The evolutionary importance of hybridization and introgression has
long been debated1. Hybrids are usually rare and unfit, but even
infrequent hybridization can aid adaptation by transferring beneficial
traits between species. Here we use genomic tools to investigate
introgression inHeliconius, a rapidly radiating genus of neotropical
butterflies widely used in studies of ecology, behaviour, mimicry and
speciation2–5. We sequenced the genome of Heliconius melpomene
and compared it with other taxa to investigate chromosomal evolution
in Lepidoptera and gene flow among multiple Heliconius
species and races. Among 12,669 predicted genes, biologically
important expansions of families of chemosensory and Hox
genes are particularly noteworthy. Chromosomal organization
has remained broadly conserved since the Cretaceous period, when
butterflies split from the Bombyx (silkmoth) lineage. Using
genomic resequencing, we show hybrid exchange of genes between
three co-mimics, Heliconius melpomene, Heliconius timareta and
Heliconius elevatus, especially at two genomic regions that control
mimicry pattern. We infer that closely related Heliconius species
exchange protective colour-pattern genes promiscuously, implying
that hybridization has an important role in adaptive radiation.