The fruit fly Drosophila melanogaster is a widely studied organism. Among its distinctive features are the drosophila sex combs, specialized structures found exclusively on the front legs of male fruit flies. These structures are important for reproduction and understanding evolutionary changes.
Anatomy and Location
Drosophila sex combs are arrays of modified, darkened bristles located on the forelegs of male fruit flies. These structures are found on the first tarsal segment of the front legs. In D. melanogaster, the sex comb consists of a single, linear row of about 8 to 14 bristles, often referred to as “teeth”. These bristles are chitinized and arranged in a tightly packed row perpendicular to the leg’s long axis.
The development of the sex comb in males involves a unique rotation of the bristle row during pupal development. In contrast, female fruit flies possess similar bristle rows on their forelegs, but these remain in a transverse orientation and do not develop into the specialized sex comb structure. This difference in leg morphology between sexes makes the sex comb a notable example of sexual dimorphism in Drosophila.
Function in Mating
The primary role of drosophila sex combs is in male courtship and mating behaviors. During courtship, male fruit flies use these structures to interact with the female. Males employ their sex combs to grasp the female’s abdomen or wings, assisting in positioning her for copulation. This grasping action helps the male maintain a secure hold throughout the mating process, important for successful reproduction.
Ablation experiments, where sex combs are removed or modified, have shown a reduction in a male’s ability to copulate successfully. While their exact function varies among Drosophila species, sex combs facilitate tactile interactions during courtship. They may also be involved in spreading the female’s wings or grasping her external genitalia before mounting.
Genetic Control and Development
The formation of drosophila sex combs is under genetic and developmental control, making them a system for studying how complex structures arise. Their development is regulated by specific genes, notably the homeotic (Hox) gene Sex combs reduced (Scr). Hox genes specify segment identity during development. In Drosophila, Scr is expressed in the first thoracic (T1) leg and is important for the patterning and differentiation of cells that form the sex comb.
The male-specific expression and morphology of the sex comb are also influenced by sex-determination genes, such as doublesex (dsx). The male-specific form of dsx promotes the development of male-specific structures, including the sex comb. The interplay between Scr and dsx, along with other genes like dachshund (dac) and bric-a-brac (bab), orchestrates the spatial and temporal expression patterns required for sex comb formation during larval and pupal stages. Studying these genetic pathways provides insights into how genetic instructions translate into morphology.
Evolutionary Significance
Drosophila sex combs offer a model for understanding evolutionary processes, especially sexual selection and speciation. The morphology of sex combs, including bristle number and orientation, varies among closely related Drosophila species. This diversification suggests that sex combs are under selective pressures, often linked to sexual selection.
Variations in sex comb morphology can influence male mating success, acting as a target of female choice or male-male competition. The ability to grasp the female during courtship is a trait favored by natural selection. Comparative studies across different Drosophila species reveal how novel traits like the sex comb can evolve and contribute to reproductive isolation, where differences in mating structures or behaviors prevent interbreeding. This makes sex combs a system for exploring the genetic basis of morphological evolution and the mechanisms driving biodiversity.