Genetic Map of Sympatric Introgression in Hybridizing Colias Butterflies

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

The sulfur butterflies, Colias eurytheme and C. philodice, common in fields and along roadsides across most of North America, are remarkable because they hybridize extensively. Up to 10% of the butterflies are hybrids, an interbreeding rate that is sufficient to blend them into a single species. Yet, they have remained distinct. Many other species pairs show similar patterns of hybridization, including Darwin's Finches and numerous plant species. These cases challenge our traditional definition of species, which do not normally interbreed. One untested hypothesis, first proposed in 1957, is that hybridizing species do in fact share part of their genome, and that selection acts to keep the remaining parts of the genomes distinct. To test this, new methods will be used to map the chromosomes of these butterflies in great detail. This genetic map will then be used for a fine-scale analysis of the rates that genes are able to spread between species. If superficially distinct, hybridizing species do share major portions of their genomes in common, then it will cause biologists to restructure their thinking about what species are. It would suggest that species identities can exist as a mosaic of genomically complex, stable polymorphisms, maintained in an otherwise dynamic, fluid genome. If this alternative explanation is not upheld, the study will provide strong support of the traditional view of species.