Behavioural elements and sensory cues involved in sexual isolation between Drosophila melanogaster strains

Sensory cues exchanged during courtship are crucial for mate choice: if they show intraspecific divergence, this may cause or reinforce sexual isolation between strains, ultimately leading to speciation. There is a strong asymmetric sexual isolation between Drosophila melanogaster females from Zimbabwe (Z) and males from all other populations (M). While M and Z flies of both sexes show different cuticular pheromones, this variation is only partly responsible for the intraspecific isolation effect. Male acoustic signals are also partly involved in sexual isolation. We examined strain-specific courtship behaviour sequences to determine which body parts and sensory appendages may be involved in sexual isolation. Using two strains representative of the Z- and M-types, we manipulated sensory cues and the social context; we then measured the consequence of these manipulations on courtship and copulation. Our data suggest that Z females mated best with males whose sensory characteristics matched those of Z males in both quantity and quality. M females were less choosy and much less influenced by the sensory and social contexts. Differences in emission and reception of sensory signals seen between Z and M flies may lead to the concerted evolution of multiple sensory channel, thereby shaping a population-specific mate recognition system.

Swimming behaviour and photoresponses of the iridescent copepods, Sapphirina gastrica and Sapphirina opalina (Copepoda: Poecilostomatoida)

The swimming behaviour and photoresponses of the iridescent epipelagic copepods, Sapphirina gastrica and S. opalina were investigated in the laboratory. In continuous dark conditions, both species showed no significant diel variation in their swimming activities. When stimulated with light, they exhibited spiral-swimming in which the males showed a significantly higher speed and frequency of turning than the females. Both sexes of S. gastrica and S. opalina showed positive phototaxis at intensities higher than 0·05×1014 quanta cm−2 s−1 for light sources of 430 nm and 580 nm. Sapphirina gastrica showed increased locomotion over a broad range of wavelengths from 430 nm to 580 nm, while S. opalina showed a gradual increase of activity with decreasing wavelength, with the highest value at 430 nm. The photoresponses of these two species suggest that light conditions play an important role in their daytime ascent and in determining the depth distributions that were observed in our previous study. It is suggested that the iridescence and fast spiral-swimming of males, and the species-specific photoresponses of both sexes constitute a putative mate recognition system in the open ocean.

Mapping of genetic loci that change pheromone discrimination in Drosophila males

Reproduction in individual animals of sexual species depends largely upon their ability to detect and distinguish specific signal(s) among those produced by various potential sexual partners. In Drosophila melanogaster males, there is a natural polymorphism for discrimination of female and male principal pheromones that segregates with chromosome 3. We have mapped two loci on chromosome 3 that change sex-pheromone discrimination in males. We successively exploited meiotic recombination, deficiencies and enhancer-trap strains; excision of the transposon in two selected enhancer-trap strains clearly reverted the discrimination phenotype. These results indicate that pheromonal discrimination is a character that can be genetically manipulated, and provide further insights into the evolution of the specific mate recognition system.