How the butterfly got its spots
12 February 2010, by Sara Coelho
The same small portion of DNA, dubbed a 'hotspot of evolution', defines the wing spots of two different species of butterfly evolved to copy each other's wing pattern. This means that evolution might be concentrated in small regions of the genome, while the rest does not change very much.
The postman butterfly (Heliconius melpomene).
Heliconius butterflies have striking wing patterns, often with yellow and red spots and bands to warn predators that they are toxic. Some species are remarkably similar because it pays to mimic patterns and bank on each other's toxic reputation with birds.
'When two species are very similar they reinforce the warning signal and gain an added protection against predation,' explains Dr Chris Jiggins, leader of the Butterfly Genetics Group at the University of Cambridge.
'Despite the many thousands of genes in the genome there are only one or two that are useful for changing this colour pattern.'
Dr Chris Jiggins,
University of Cambridge
Heliconius melpomene, known as the postman butterfly, and Heliconius erato are a good example of this mimicry: they are distantly related and yet it's difficult to tell them apart just by looking at their wings.
But despite knowing about the practical benefits of copying warning signals, scientists were puzzled by the mechanisms behind the mimicry. The wing pattern evolved separately in separate species, but is it controlled by the same genes? Or are different genes doing the same thing?
To find out, Jiggins and colleagues isolated the region of the Heliconius butterflies' DNA and looked at the genes that control wing pattern variation.
They found that major changes are controlled by a small number of genes. 'It seems that evolution might be concentrated in small hotspot regions of the genome, while the rest does not change very much,' says Jiggins.
'This tells us something about the limitations on evolution, and how predictable it is,' he says. '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 colour pattern.'
'We also don't see a strong signal of natural selection,' he says. When a species is selected according to a specific trait, for example resistance to insecticides, the selection leaves behind little genetic variation in the genes that control the feature.
The findings published this week in PLoS Genetics show that this is not the case for Heliconius wing patterns: there is no reduction in genetic variation, which suggests that the genetic changes controlling mimicry are old.
Baxter SW, Nadeau NJ, Maroja LS, Wilkinson P, Counterman BA, et al. (2010) Genomic Hotspots for Adaptation: The Population Genetics of Müllerian Mimicry in the Heliconius melpomene Clade. PLoS Genet 6(2): e1000794. doi:10.1371/journal.pgen.1000794
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