13) Co-option and the evolution of novel characters (by Rey J. Rosa Morales)

        Have you ever questioned, how is possible that an organism acquires new features based on their own genes? Many genes and signaling pathways have multiple developmental roles. For example, the transcription factor Distalless is required to organize the development of legs, wings, and antennae of all insects, but in some butterflies, it is also expressed later in specific positions on the developing wing, where it is involved in setting up the color patterns known as "eyespots" (see Figure 1). Such cases suggest that, over the course of evolution, genes and pathways have been redeployed to serve new functions such as in the case of reptiles scales that through million of years become bird’s feathers (see figure 2 and Youtube video). Change in the function of pre-existing features in adaptive evolution has been known ever since Darwin. Gould and Vrba (1982) coined the term exaptations to refer to novel uses of pre-existing morphological traits. Developmental biologists have used the terms recruitment (Wilkins 2002) and co-option (True and Carroll 2002) to refer to the evolution of novel functions for pre-existing genes and developmental pathways.

Figure 1. Co-option of developmental circuits in the evolution of novelties. (A) Butterfly "eyespots" are the developmental products not only of genes for pigmentation, but also of many co-opted genes and pathways that play important roles in establishing the body plan. (B) Co-option of the vertebrate Hoxa genes during evolution of the tetrapod appendage. Ancestrally, Hox genes were expressed only along the anterior-posterior axis of the developing body. The evolution of paired fore-and hindlimbs involved novel gene expression, presumably using novel enhancer sequences of Hoxa 9-13. 

Figure 2. Wallace’s flying frog (Rhacophorus nigropalmatus), an inhabitant of the rain forest canopy in southeastern Asia, glides from tree to tree with the aid of its toe webbing, which is much more extensive than in most other tree frog species. (Photo by Stephen Dalton/Minden Pictures, Inc.)

The Origin of the brain

Feather Evolution

        Co-option of single genes for new functions may be common. The members of many gene families have diversified into different developmental and physiological roles. In one of the most interesting such cases, the diverse crystals in proteins of animal eye lenses have been co-opted from a number of genes (Figure 3). Two types of crystallins, α. and β, which are common to all vertebrates, are derived from stress proteins, which have help stabilize cellular functioning during environmental stresses, such as excess heat. Various lineages of animals have also derived taxon-specific crystalline from distinct enzymes, such as lactate dehydrogenase in reptiles and glutathione-S-transferase in cephalopods. In the eye lens, crystallins are expressed at high levels and packed tightly into transparent matrices that are resistant to environmental stress and are designed to endure for the entire adult life of the animal. To achieve this function, many crystallins have undergone amino acid substitutions since they were co-opted from their ancestral function, but in most cases these proteins still share extensive homology with the ancestral proteins. These crystallins are derived from duplicated enzyme genes. In other cases, gene duplication has not occurred, and both the crystalline and the enzyme are encoded by the same gene (e.g., τ crystallin: α enolase in fishes, reptiles, and birds).

Figure 3. Two modes by which variation in the expression of a developmental regulatory gene may be co-opted to promote novel morphogenetic features. (A) Temporal co-option. Gene expression that persists after the critical period may be utilized during evolution to regulate novel morphogenetic processes. (B) Spatial co-option. This novel expression may be co-opted to promote morphogenesis of an ancestral feature in the novel region of the body.

References:

1. Bock, W.J. (2000). "Explanatory History of the Origin of Feathers". Amer. Zool. 40 (4): 478–485.
2. Futuyma, D.J. (2005). Evolution. Sinauer Associates, Inc. Publishers Sunderland, Massachusetts U.S.A.
3. Sumida, SS & CA Brochu (2000). "Phylogenetic context for the origin of feathers". American Zoologist 40 (4): 486–503. 
4. True, J. R, and S. B. Carroll. (2002). Gene co-option in physiological and morphological evolution. All/III. Rev. Cell. Oev. BioI. 18: 53-80.
5. Wilkins, A. S. (2002). Tile Evolution of Developmetal Pathways. Sinhauer Associates, Sunderland, MA.
6. Wistow, G.(1993). Lens crystallins: Gene recruitment and evolutionary dynamism. Trends Biochem. Sci. 18: 301-306.
7. Wistow, G., and J. Piatigorsky, J. (1988). Lens crystallins: The evolution and expression of proteins for a highly specialized tissue. Annu. Rev. Biochem. 57: 479-504.