Figure 1.

The premise of interspecies gene transfer. (A) Transferring a locus from the blue-flowered donor species is able to transform the color of the white flowers of the recipient; note that the transfer does not recapitulate the ornamentation (the nectar guides) of the donor in the recipient. (B–D) Hypothetical network controlling color and ornamentation expression. (B) Light and dark blue transcription factors (TFs) interact directly with the cis-regulatory elements of a key blue pigment synthesis gene, and control the color and the ornamentation of the blue flower, respectively. These TFs are in turn regulated by upstream regulators. (C) In the white-flowered species, pigment synthesis is abolished via inactivation mutations in TFs but the blue pigment synthesis genes (gray) and the upstream regulator for blue color (light blue) remain intact. (D) Introducing the light blue TF (arrowhead) restores blue pigment synthesis in a white-flowered recipient but does not transfer the ornamentation pattern. (E) Possible outcomes of an IGT experiment designed to test the contribution of a candidate locus to the divergence in color. In protein divergence, expression of the coding sequence of the donor under the recipient's promoter will result in change of phenotype. If cis-regulatory evolution underlies phenotype divergence, the coding sequence of the recipient expressed under the donor's promoter and the entire locus of the donor may be sufficient for phenotypic change. Trans-regulatory mutation, a combination of cis- and trans-mutations, or lack of involvement of the locus are possible when transferring the entire locus of the donor does not reconstitute the phenotype in the recipient.