Sex in the fruit fly Drosophila melanogaster

Males and females of many species differ in their morphology, patterns of gene activity, biochemistry, and behavior. All of our research concerns aspects of the differences between females and males in the fruit fly Drosophila melanogaster. One major area of our research addresses sex as a developmental process. Our studies in this area are aimed at elucidating at the genetic, molecular, and developmental levels how sexual differences arise and the roles they play in the life of the organism. Among the questions being addressed here are: (1) How are sex-specific tissues and organs specified and constructed during development? and (2) How is the functioning of genes that specify sex-specific differences in parts of body integrated with the functioning of the genes that specify the basic body plan common to males and females? A second major area of research is focused on sex-specific courtship behaviors. In the fruit fly the male’s complex courtship behavior is an innate behavior, and thus genetically specified during development. Our studies of male courtship behavior address: (1) How is potential for male courtship behavior is established in the central nervous system (CNS)? and (2) How do the cells subserving male courtship behavior function together in the adult to insure the ordered manifestation of the events comprising this behavior. A third area of research concerns the process of dosage compensation that serves to make the one X chromosome of males equivalent to the two X chromosomes of females in terms of the amounts of gene products they produce.

The somatic sex hierarchy. A single multi-branched regulatory hierarchy controls all somatic sexual differences in Drosophila melanogaster (Figure 1). The three branches of the hierarchy govern (1) X chromosome dosage compensation, (2) somatic sexual differentiation, and (3) male sexual behavior.

The initial steps in sex determination regulatory hierarchy assess the X chromosome: Autosome (X:A) ratio and establish sex by setting the activity of Sex lethal (Sxl) to "on-produces and SXL protein" in females (XX flies) and "off-does not produce a protein" in males (XY flies). In females SXL protein turns off dosage compensation by preventing the translation of the mRNA from the male-specific lethal-2 (msl-2) gene which is required for dosage compensation. In males, where there is no SXL protein, the msl-2 mRNA is translated and dosage compensation occurs. In addition, the presence (in females) or absence (in males) of SXL triggers cascades of sex-specific alternative mRNA splicing events that lead to the production of sex-specific doublesex (dsx) and fruitless (fru) products (Figure 2).

In the case of dsx both the male- and female-specific dsx mRNAs encode sex-specific zinc finger proteins (DSXM and DSXF, respectively), which have identical DNA binding domains, but different carboxy termini. The dsx gene is the last sex determination regulatory gene in its branch of the hierarchy.

In the case of fru only the products of the P1 fru promoter are expressed sex-specifically as part of the sex determination hierarchy. Transcripts from the P1 fru promoter are sex-specifically spliced in females, like dsx, under the direct control of the TRA and TRA-2 proteins. In males, which lack TRA protein, a default splice occurs. Conceptual translation of the P1 derived fru mRNAs reveals that they encode proteins with a BTB domain at their amino termini and one of three alternative Zn-finger pairs at their carboxy termini. The P1-derived male-specific mRNAs encode proteins that differ from those predicted from the female mRNAs by the presence of 101 amino acids N-terminal to the BTB domain. Immunohistochemistry revealed that the male-specific P1 mRNAs are translated; whereas the P1-derived female-specific mRNAs are not translated. We proposed that it is these male-specific FRU proteins that function to establish the potential for male courtship behavior .

With respect to the processes controlled by the dsx vs. fru branches of the hierarchy we know the following. The dsx branch is responsible for all aspects of somatic sexual differentiation outside of the CNS. Thus dsx governs sexual dimorphism in features of external morphology such as pigmentation, sex-specific aspects of abdominal segmentation, and bristle patterns, as well as all the structures that comprise the internal and external genitalia. Wild-type dsx function is also required for internal biochemical differences that distinguish the somatic cells of the two sexes. In addition, wild-type dsx function is required for several aspects of sexual differentiation that may, or do, have behavioral consequences. These include (1) the sex-specific division patterns of a set of neuroblasts found in the abdominal ganglion of both sexes, (2) the sex-specific patterns of pheromones, (3) the production of courtship-specific humming sounds, and (4) female courtship behavior.

Figure 1. The regulatory hierarchy controlling somatic sexual differences in Drosophila melanogaster.
The fru branch of the somatic sex determination hierarchy is necessary for all, or nearly all, steps of male courtship behavior. Wild-type fru function is also required for the differentiation of a male-specific abdominal muscle, termed the Muscle of Lawrence (MOL). It is the sex of the innervating motorneurons, rather than the sex of the muscle cells, that determine whether a MOL will form. Since both the MOL and male courtship phenotypes are determined by the genotype of the CNS, this suggested that the function of the fru branch of the sex hierarchy is to control aspects of differentiation in the CNS. This hypothesis is strongly supported by the finding that the P1 derived male-specific FRU proteins are expressed almost exclusively in a limited number of CNS cells.
Figure 2. Sex-specific splicing patterns of pre-mRNAs from sex determination regulatory genes

Publications on regulatory genes controlling sex

Research Publications

Baker, B. S. and K. Ridge (1980). Sex and the single cell: I. On the action of major loci affecting sex determination in Drosophila melanogaster. Genetics 92: 383-423.

Baker, B. S., and D. L. Lindsley (1982). The genetic control of sex determination and male fertility in Drosophila melanogaster. In: Genetic Control of Gamete Production and Function, No. 3 P. G. Crosignani and B. L. Rubin, eds., Academic Press, pp. 152-170.

Belote, J. M., and B. S. Baker (1982). Sex determination in Drosophila melanogaster: analysis of trasnsformer-2 , a sex-transforming locus. Proc. Natl. Acad. Sci. (USA). 79: 1568-1572.

McKeown, M., Belote, J. M., and B. S. Baker (1987) A molecular analysis of transformer, a gene in Drosophila that controls female sexual differentiation. Cell, 48: 489-499.

Baker, B. S. and M. A. Wolfner (1988). A molecular analysis of doublesex, a bifunctional gene that controls both male and female sexual differentiation in Drosophila melanogaster. Genes and Develop. 2: 477-489.

Nagoshi, R., McKeown, M., Burtis, K., Belote, J. M., and Baker, B. S. (1988). The Control of alternative splicing at the genes regulating sexual differentiation in D. melanogaster. Cell, 53: 229-236.

Goralski, T. J., J.-E. Edstrom, and B. S. Baker (1989). The sex determination locus transformer-2 of Drosophila encodes a polypeptide with similarity to RNA binding proteins. Cell, 56: 1011-1018.

Burtis, K. C. and B. S. Baker (1989). Drosophila doublesex gene controls somatic sexual differentiation by producing alternatively spliced mRNAs encoding related sex-specific polypeptides. Cell 56: 997-1010.

Nagoshi, R. N., and Baker, B. S. (1990) The regulation of sex-specific RNA splicing at the Drosophila doublesex gene: Cis-acting mutations in exon sequences alter sex-specific RNA splicing patterns. Genes and Develop., 4: 89-97.

Mattox, W., Palmer, M. J., Baker, B. S., (1990). Alternative splicing of the sex determination gene transformer-2 is sex-specific in the germ line but not in the Soma. Genes and Develop., 4: 789-805.

Belote, J. m., F. M. Hoffmann, M. McKeown, R. Chorsky, and B. S. Baker. (1990). Cytogenetic analysis of chromosome region 73AD of Drosophila melanlgaster. Genetics 125: 783-793.

Baker, B. S., Hoff, G., Kaufman, T. C., Wolfner, M. W., and Hazelrigg, T. (1991). A cytopgenetic analysis of the doublesex locus and its flanking regions. Genetics 127: 125-138.

K. C. Burtis, K. T. Coshigano, B. S. Baker and P. C. Wensink (1991). Drosophila doublesex proteins bind to a sex-specific yolk protein gene enhancer. EMBO J, 10: 2577-2582.

Mattox, W. and B. S. Baker (1991). Autoregulation of the splicing of transcripts from the transformer-2 gene of Drosophila. Genes and Dev., 5: 786-796.

Ryner, L., and Baker, B. S. 1991. Regulation of doublesex pre-mRNA splicing occurs by 3' splice site activation. Genes and Dev. 5: 2071-2085.

Oliver, B., Kim, Y.-J., and Baker, B. S. (1993) Sex-lethal, master and slave: The hierarchy of germline sex determination in Drosophila. Development,119: 897-908.

Pultz, M. A., Carson, G., and Baker, B. S. (1994) A genetic analysis of hermaphrodite, a pleiotropic sex determination gene in Drosophila. Genetics, 136: 195-207.

Pultz, M. A., and Baker, B. S. (1995). The dual role of hermaphrodite in the Drosophila sex determination hierarchy. Development 121: 99-111.

Chase, B. A., and Baker, B. S., (1995) A genetic analysis of intersex, a gene regulating sexual differentiation in Drosophila melanogaster females. Genetics, 139:1649-1661.

Heinrichs, V., and Baker, B. S. (1995). The Drosophila SR protein RBP1 contributes to the regulation of doublesex alternative splicing by recognizing RBP1 RNA target sequences. EMBO, 14: 3987-4000.

Mattox, M., McGuffin, M. E., and Baker, B. S., (1996). A negative feedback mechanism revealed by functional analysis of the alternative isoforms of the Drosophila splicing regulator transformer-2. Genetics 143: 303-314.

Ryner, L. C., Goodwin, S. F., Castrillon, D. H., Anand, A., Villella, A., Baker, B., Hall, J. C., Taylor, B. J., and Wasserman, S. A. (1996). Control of male sexual behavior and sexual orientation in Drosophila by the fruitless gene. Cell, 87:1079-1089.

Heinrichs, V., and Baker, B. S., (1997). In vivo analysis of functional domains of the Drosophila alternative splicing factor RBP1. Proc. Natl. Acad. Sci., 94: 115-120.

Chandler, D., McGuffiin, M. E., Piskur, J., Yao, J., Baker, B. S., and Mattox, W., (1997) Evolutionary conservation of regulatory strategies for the sex determining factor transformer-2. Mol. Cell. Biol.,17: 2908-2919.

Li, H., and Baker, B. S., (1998). her, a sex differentiation gene of Drosophila, encodes a zinc finger protein with characteristics of ZFY-like proteins and is expressed independently of the sex determination hierarchy. Development, 125: 225-235.

Heinrichs, V., Ryner, L., and Baker, B. S., (1998). Regulation of fruitless sex-specific 5' splice site selection by transformer and transformer-2. Mol. Cell. Biol.,18: 450-458.

Li, H., and Baker, B., (1998). her and dsx iact both dependently and independently to control various aspects of sexual differentiation in Drosophila. Development, 125: 2641-2651.

Garrett-Engele* C. M., Siegal*, M. L., Manoli, D. S., Williams, B. C., Li. H., and Baker, B. S., (2002). intersex, a gene required for female sexual development in Drosophila, is expressed in both sexes and functions together with doublesex to regulate terminal differentiation. Development, 129: 4661-4675. (* co-first authors)

Reviews

Baker, B. S., and J. M. Belote (1983). Sex determination and dosage compensation in melanogaster. Ann. Rev. Genet. 17: 345-393.

Baker, B. S., M. Wolfner, and J. Belote (1984). Sex determination in Drosophila melanogaster. Proc XV Int. Cong. Genetics, New Delhi, pp.223-232.

Belote, J. M., M. B. McKeown, D. J. Andrew, T. N. Scott, M. F. Wolfner, and B. S. Baker (1985). Control of sexual diferentiation in Drosophila melanogaster. CSHSQB 50: 605-614.

McKeown, M. B., J.M. Belote, D. J. Andrew, T. N. Scott, M. Wolfner, and B. S. Baker (1986). Molecular genetics of sex determination in Drosophila. In: Gametogenesis and the early embryo ( 44th Symp. Soc. for Devel. Biol.), Alan R. Liss, Inc. pp3-17

Baker, B. S., Nagoshi, R. N., and K. C. Burtis (1987) Molecular genetic aspects of sex determination in Drosophila. BioEssays, 6: 66-70.

Baker, B. S., K. Burtis, T. Goralski, W. Mattox and R. Nagoshi (1988). Molecular geneitc aspects of sex determination in Drosophila melanogaster. In: Proceedings of the XVI international congress of genetics. Genome 31: 638-645.

Baker, B. S., (1989). Sex in flies: the splice of life. Nature. 340: 521-524.

Mattox, W., Ryner, L. and Baker, B. S. 1992. Autoregulation and multifunctionality among trans-acting factors that regulate alternative pre-mRNA processing. J. Biol. Chem., 267:19023-19026.

Marin, I., and Baker, B. S., 1998 The evolutionary dynamics of sex determination. Science, 281: 1990-1994.