“Most colorful example of genetic assimilation? Exploring the evolutionary destiny of recurrent phenotypic accommodation”

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Alexander V. Badyaev, Ahva L. Potticary, and Erin S. Morrison

Evidence that carotenoid coloration may have evolved by genetic assimilation

A controversial evolutionary phenomenon might be ubiquitous in the evolution of diet-dependent animal coloration a new study reports

Feather accommodation of external carotenoids, as in display plumage of the house finch (Haemorhous mexicanus), likely evolves by genetic assimilation.
(Credit: Alex Badyaev)

How can fully-grown multicellular organisms accommodate and retain novel inputs without losing functionality of already evolved structures? Framers of evolutionary theory envisioned a process – termed genetic assimilation – in which changes induced by the environment at peripheries of developmental trajectories get progressively stabilized by genetic controls of existing traits. Such a view of evolution as “the direction of ontogenetic accommodations of earlier generations” (Baldwin 1896, Am. Nat. 30:441–451) predicts developmental shifts in the determinants of a trait between ancestral and descendent forms. Yet we now know that such a shift is an inevitable consequence of the reshuffling of ancient genes for contemporary functions and thus an insufficient proof of any particular evolutionary mechanism.

A new study by researchers from the University of Arizona overcomes this constraint by studying genetic assimilation as a recurrent process. Using one of the largest datasets ever assembled for such a study, they examined changes in feather structure caused by accommodation of carotenoid pigments with the same physical properties but distinct evolutionary histories, capitalizing on the fact that highly metabolically derived (i.e., internalized by an organism over a long evolutionary history) carotenoids in one species can be recent dietary additions in another species. The researchers applied a metabolic network view of carotenoid evolution to show that some derived carotenoids are biochemically redundant and thus co-evolve with feathers over longer time, despite having diverse dietary precursors in different environments.

The researchers found that feathers repeatedly evolved the ability to integrate a diverse array of recurrent and metabolically internalized carotenoids into their growth, whereas inclusion of novel or dietary carotenoids of the same molecular weight induced significant aberrations in feather shape. Thus, the evolution of carotenoid-based ornamentation within avian lineages may be thought of as an arena for ongoing genetic assimilation – passing through stages of this process from phenotypic induction by external carotenoids to a progressively internalized metabolism of derived carotenoids and their integration with feather growth and restarting when the avian lineage switches to novel dietary precursors. Read the Article