One of my favourite conferences is always the international biology of butterflies meeting – the general enthusiasm of butterfly people always makes it a stimulating event, and as it only happens every four years or so, there is plenty of time to recover for the next one. The most recent, and now the 6th time the congress has been held, was in Edmonton, Canada last week.
The Edmonton meeting was no exception to the rule, and there was a generous helping of Heliconius biology to boot. Much of this revolved around wing pattern genetics, with ongoing efforts to identify the genes that control wing patterns finally starting to come to fruition after some ten years of effort in labs on several continents. Perhaps most exciting, the identification of striking expression patterns at a transcription factor previously associated with eye development, by Arnaud Martin and Riccardo Papa, appear to signal the identification of the loci controlling red pattern elements in both Heliconius erato and melpomene. Similarly, Marcus Kronforst has now identified a regulatory gene (transcription factor) that controls the switch between yellow and white in the Heliconius cydno group, while Nicola Nadeau presented data for a cell cycle regulator that is prime candidate for the switching of yellow patterns in Heliconius melpomene.
For me, the identification of these genes has been something of an obsession over the past few years and it is a relief that we are finally close to an answer. The results seem to confirm the hunch that we have had for a while now, that the same genes are involved in producing similar patterns across many Heliconius species. There really is a common ‘toolkit’ of wing patterning genes that are shared across many species. What is surprising is that the genes identified so far do not seem to obviously fall into a single signalling pathway, or interacting network. The challenge now is to figure out how they have evolved to produce the remarkable patterns of Heliconius from their fritillary ancestors, and whether there is something specific to the way patterns are generated that allows such remarkable diversity to be produced in Heliconius.
There was another surprising result presented at the meeting, from linkage mapping projects in Bicyclus (a satyrid or brown butterfly) and the peppered moth. In both species, genes affecting wing patterns (eyespots and melanic wing colouration respectively) map to the same regions of the genome as one of the Heliconius wing patterning genes that was first mapped in our lab. If it really turns out that the same genes are involved in these species, this would imply that there is a shared mechanism for wing evolution across moths and butterflies, which dates back to the time of the dinosaurs. Now that is pretty cool.
Hissing peacocks and new dimensions to communication
One of the most surprising talks for me was a tale of hissing peacocks and mice. Peacock butterflies are known for the stunning eyespots on their wings, which are used to startle predators when they are approached. What is less well known, is that they also make a hissing sound with their wings. This proves remarkably effective at deterring mice, that would otherwise make a meal of them during hibernation. Videos of hibernating butterflies presented by Martin Olofsson showed mice approaching sleeping butterflies, and being startled off by wing flaps, in the complete darkness of a winter roost. It seems that the startle display of the stunning peacock eyespots is complemented by an equally scary audible signal generating by rubbing the wings together. Heliconius cydno are also supposed to make clicking noises (although I have never heard them myself) – perhaps they serve a similar function.
While sounds may be important for some butterflies, a far more ubiquitous mode of communication is through scents. However this remains a black box of butterfly biology, which is only just beginning to be opened. Fritz Muller (famous for the discovery of Mullerian mimicry), was the first to described the feathery ‘androconial’ scales on butterfly wings that diffuse their scents, in the 1860s. In Edmonton, Carla Penz and Naomi Pierce, discussing owl butterflies and blues respectively, both described remarkable patterns of diversity in the placement, shape and form of these androconial patches on the wings. Such rapid evolution generally signals strong selection, and in this case it is likely driven by sexual selection. This was confirmed by Caroline Nieberding, who has identified the compounds that the African butterfly, Bicyclus anynana, uses for sex. The females of this species choose to mate with males based in part on their chemical bouquet, with the older males having a distinct smell that is preferred by females. This makes sense, as older males have survived the trials of life on the African savannah and are therefore more likely to have successful combinations of genes that will produce fit caterpillars. However, the puzzle is why males don’t cheat the system and pretend to be older than they really are by producing the characteristic chemical signal. Perhaps the signals are expensive or difficult to produce, but there is clearly potential for conflict between the sexes in the use of these chemical signals that could lead to an evolutionary arms race and perhaps explain the rapid evolution of the androconial patches and their chemical bouquet.
In Heliconius, a poster by Catalina Estrada described a different set of chemicals known as anti-aphrodisiac pheromones. These have the opposite effect to the Bicyclus sex phermones, and as their name suggests deter mating by males. The chemicals are transferred from males to females at mating, and presumably serve the interests of females who benefit by avoiding pestering by ardent suitors (although such a benefit has yet to be demonstrated). Strikingly there is also a similar pattern of rapid evolution in these chemicals, with closely related species commonly differing in their chemical composition. Catalina has demonstrated that species in the pupal-mating clade evolve more slowly than the non-pupal-maters. This also fits with the idea that conflict could lead to rapid evolution, as among pupal-maters the males call the shots and there is little opportunity for females to choose their partners, while in non-pupal-maters both species can choose so there is more chance for them to differ in opinion – hence more conflict. Once again, sexual conflict is associated with faster evolution of chemical signals.
Researchers are just beginning to explore the world of butterfly scents, and this is clearly an exciting area for future research. No doubt ‘Biology of Butterflies’ in 2014 will have more on this topic…
Overall the meeting included many more aspects of butterfly biology than I can even begin to mention here. Butterflies continue to be at the forefront of our understanding of the biological response to climate change, with striking range changes very evident at both the warm and cold fringes of species ranges. However, at this meeting there was a shift in emphasis away from simply documenting range changes in response to climate, towards investigating how we might best respond to such changes. Other talks covered so many aspects of butterfly ecology, population biology and evolution that I was exhausted by the end of the week. To recover, we headed south to the Alberta prairies and Drumheller, for some dinosaur therapy.