Heliconius

Heliconius comprises a colorful and widespread genus of brush-footed butterflies commonly known as the longwings or heliconians. This genus is distributed throughout the tropical and subtropical regions of the New World, from South America as far north as the southern United States. The larvae of these butterflies eat passion flower vines (Passifloraceae). Adults exhibit bright wing color patterns to signal their distastefulness to potential predators. Brought to the forefront of scientific attention by Victorian naturalists, these butterflies exhibit a striking diversity and mimicry, both amongst themselves and with species in other groups of butterflies and moths. The study of Heliconius and other groups of mimetic butterflies allowed the English naturalist Henry Walter Bates, following his return from Brazil in 1859, to lend support to Charles Darwin, who had found similar diversity amongst the Galapagos finches. Heliconius butterflies have been a subject of many studies, due partly to their abundance and the relative ease of breeding them under laboratory conditions, but also because of the extensive mimicry that occurs in this group. From the nineteenth century to the present day, their study has helped scientists to understand how new species are formed and why nature is so diverse. In particular, the genus is suitable for the study of both Batesian mimicry and Müllerian mimicry. Because of the type of plant material that Heliconius caterpillars favor and the resulting poisons they store in their tissues, the adult butterflies are usually unpalatable to predators. This warning is announced, to the mutual benefit of both parties, by bright colors and contrasting wing patterns, a phenomenon known as aposematism. Heliconius butterflies are thus Müllerian mimics of one another, and are also involved in Müllerian mimicry with various species of Ithomiini, Danaini, Riodinidae (Ithomeis and Stalachtis) and Acraeini as well as pericopine arctiid moths. They are probably the models for various palatable Batesian mimics, including Papilio zagreus and various Phyciodina. Heliconius butterflies such as Heliconius numata benefit from mimicking other unpalatable species of butterfly in their local habitat, such as Melinaea, because doing so spreads the cost of educating predators. Such mimicry is termed Müllerian and may result in convergent evolution. Work has been done to try to understand the genetic changes responsible for the convergent evolution of wing patterns in comimetic species. Molecular work on two distantly related Heliconius comimics, Heliconius melpomene and Heliconius erato, has revealed that homologous genomic regions in the species are responsible for the convergence in wing patterns. Also, Supple had found evidence of two co-mimics H. erato and H. melpomene having no shared single-nucleotide polymorphisms (SNPs), which would be indicative of introgression, and hypothesized the same regulatory genes for color/pattern had comparably changed in response to the same selective forces. Similarly, molecular evidence indicates that Heliconius numata shares the same patterning homologues, but that these loci are locked into a wing patterning supergene that results in a lack of recombination and a finite set of wing pattern morphs. One puzzling thought with Müllerian mimicry/convergence is it would be predicted the butterflies would all eventually converge on the same color and pattern for the highest predator education. Instead Heliconius butterflies are greatly diverse, and even form multiple 'mimicry rings' within the same geographical area. Additional evolutionary forces are likely at work. Heliconius butterflies are models for the study of speciation. Hybrid speciation has been hypothesized to occur in this genus and may contribute to the diverse mimicry found in Heliconius butterflies. It has been proposed that two closely related species, H. cydno and H. melpomene, hybridized to create the species H. heurippa. In addition, the clade containing Heliconius erato radiated before Heliconius melpomene, establishing the wing pattern diversity found in both species of butterfly. In a DNA sequencing comparison involving species H. m. aglope, H. timareta, and H. m. amaryllis, it was found that gene sequences around mimicry loci were more recently diverged in comparison with the rest of the genome, providing evidence for speciation by hybridization over speciation by ancestral polymorphism. Hybridization is correlated with introgression. Results from Supple and her team have revealed showed SNP's being polymorphic mostly around hybrid zones of a genome, and they claimed this supported the mechanism of introgression over ancestral variation for genetic material exchange for certain species. Selection factors can drive introgression to revolve around genes correlated with wing pattern and color. Research has shown introgression centering on two known chromosomes that contain mimicry alleles.