The field I work in is Developmental Biology -- a discipline that attempts to investigate how all the strikingly different and specialized cells making up a complex living organism come into being. This is a subtler question than might appear at first glance: for, in almost all cases, all the cells in an organism contain the same genetic blueprint, DNA, yet end up choosing different fates. Understanding the manner in which this is achieved -- the signals and mechanisms used -- is the primary goal in developmental biology.
My previous work (as a graduate student completing his PhD) dealt with certain aspects of sex determination and sexual differentiation in the fruit-fly Drosophila melanogaster. My current research is focused on the genes involved in the proper development of the heart and muscles using Drosophila once again as a model system. A brief description of both these research projects is presented below.
And in all these cases of independent evolution, the mechanisms by which sex is determined have, naturally, been quite different. In mammals like us, for instance, it is the presence or absence of the Y-chromosome that determines whether a particular organism is male or female. In ants and bees, males are the individuals that are produced parthenogenetically, that is, without fertilization; females are those that emerge from fertilized eggs. In some species of reptiles like turtles, lizards, and crocodiles, sex is determined by the temperature at which the eggs develop. And there are schools of fish in which the largest animal is the male; if he were to meet an untimely end, the largest female in the school would gradually become transformed into a male. Similarly, in the marine worm Bonellia, young worms are indeterminate sexually, but are attracted to females. If they can attach themselves to a female, they become transformed into males and live inside the female body as a parasite. If they are unable to attach themselves to any female, they develop into females themselves. Obviously all these systems present the developmental biologist with an absolute plethora of mechanisms by which the fates of cells become determined -- making the study of sex a very promising undertaking indeed.
There remains, however, a more specific reason for studying sex determination in Drosophila: the fact that it uses quite a novel and interesting scheme. Sex in Drosophila is not brought about by the presence or absence of the Y-chromosome (though generally males have one while females lack any); no, it is instead the ratio of the X-chromosomes to the other chromosomes (autosomes) that determines sex. When the ratio is 1:1, that is, the individual contains two X-chromosomes and a pair of each autosome, the fly is female; when the ratio is 1:2 (one X-chromosome and a pair of each autosome), the fly is male.
This entire process of sex determination is mediated through a gene called Sex-lethal that acts as a master switch. In response to the appropriate X-chromosome:autosome ratio, the gene remains in either an active or an inactive state, leading respectively to either female or male development. In both cases, Sex-lethal lies at the head of a hierarchy of genes, a hierarchy that gradually branches out to control different aspects of sex: dosage compensation (the mechanism that ensures the same amount of RNA and thus protein is made despite the female fruit-fly having twice as many X-chromosomes), sexual behaviour, germline sex differentiation (sex determination of the cells that will ultimately give rise to sperm or eggs), and somatic sex differentiation (sex determination of all other cells).
The study of sex in Drosophila revolves around indentifying all the agents that are involved in this hierarchy (more and more are gradually being discovered) and investigating the mechanisms by which they interact.
It is these other genes, direct or indirect targets of doublesex, that I was interested in. doublesex and all the genes above it in the hierarchy function essentially as regulatory switches, serving to fine-tune and transmit the X-chromosome:autosome sex determining signal along different pathways. Unlike them, I expected these target genes to be the actual effectors of sex determination, the genes which directly bring about one or more particular sex-specific fates. I decided to look for these genes in the genital imaginal disc, the precursor to the structures that show the most extreme degree of sexual dimorphism in flies -- the male and female genitalia and anal structures.
My project involved identifying these target genes and understanding both how they are utilised to bring about sexual differentiation and how they are regulated by both the sex determination hierarchy and other hierarchies (for while the sex determination hierarchy will determine whether they are on in say, males or in females, clearly it is the influence of other hierarchies that will determine the spatial and temporal patterns in which they are expressed). Some of this work is described in more detail in the following papers (PDF format):