Evolution of a New Function by Degenerative Mutation in Cephalochordate Steroid Receptors
A) In the classic model, duplicate genes are redundant and do not interact, so mutation in one copy causes no change in the function (F) and evolve neutrally. B) In the DDC model, duplicate genes, which have two or more modular subfunctions (F1 and F2, blue and yellow), are redundant and do not interact. Degenerative mutations in subfunctions cause no change in function if the subfunction is retained in the other copy and evolve neutrally. C) If a gene depends upon molecular interactions for its function, degenerative mutations in one copy can affect the function of the other and therefore be selected for or against. i) In the case shown, a gene product must dimerize and interact with DNA (white bar) and an accessory factor (orange triangle) for its function (transcriptional activation). ii) After duplication, degenerative mutations that impair the interactions of one copy with the accessory factor yield a nonfunctional product that competes for DNA binding sites, reducing the activity of the other copy. iii) Degenerative mutations that impair DNA binding in one copy yield nonfunctional products that compete for accessory factors and reduce the activity of the other copy. Inhibition also occurs if the duplicate genes do not dimerize but compete for other binding partners.