously required for female sexual development, constitutive mutations in Sxl (SxlM) that cause XY animals to undergo sexual transformation from male to female do not necessarily interfere with male germline development. Moreover, tra, tra-2, and dsx are not required for female germline development. Finally, female-specific Sxl expression has been detected later in gametogenesis, but not in early germline development (Hashiyama, 2011).
Contrary to previous observations, this study found that Sxl was expressed in XX but not XY pole cells during their migration to the gonads. In the soma, Sxl transcripts are first expressed from the establishment promoter (Sxl-Pe) in a female-specific manner. Using a probe specific to the early transcript derived from Sxl-Pe, in situ hybridization signals were detected in migrating XX pole cells at around stage 9/10. Transgenic embryos, which expressed enhanced green fluorescent protein (EGFP) under the control of the Sxl-Pe promoter, were used to further confirm this female-specific Sxl-Pe activation. RTPE and sequencing analyses in pole cells were used to detect early Sxl transcripts that had the same sequence as the transcripts expressed in the soma (Hashiyama, 2011).
Next, it was determined whether Sxl feminize early pole cells using f-gal as a marker for female identity. It was found that the loss-of-function mutation SxlfP7B0 represses f-gal in XX pole cells. This repression is unlikely to result from sexual transformation of the soma, because an amorphic tra-2 mutation, which alters somatic sex, did not affect f-gal. Conversely, when the expression of Sxl together with lwrDN is forced in pole cells from stage 9 onward by using nanos-Gal4 and UAS-Sxl, f-gal is ectopically observed in XY pole cells. Sxl alone does not induce apoptosis or developmental defects in pole cells. These observations suggest that female sexual identity of migrating pole cells is regulated cell-autonomously by Sxl (Hashiyama, 2011).
It was then determined whether Sxl induces female development in XY pole cells. Because XY soma produces signals that direct XX germline cells to a male fate, XY pole cells expressing Sxl were transplanted into XX females, and their developmental fate was examined. Even in the presence of a gain-of-function Sxl mutation (SxlM1) that causes XY soma to transform from male to female, XY (or XO) pole cells enter the spermatogenic pathway when transplanted into XX females. These results suggest that Sxl is not sufficient to activate female germline development. SxlM1 mutations, however, do not affect transcription from the Sxl-Pe promoter, but instead structurally alter the late transcript from the Sxl maintenance promoter (Sxl-Pm), which allows Sxl protein production in both males and females. Consistent with this observation, Sxl transcripts derived from Sxl-Pe were detected in the pole cells of only female SxlM1 embryos. Thus, the SxlM1 mutation does not result in Sxl expression in XY pole cells as early as in XX pole cells (Hashiyama, 2011).
Instead, nanos-Gal4 and UAS-Sxl were used to induce Sxl expression in XY pole cells. Three types of XY pole cells were transplanted, each characterized by a different duration of Sxl expression: (1) XY pole cells in which Sxl was expressed from stage 9 until stage 16/17 using maternal nanos-Gal4 (XY-mSxl), (2) XY pole cells in which Sxl was expressed from stage 15/16 onward using zygotic nanos-Gal4 (XY-zSxl), and (3) XY pole cells in which Sxl was expressed from stage 9 onward using both maternal and zygotic nanos-Gal4 (XY-mzSxl). XY-mzSxl and XY-mSxl pole cells entered the oogenic pathway and produced mature oocytes in XX females. These oocytes contributed to progeny production. Thus, the XY pole cells produced functional eggs, even though oogenesis and egg production were reduced compared with XX pole cells. In contrast, XY-zSxl pole cells did not enter the oogenic pathway in almost all (92.3%) of the XX female hosts and instead were characterized by a tumorous phenotype, an indication of XY germline cells that have maintained male characteristics. Control XY pole cells from the embryos expressing Sxl only in the soma (XY-nullo-Sxl) showed a similar phenotype to that of XY-zSxl pole cells. These observations demonstrate that Sxl expression in XY pole cells during embryogenesis induces functional egg differentiation in the female soma (Hashiyama, 2011).
Sxl-specific double-stranded RNA (UAS-SxlRNAi) under the control of maternal nanos-Gal4 was used to reduce Sxl activity in XX pole cells during embryogenesis. Introducing UAS-SxlRNAi resulted in tumorous and agametic phenotypes in female adults, indicating that the XX germ line lost female characteristics. Taken together, these results show that Sxl acts as a master gene necessary and sufficient to induce female development in pole cells (Hashiyama, 2011).
XY-mzSxl pole cells adopted a male fate and executed spermatogenesis when they developed in an XY male soma. This observation suggests that the male soma plays a dominant role in determining the male germline fate, overriding the feminizing effect of Sxl. Another possibility is that the XX female soma plays a critical role in maintaining the Sxl-initiated female germline fate. Indeed, an XX germ line in the male soma shows a male gene-expression profile, whereas an XY germ line in the female soma exhibits a female expression profile, although these germ lines does not execute gametogenesis. Thus, female germline development requires interactions between the germline and somatic cells, in addition to germline-autonomous mechanisms involving Sxl (Hashiyama, 2011).
In mice, germline sexual identity is also regulated by both germline-autonomous and somatic signals. In the coelenterate Hydra, the germline sex is not influenced by the surrounding soma, and the germ line determines the phenotypic sex of the polyp. Thus, germline-autonomous regulation of sex has probably been present throughout the evolution of animals, and somatic control may have evolved with the emergence of mesodermal tissues, including gonadal soma. Sxl does not appear to play a key role in sex determination in non-drosophilid animals. Nevertheless, future studies should determine whether Sxl homologs are expressed in the germ line of non-drosophilids. Moreover, it would be of particular interest to identify downstream targets of Sxl in the Drosophila germ line and to test whether these genes have a widespread role in germline sex determination (Hashiyama, 2011).
Sex-lethal expresses aset of three early transcripts and a set of seven late transcriptsoccurring from midembryogenesis through adulthood. Among the late transcripts, male-specificmRNAs have been distinguished from their female counterparts by the presence of an extra exoninterrupting an otherwise long open reading frame (ORF). The late transcripts appear to use a common 5' end but differ at their 3' ends bythe use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences,and their use correlates in females with the presence of a functional germ line, suggesting possibletissue-specific polyadenylation. A number of non-sex-specific splicing variantshave been observed. In females, the various forms of late SXL transcript potentially encode up to sixslightly different polypeptides (Samuels, 1991).
Sex lethal has two transcripts, early and late, with different promoters and dramatically different splicing patterns. The Sxl early transcripts are activated transiently in early embryos by a female-specific promoter and have a unique 5' exon (E1) located between late exons 1 and 2. Exon E1 is spliced to exon 4, which is common to all SXL transcripts, skipping both exons 2 and 3 (Keyes, 1992). In contrast, the late SXL transcripts derive from an essentially constitutive promoter but are spliced sex specifically. The male-specific exon, exon 3, is included by default in all male transcripts and contains in-frame nonsense codons that block SXL protein production. In the presence of SXL protein, the late transcripts skip exon 3 and splice in the female pattern. No embryo-specific splicing factors are needed for the early splice. Neither are sex-specific factors required. Instead, the early splicing pattern is dependent on whether the 5' splice site region originates from exon E1 or exon 2 (Zhu, 1997).
The female SXL protein has large duplicate domains each with 74 residues that are 35% identical to one another. These domains are common to RNA binding proteins. SXL shows greatest homology to yeast poly A binding protein (Bell, 1988).
date revised: 20 June 98
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