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Butterfly Sequencing Finds Adaptive Features Stemming from Hybrid Sequence Swaps

NEW YORK (GenomeWeb News) – A new study is highlighting contributions that hybridization has made to adaptive traits in South American butterflies.

Members of the Heliconius Genome Consortium sequenced the draft genome of the Postman butterfly, Heliconius melpomene, comparing sequences in the genome with those from related butterfly species such as H. timareta and H. elevatus and other insects.

The study, appearing online today in Nature, suggests that the wing pattern mimicry observed in the three butterfly species is due to shared chunks of sequence passed around through past interbreeding events between species.

"Evolutionary biologists often wonder whether different species use the same genes to generate similar traits, like the mimetic wing patterns of Heliconius butterflies," Harvard University researcher Marcus Kronforst, one of the study's co-authors, said in a statement. "This study shows us that sometimes different species not only use the same genes, but the exact same stretches of DNA, which they pass around by hybridization."

The Heliconius genus is comprised of almost four-dozen butterfly species and hundreds of butterfly races noted for their diverse color patterns.

For instance, the H. melpomene species includes butterflies with a range of color patterns that are present in related species as well, the researchers explained. And because butterflies in the genus have an unpleasant taste, the color patterning is believed to both deter would-be predators and attract potential mates.

But while some interbreeding has been detected between the species, not much was known about how important hybridization is in the acquisition of adaptive traits in butterflies, if at all.

To look at these and other questions related to butterfly biology in more detail in the current study, the international team used a combination of Roche 454 and Illumina sequencing platforms to generate the 269 million draft genome sequence of H. melpomene, using DNA from an inbred male H. melpomene melpomene butterfly stemming from crosses between stock butterflies from Panama.

The researchers' analyses of the draft genome, sequenced to a depth of around 38 times, uncovered an estimated 12,669 protein coding genes organized on 21 chromosomes.

Based on comparisons with the silkmoth, Monarch butterfly, and fruit fly sequences, the team determined that both butterfly species show expansions to families of genes participating in sensory perception genes such as those involved in smell and taste.

The analysis also uncovered shared duplications in Hox body plan genes in the butterfly species and the silkmoth.

Using restriction-site associated DNA sequencing and other analyses, meanwhile, researchers were able to do phylogenetic analyses of H. melpomene and related butterflies that helped uncover introgression events.

Based on such tests, the team concluded that "closely related Heliconius species exchange protective color-pattern genes promiscuously, implying that hybridization has an important role in adaptive radiation."

"What we show is that one butterfly species can gain its protective color pattern genes ready-made from a different species by interbreeding with it — a much faster process than having to evolve one's color patterns from scratch," co-author Kanchon Dasmahapatra, a post-doctoral researcher at the University College London, said in a statement.

Additional information on the Heliconius Genome Project and an annotated version of the new butterfly draft genome is available online. The same version of the genome, 1.1, is expected in an upcoming ENSEMBL Genomes release.

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