Male butterflies take a particular liking to females which look just like them, researchers found.
Butterflies are weird creatures -- there, I've said it. Their very existence is tied to one of the most bizarre processes in the natural world (metamorphosis), they have crazy long tongues, and they evolved at least 55 million years ago -- making them much, much older than mankind. They are also beautiful creatures with remarkably colorful wings which have been admired by mankind since the dawn of our civilization.
But, for all our admiration, there's still much we don't know about them -- particularly at the genetic level. In order to address that, researchers from the University of Cambridge, in collaboration with the Smithsonian Tropical Research Institute in Panama, observed the courtship rituals of two Colombian species of Heliconius -- a colorful and widespread genus commonly known as longwings. The team also sequenced the DNA from nearly 300 butterflies to find out how much of the genome was responsible for their mating behavior. Their results brought forth a few surprises. Professor Chris Jiggins, one of the lead authors on the paper and a Fellow of St John's College, explains:
"There has previously been lots of research done on finding genes for things like colour patterns on the butterfly wing, but it's been more difficult to locate the genes that underlie changes in behaviour.
"What we found was surprisingly simple - three regions of the genome explain a lot of their behaviours. There's a small region of the genome that has some very big effects."
Unlike most butterflies, which use chemical signals to find a mate, Heliconian males use their long-range vision to locate females -- which also explains why they have distinctive wing markings. Researchers took advantage of this fact and carried out another experiment, introducing male butterflies of one species to females from both species. They then followed the males, noting their levels of sexual interests towards each of them (yes, for science).
They found that males would most often choose females with similar wing markings -- again, a rather surprising fact. 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."
Researchers also published a second paper on the subject, reporting that although hybrids are very rare, there is a surprisingly large amount of DNA shared between both species, DNA that has been shared through hybridization -- ten times more than Neanderthals and humans share, for instance. The reason for this, researchers suspect, is that the lifespan of butterflies is shorter than that of humans, which allows for a much higher number of generations over the same period.
"Over a million years a very small number of hybrids in a generation is enough to significantly reshape the genomes of the these butterflies," says Simon Martin, another one of the authors.
But despite this genetic mixing, the two species retain different behaviors and have not become blended. The part of the genome that defines the sex of the butterflies is protected from the effects of inter-species mating, but more importantly, their genome is tweaked and shaped by natural selection and cultural preferences, which allow species to remain distinct and unique.
Professor Jiggins says that ultimately, this type of study suggests that humans are not as unique as we used to think.
"In terms of behaviour, humans are unique in their capacity for learning and cultural changes but our behaviour is also influenced by our genes. Studies of simpler organisms such as butterflies can shed light on how our own behaviour 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," he concludes.
The two papers have been published in PLoS Biology and are freely available:
- "Genetic dissection of assortative mating behavior"
- "Recombination rate variation shapes barriers to introgression across butterfly genomes"