Evolution, 4th Edition

EvoluTion And dEvEloPmEnT 375

phenotypes. These ideas were very similar to what was later

learned about the developmental transition from genes to

phenotypes.

Gene Regulation

In eukaryotes, transcription of a protein-coding gene is initi-

ated when RNA polymerase II binds to an upstream region, the

promoter. This occurs when certain regulatory proteins—tran-

scription factors (TFs)—that determine transcription of a gene

bind to a short upstream region called an enhancer (FIGURE

15.7). Several transcription factors form a complex, so different

parts (domains) of a TF interact with the enhancer and with

other TFs (or other proteins). A single gene often has multi-

ple enhancers, each with a different sequence that binds dif-

ferent transcription factors. Consequently, the gene may be

transcribed in different cell types, at certain times, if the cells

include different transcription factors that are specific to one

enhancer or another. Enhancers are often called cis-regulatory

elements. Cis means that the element regulates a gene on the

same stretch of DNA. Transcription factors are trans-regula-

tory elements, meaning that they are encoded by DNA distant

from the genes that they regulate.

Much of the research on the evolution of development con-

cerns enhancers and transcription factors, but several other

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Interacting morphogens

Concentration

Position on organism Position on organism

Concentration

Product (e.g., pigment)

(A)

(B)

Cone snail Puffer sh

FIGURE 15.6 A) Alan Turing’s model of diffusion of interacting (

chemical morphogens (left graph) shows that regular patterns of

a product may be produced across a part of a developing em-

bryo (right graph), which may induce the development of regular

patterns in the distribution of pigments, hairs, or other features.

(B) Patterns of coloration that may result from the reaction-

diffusion process described in (A). At left is an olive shell (Oliva

porphyria) and at right a puffer fish (Arothron mappa). A photo

of each organism is shown to the left of patterns produced by

computer simulations of a reaction-diffusion model. (A after [32];

B shell simulation from [47], fish eye simulation from [65].)

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Ldb1

Enhancer

No transcription

RNA

polymerase II

Enhancer Promoter

Promoter

Ldb1

Ldb1

Transcription

FIGURE 15.7 RNA polymerase II binds to a gene’s promoter

and initiates transcription of the gene into mRNA, but only after

transcription factor proteins bind to both the promoter and an

enhancer sequence. The promoter and enhancer may be linked

to each other by other transcription factors (labeled Ldb1 here).

(From [19], after [13].)

15_EVOL4E_CH15.indd 375 3/22/17 1:30 PM