Heliconius erato and H. melpomene have evolved over 20 distinct geographic races that are convergent between the species due to mimicry, offering an excellent opportunity to study the genetic basis of phenotypic adaptation.
Different races can be crossed together and show that colour pattern divergence has a relatively simple genetic basis – few genes control most of the dramatic colour pattern differences between races. Furthermore, the same genes are repeatedly involved in convergent evolution. Between H. melpomene and H. erato, this is a result of independent evolution of convergent patterns, but in more closely related species this is through exchange of patterns by hybridization.
We are studying the expression of genes during the formation of wing patterns. The pigment pathways involved in producing Heliconius colours are well characterised in Drosophila, so we can identify homologues in Heliconius and study their expression.
This raise the question of why particular genes are targeted by evolution – we hope that by understanding the gene networks that control patterning, we will be able to address this question of why evolution is constrained to particular genetic solutions, but at the same time these few loci can control such a vast array pf diversity in patterns.
Laura Ferguson has identified pigmentation enzymes that have localized expression patterns correlated with pigment distribution. These genes are the downstream targets of the loci that control presence or absence of wing pattern elements.
The loci that switch between major phenotypic pattern elements are controlled by cis-regulatory elements that have a very modular structure. Particular elements turn genes on in specific regions of the developing wing.