In 1878, German naturalist Fritz Müller observed an evolutionary phenomenon where two distinct species develop the same bright coloring and patterns of pigmentation, despite their inability to interbreed. The physical appearance of these organisms indicates to predators that the organisms are toxic, and therefore unpalatable. Exhibited by a variety of species, this phenomenon, called Müllerian mimicry, has baffled scientists searching for the mechanism that would enable separate species to evolve in the same way. New research from an international team has found a series of genes on a specific part of the genome, which they are calling a “hotspot,” that is responsible for this type of mimicry.
According to the University of Cambridge, which was involved in the study, the researchers conducted a genetic analysis of two species of Heliconius butterflies, which are known for exhibiting this type of mimicry. In their papers on PLoS Genetics the researchers explain that Heliconius butterflies are renowned as a textbook example of adaptation by natural selection, mimicry, and speciation, which made them a logical choice as model organisms. The researches chose two species, Heliconius melpomene and Heliconius erato, which are distantly related yet have the same bright streaks of red and yellow on their otherwise black wings. H. melpomene (called the Postman) and H. erato (called the Red Postman, or Red Passion Flower Butterfly) are found in eastern Peru over a range of more than 10 km.
"The mimicry is remarkable; the two species that we study, H. erato and H. melpomene, are quite distantly related, yet you can't tell them apart until you get them in your hand,” coauthor Chris Jiggins of the Department of Zoology at the University of Cambridge, said in a press release. “The similarity is incredible, even down to the spots on the body and the minute details of the wing pattern."
According to Jiggins, though it was possible that the butterflies had developed an identical, but independently evolved wing pattern, they did not expect to find the same genes active in the pigmentation of the different species. The researchers sampled DNA from the butterflies using Qiagen’s Plant DNeasy Tissue Kit, amplified the samples with PCR, and sequenced the samples using Applied Biosystems’ 3700 DNA Sequence Analyzer. The results indicated that there was a specific region of the genome of both butterflies species responsible for the appearance of the butterflies’ wings. The small region, or hotspot, contained the genes vangogh, slu7, gpcr, dna-J, and kinesin, which were not previously known to play a role in the bright pigmentation of toxic butterflies.
"This tells us something about the limitations on evolution, and how predictable it is,” said Jiggins. “Our results imply that despite the many thousands of genes in the genome there are only one or two that are useful for changing this color pattern. We think it's [the hotspot] likely to be some novel method of cellular signaling, which is quite intriguing and could be important in many other insect species," said Jiggins.
According to the researchers, the consistency of nucleotide variation in the very narrow genomic region containing the hotspot indicates that the hot spot is tightly linked to the site that controls adaptive color pattern variation. “This pattern is consistent with the hypothesis that pattern diversification in H. erato is quite ancient, dating perhaps into the Pliocene,” wrote the researchers in their paper on H. erato. “Interstingly we see a very similar pattern in H. melpomene, which likely radiated much more recently.” Jiggins said the researchers will explore this possibility as they continue their research.
The international research team included scientists from the University of Cambridge (Cambridge, UK), the Smithsonian Tropical Research Institute (Balboa, Panama), the University of Exeter (Cornwall, UK), North Carolina State University (Raleigh, NC, USA), The University of Bristol (Bristol, UK), Harvard University (Cambridge, MA, USA), The Wellcome Trust Sanger Institute (Cambridge, UK), University College London (London, UK), the University of Puetro Rico (San Juan, Puerto Rico), the University of Texas (Houston, TX, USA), the University of California Irvine, (Irvine, CA, USA), the Baylor Human Genome Sequencing Center (Houston Texas), and the Museum National d’Histoire Naturelle (Paris, France).
The papers, “Genomic hotspots for adaptation: the population genetics of Müllerian mimicry in the Heliconius melpomene clade,” and “Genomic hotspots for adaptation: the population genetics of Müllerian mimicry in Heliconius erato,” were published Feb. 5 in PLoS Genetics.