The fruit fly, Drosophila melanogaster, is a powerful model organism in biological research, offering insights into the relationship between genes and behavior. Its mating process, a structured and predictable series of actions, has been extensively studied. This ritual helps scientists understand how innate behaviors are encoded by genetics and orchestrated by the nervous system. Studying this process provides understanding of biological principles governing reproduction and species survival.
The Courtship Ritual
The male Drosophila melanogaster initiates a precise sequence of behaviors when encountering a potential female mate. This courtship ritual begins with orientation, where the male positions himself towards the female. Next, he performs tapping, gently touching her abdomen with one of his forelegs.
Following tapping, the male engages in wing vibration, extending one wing and vibrating it to produce a species-specific “courtship song”. This song is composed of distinct patterns, including a “pulse song” with short vibrations and a “sine song” characterized by longer humming bursts. The male then proceeds to lick the female’s genitalia with his proboscis.
The final step in the sequence is attempted copulation, where the male bends his abdomen to try and connect with the female. Throughout these stages, the female’s receptivity plays a significant role in determining the ritual’s progression. A receptive female may slow her movement, partially extrude her ovipositor, or even emit a droplet that excites the male. Conversely, a non-receptive female can reject the male by kicking him, moving her abdomen up and down, running away, or keeping her wings closed to prevent copulation.
Sensory Communication in Mating
The Drosophila mating ritual relies on an interplay of sensory signals exchanged between the male and female. Chemical cues, specifically pheromones, are detected through taste and smell. Males identify females by non-volatile hydrocarbon pheromones on their cuticle. Olfactory receptors also detect male-transferred pheromones that can influence male behavior.
Auditory cues are central, primarily through the male’s courtship song, produced by wing vibrations. This species-specific song conveys information to the female, influencing her decision to accept or reject the male. Visual cues enable the male to locate and track the female, particularly in the early stages of courtship.
Tactile cues involve physical contact during tapping. These multisensory inputs are integrated by the male’s nervous system, guiding his courtship decisions and the ritual’s progression. This sensory communication ensures the male targets an appropriate mate and the female assesses his suitability.
Genetic Control of Mating Behavior
The fruitless (fru) gene regulates male courtship behavior in Drosophila melanogaster. This gene is significant because it undergoes sex-specific alternative splicing, producing different protein versions in males versus females. Male-specific splice variants of the fru gene product are essential for the development and function of neural circuits that underpin male-typical behaviors, including courtship.
If males lack the male-specific fru product, they often fail to court females effectively or may even direct courtship towards other males. Conversely, if females are genetically engineered to produce the male-specific fru variant, they can exhibit male-like courtship behaviors, often directing them towards other females. This demonstrates a direct link between a single gene’s sex-specific expression and the manifestation of an innate behavioral pattern.
The fruitless gene encodes a transcription factor, meaning it controls the activity of many other genes involved in shaping the nervous system for male behaviors. Its influence extends to various aspects of courtship, from the proper ordering of behavioral steps to the production of the courtship song. Research on fruitless has contributed to understanding how genetic information is translated into behavioral outputs.
Post-Mating Changes and Competition
Following successful copulation, changes occur in the female Drosophila, largely driven by components of the male’s seminal fluid. Males transfer sperm along with accessory gland proteins (Acps) during mating. These Acps induce physiological and behavioral shifts in the mated female, including a reduction in her receptivity to remating and an increase in egg-laying.
These post-mating changes are biphasic; a short-term response, lasting about 24 hours, is primarily dependent on Acps. A long-term response, which can persist for one to two weeks, requires the continued presence of both Acps and stored sperm. Specific Acps, like sex peptide (SP), decrease female receptivity and stimulate egg production.
Sperm competition becomes relevant when a female mates with multiple males. In such cases, sperm from different males may compete within the female’s reproductive tract to fertilize her eggs. This competition can lead to “last male sperm precedence,” where the sperm from the most recent mate has a higher chance of fertilizing the eggs. This post-mating dynamic influences male reproductive success and female reproductive strategy.