Sex determination in Drosophila: The view from the top

Abstract

One of the most important decisions in development is whether to be male or female. In Drosophila melanogaster, most cells make this choice independent of their neighbors such that diploid cells with one X chromosome (XY) are male and those with two X chromosomes (XX) are female. X-chromosome number is relayed through regulatory proteins that act together to activate Sex-lethal (Sxl) in XX animals. The resulting SXL female specific RNA binding protein modulates the expression of a set of downstream genes, ultimately leading to sexually dimorphic structures and behaviors. Despite the apparent simplicity of this mechanism, Sxl activity is controlled by a host of transcriptional and posttranscriptional mechanisms that tailor its function to specific developmental scenarios. This review describes recent advances in our understanding of Sxl regulation and function, highlighting work that challenges some of the textbook views about this classical (often cited, yet poorly understood) binary switch gene.

Figure 1

Sxl is a sexually-dimorphic genetic switch. Sxl is expressed in XX but not in XY animals. Once expressed, an autoregulatory feedback loop ensures continued expression throughout the remainder of development. The presence or absence of Sxl modulates expression of a set of downstream genes whose products are required for control of somatic sex determination/sexual behavior and dosage compensation.

Figure 2

Sxl gene structure & products. Schematic illustrating the portion of the 11 exon ~20 kb Sxl gene that gives rise to the three major classes of sex-specific transcripts through the differential use of two promoters and alternative splicing. The embryo-specific exon (E1) is blue. The translation-terminating male-specific third exon is red. Other exons are gray. The mechanism that drives splicing of the SxlPe pre-mRNAs such that exon E1 is joined directly to exon 4 is not understood.,

Figure 3

Overview of the regulatory logic that guarantees Sxl protein expression in XX animals. During the initiation phase, which takes place during syncytial blastoderm, SxlPe transcription is activated in response to two X-chromosomes worth of XSE products. The Sxl protein produced from the SxlPe transcripts directs the splicing of the newly transcribed RNA from the SxlPm promoter. During the maintenance phase, after SxlPe is shut down and SxlPm is expressed in both males and females, the autoregulatory splicing loop converts the decision to activate Sxl into an irreversible commitment.

Figure 4

Threshold response model. The maternally provided Gro corepressor establishes the initial threshold against which the dose of XSE elements is measured. In XX embryos the levels of the XSE proteins exceeds this threshold in cycle 12. Once SxlPe transcription is initiated, repression is dampened to allow the XSE proteins to more efficiently stimulate SxlPe transcription during cycles 13 and 14. This might occur directly, if Gro activity is antagonized by an XSE, or indirectly, if Gro binding is reduced in the face of transcription-induced changes in chromatin architecture. In XY embryos, gro-mediated repression is sufficient to keep SxlPe silent in the face of XY levels of XSE proteins until cycle 13. Thereafter, zygotic expression of Dpn, combined with Gro, increases the threshold, thereby insuring that SxlPe will remain silent through cellular blastoderm.

Figure 5

Sxl splicing autoregulation via SXL-mediated exon skipping. In both male and females, the spliceosome begins to assemble on the male specific exon 3 (red), with the binding of the U1 snRNP to the 5′ splice site (ss) and the binding of the U2AF/SPF45 proteins near the 3′ splice site. In females, SXL forces the exon 3 to be skipped by binding to sequences (blue ovals) in the flanking introns and antagonizing the function of general splicing factors, including the U1 snRNP, the U2AF complex, SPF45, FL(2)d (not shown) and Vir (not shown).

Figure 6

Sxl controls tra expression by regulating 3′ splice site selection. In the absence of SXL, the U2AF complex binds preferentially to the proximal 3′ splice site (ss) and a non-coding mRNA is produced. The SXL binding site (blue oval) overlaps with the proximal U2AF binding site. SXL out competes U2AF for binding to this site, thereby allowing U2AF to bind the weaker distal 3′ splice site. The arrow indicates that SXL may promote U2AF binding to this alternate 3′ splice site.

Figure 7

SXL-mediated msl-2 translational repression. SXL associates with the 5′ and 3′ UTR of msl-2 mRNA. SXL protein recruits UNR to the 3′ UTR where it interacts with PABP. The SXL/UNR/PABP complex then represses translation initiation by blocking the association of the 43S ribosomal preinitiation complex with the 5′ end of the msl-2 mRNA. SXL can also inhibit scanning of any 43S subunits that escape the SXL/UNR/PABP blockage.