Science - May 16, 2013

The scent of inbreeding

Female butterfly can smell whether the male is a product of inbreeding.
Mechanism revealed by nail varnish on butterfly antennae.

Female Squinting Bush Brown choses partner by his scent.
Men who smell ill will not get much attention from the ladies but at least they have the option of a good wash. But males of the African butterfly species the Squinting Bush Brown are unable to hide their defects. If they are the product of inbreeding, the female butterflies will smell this and give them a wide berth. Rather a neat mechanism given that 50 percent of such males are infertile. A team from Leiden and Wageningen published these results on 2 May in the journal Proceedings of the Royal Society B.     
The researchers compared the reproductive success of healthy males with males that are the product of one or two generations of inbreeding. The healthy specimens turned out to mate twice as often as butterfly males with mild or serious inbreeding. The underlying mechanism became clear in the second part of the study. The antennae of the females were dabbed with nail varnish so that they were no longer able to perceive the male pheromones. The result was that the females made no distinction between inbred males and the rest. 'The negative effect of inbreeding on males' mating success is due to their scent. They are literally less likely to score,' says Bas Zwaan, professor of Genetics.  
The researchers were able to eliminate another possible explanation, namely that the inbred males were in poorer condition. The males' ability to fly was only affected after two generations of inbreeding but the females were wise to their inbreeding after only one generation.
The males are betrayed by a single chemical compound from their cocktail of three pheromones. Males that are the product of inbreeding produce 17 percent less MSP2. The reduction was already clear following mild inbreeding and did not become significantly worse after further cross-breeding of brothers and sisters. Zwaan regrets not having carried out one particular experiment: sprinkling the males with a mixture of pheromones to see if they turned back into true Don Juans. 'If we'd done that, I reckon we might have got into Nature or Science.'