Courtship and mating behaviours in animals are distinctive, characteristic and often very dramatic. Multiple interacting factors direct their progression. Now, two independent research groups uncover new information about some of these factors in Drosophila melanogaster.
Following mating, wild-type female Drosophila become unreceptive to additional advances and start laying eggs. This response is primarily triggered by sex peptide (SP), which is released by the male, but it was unknown how this peptide exerts its effects in females. Using a genome-wide RNA interference (RNAi) screen, Dickson and colleagues identified a gene that is required for the behavioural response to SP — they named the product of this gene sex peptide receptor (SPR).
Knocking down SPR mRNA with RNAi caused females to retain their eggs after a primary mating encounter and to continue mating at high frequency. When the authors injected SP into wild-type, SPR-mRNA-knockdown and SPR-deficient females, only the wild-type flies became unreceptive to subsequent mating approaches, indicating that SPR is essential for the behavioural effects of SP.
Immunostaining against SPR showed that the gene is expressed in the female's reproductive organs and CNS. The expression in the CNS substantially overlapped with that of fruitless, which also modulates post-mating behaviour. The authors therefore investigated whether SPR function is specifically required in fruitless-expressing neurons. They found that knockdown of SPR in these neurons was sufficient to cause the abnormal post-mating behaviour, and re-introduction of SPR into only fruitless-expressing neurons in SPR-deficient flies partially rescued the phenotype. Together, these results show that the post-mating behavioural switch is mediated primarily by SP actions on SPR in fruitless-expressing neurons.
A second study, by Featherstone and colleagues, examined mate choice in male Drosophila. Drosophila that lack a protein called genderblind are unable to correctly choose mates; genderblind-mutant males attempt to copulate with both males and females equally. Multiple types of sensory cue are used by Drosophila when selecting a mate. The authors tested several possibilities and determined that genderblind mutants are attracted to normally unattractive male-specific pheromones, including the chemical 7-tricosene. Consistent with the idea that genderblind regulates olfactory information processing, they showed that genderblind is expressed in fly olfactory centres.
Genderblind is a glial protein that is involved in glutamate secretion. Genderblind mutants show a 50% reduction in ambient extracellular glutamate. Because ambient extracellular glutamate desensitizes glutamate receptors and suppresses synaptic transmission, the authors thought that the homosexual courtship might be due to increased glutamatergic synapse strength in neural circuits controlling olfactory processing. Accordingly, they found that overexpressing the vesicular glutamate transporter DVGluT in wild-type flies increased homosexual courtship. Homosexual courtship could also be induced in wild-type flies by feeding them glutamate receptor agonists; this showed for the first time that Drosophila mate choice is not completely determined during brain development, as previously thought.
These studies shed light on the signalling pathways that regulate mating behaviour in Drosophila. This information might prove useful for the development of new methods to control breeding in insect pests.