(This exercise is based on Monteiro, A., and O. Podlaha. 2009. Wings, horns, and butterfly eyespots: How do complex traits evolve? PLoS Biology 7: 209–216.)
(Note: The reference above links directly to the article on the journal’s website. To access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
One of the arguments that gets creationists most excited is the notion that natural selection is incapable of resulting in complex structures like eyes or wings because there are no useful intermediates for selection to act on; you either have a functional eye or you don’t. This is a spurious argument for several reasons, one of which is discussed by the authors of this paper.
One of the ways that a complex structure can evolve without the need for a series of intermediate forms is by co-opting gene networks that have already evolved for other functions or structures. Co-option certainly occurs in nature, but distinguishing co-option from de novo evolution of similar gene networks (independently of each other) is not so straightforward. The authors of this paper propose a test by which the two scenarios (emergence of similar gene networks by co-option versus de novo evolution) could be distinguished by looking at the cis-regulatory elements (CREs) involved in the networks.
A CRE is a part of a gene that doesn’t code for a protein but rather acts as a promoter or enhancer that regulates the production of the protein at precise times and places in a developing organism. The cis- prefix refers to the fact that the regulatory element in question is located on the same DNA molecule as and relatively close to the protein-coding sequence that it regulates. Regulatory elements that are located more distantly or on a different DNA molecule (a different chromosome) are called trans-regulatory elements. Trans-regulatory elements are usually genes that encode proteins that bind to cis-regulatory elements of a different gene in order to activate or repress the activity of that gene. Gene networks are sets of genes that participate in such regulatory interactions over the course of development. The networks are usually represented with arrows connecting different genes (see Figure 1A).
The authors of this paper argue that if a gene network is co-opted for another function (say in a different tissue), then genes in the middle of the network will use the same CREs for two different functions (i.e., these CREs will become pleiotropic) (see Figure 1B). In other words, the same CREs will affect two phenotypes—the one in the original tissue and the one in the new tissue. On the other hand, if the gene network evolves independently in the two different tissues, the regulatory elements will not be pleiotropic, but rather each tissue will have its own CREs (see Figure 1C). Figure 2 shows some examples of pleiotropic CREs.
Question 1. Why would co-opted gene networks help in the rapid evolution of complex structures?
Question 2. When a phenotypic trait is controlled by a gene network such as that shown in Figure 1, is that trait more likely or less likely to be modified by random mutation than a trait that is controlled by a single gene? Why?
Question 3. How does the gene network responsible for insect leg development act in developing butterfly wing tissue (in other words, what are the analogous structures in each of these tissues)?
Question 4. Looking at Figure 1, what would be another term for what are called “enhancers” in the figure?
Question 5. Cis-regulatory elements (CREs) can easily be duplicated along with the genes that they control. Is this also true of trans-regulatory elements? Why or why not?
Question 6. Refer to Figure 2 above. Why do the arrows that define the directions of the genes on the DNA strand point in both directions?
Question 7. Thinking about your answer to Question 6, is a gene on one DNA strand matched by a copy of itself on the complementary strand?
Question 8. How many of the CREs shown in Figure 2A (the spalt gene complex) are not pleiotropic, and which tissues do they operate in?
Question 9. Of the 5 CREs shown in Figure 2A, how many are located upstream of either the salr or the sal gene?
Question 10. In Figure 2D, is the CRE shown for the odd gene pleiotropic or not?