<p>Evolutionary innovations, at the molecular level, represent the novel establishment of regulation networks among previously unconnected genes. Understanding the cellular and molecular mechanisms that underlies the development of such innovations is of central importance in evolutionary-developmental research (evo-devo). The sex comb of <em>Drosophila</em> is an excellent model to study the molecular basis of evolutionary innovations. Highline and Lowline are two artificial selected <em>D. melanogaster</em> lines differing in the number of sex comb bristles. It was expected that the “cross-regulation loop” between two transcription factors, <em>Doublesex</em> male isoform (DSX^M) and <em>Sexcombs reduced</em> (SCR), evolves rapidly and promotes the morphological evolution of sex combs. We used immunofluorescent technique (antibody staining) to compare the expression of DSX^M and SCR in the forelegs of three different lines (Highline, Wildtype and Lowline). We hypothesized that artificial selection will increase expression of DSX^Mand SCR in the Highline and reduce expression in the Lowline. The fluorescent pictures of antibody staining experiments indicate that the expression region of DSX^M in the Highline is significantly higher than the expression region in the Lowline, and the expression levels of SCR has minor difference among the three lines. DSX^M expression is altered by the artificial selection, but SCR expression is not. The influence of artificial selection appears to have been constrained by development. Our investigation provides an approach to test the validity of the models of cross-regulation s between SCR and DSX^M during development.</p> / Master of Science (MSc)