Scientists at the University of Cambridge, in collaboration with the Smithsonian Tropical Research Institute in Panama, have discovered that male butterflies have genes which give them a sexual preference for a partner with a similar appearance to themselves. This discovery suggests how the genome is responsible for the mating behavior in butterflies.
This is the first genome study that observed butterfly behavior. It also unleashes the secrets of evolution that can explain how new species are formed.
For the study, scientists observed the courtship rituals and sequenced the DNA from nearly 300 butterflies with two different species of Heliconius. Heliconians have evolved to produce their own cyanide which makes them highly poisonous and they have distinct and brightly colored wings which act as a warning to would-be predators.
The male butterflies were introduced to female butterflies of two species and were scored for their levels of sexual interest directed towards each. The scientists rated each session based on the number of minutes of courtship by the male – shown by sustained hovering near or actively chasing the females.
Professor Chris Jiggins, one of the lead authors on the paper and a Fellow of St John’s College, said: “There has previously been lots of research done on finding genes for things like color patterns on the butterfly wing, but it’s been more difficult to locate the genes that underlie changes in behavior. What we found was surprisingly simple – three regions of the genome explain a lot of their behaviors. There’s a small region of the genome that has some very big effects.”
Heliconians use their long-range vision to locate the females, which is why it’s important each species has distinct wing markings. When a hybrid between the two species was introduced, the male would most commonly show a preference for a mate with similar markings to itself. The research showed the same area of the genome that controlled the coloration of the wings was responsible for defining a sexual preference for those same wing patterns.
Despite the rarity of hybrid butterflies – as a result of their reluctance to mate with one another – a surprisingly large amount of DNA has been shared between the species through hybridization. There has been ten times more sharing between these butterfly species than occurred between Neanderthals and humans.
Dr. Richard Merrill, one of the authors of the paper, based at Ludwig-Maximilians-Universität, Munich, said: “It explains why hybrid butterflies are so rare – there is a strong genetic preference for similar partners which mostly stops inter-species breeding. This genetic structure promotes long-term evolution of new species by reducing intermixing with others.”
Many areas of the genome that define each species- are maintained by natural selection, which weeds out the foreign genes. Particular, the genome is protected from the effects of inter-species mating. As with the genetics that controls mating behavior, these genes enable each butterfly type to maintain its distinctiveness and help ensure the long-term survival of the species.
Professor Jiggins said: “In terms of behavior, humans are unique in their capacity for learning and cultural changes but our behavior is also influenced by our genes. Studies of simpler organisms such as butterflies can shed light on how our own behavior has evolved. Some of the patterns of gene sharing we see between the butterflies have also been documented in comparisons of the human and Neanderthal genomes, so there is another link to our own evolution.”
“Next we would like to know how novel behavior can arise and what kind of genetic changes you need to alter behavior. We already know that you can make different wing patterns by editing the genes. These studies suggest that potentially new behaviors could come about by putting different genes together in new combinations.”