This past weekend we were assigned a take home exam. One of the prompts addressed an article that I had commented on two weeks ago. Here is the prompt:
We went over the experiment to test the role of enhancers of the Prx1 locus which showed their role in regulating limb length in bats and mice. Explain it again, going over the details of the experiment, the results, and the interpretation...but without using any scientific jargon. If you do use any jargon (like "locus", "regulation", "enhancer"), you must also define it in simple English. Make the story comprehensible to a non-biologist!
And here was my response:
The article "Regulatory divergence modifies limb length between mammals" by Chris J. Cretekos et al. set out to investigate the cause of length differences between the limbs of short-tail fruit bats and mice. These organisms represent the two largest orders of mammals and it is thought that they shared a common ancestor between 80 and 100 million years ago. The "lay-out" of their limbs is extremely similar and includes the general one-bone, two-bones, cluster of bones, digits pattern (as also seen in our limbs). However, there are also differences in their limb structure such as the relative length of the digits, the presences or absence of tissue between digits (similar to "webbing") and between limbs (forming wings). In the development of bat and mice embryos the initial formation of the fore-limb show similar timing and position, however as the embryo grows and develops these differences become more apparent.
So what causes this difference in development between mice and bats? Well, before I address this question it is also important to consider the two types of changes that can cause morphological differences. First, it is possible that there are changes in the DNA sequence itself (that code for the parts of the limbs). Second, it is possible that there are changes in the DNA sequence that are non-coding (do not code for limb itself) but are rather the regulatory elements in limb development. A regulatory element is a molecule that helps control the development of some aspect of the organism (in this case the forelimb). This second type of mutation is called a cis-mutation, and can cause modification in specific aspects of the patterns or the level of gene expression.
Prx1 is a gene that is found in both mice and bats and it is a developmental control gene that has been shown to increase skeletal elongation in limbs. When scientists deleted this gene in mice, the resulting embryo initially developed normally, however, the long bones of limbs were significantly shorter and these mutants died at birth. This suggests that Prx1 is essential in regulating long bone elongation. Cretekos et al. hypothesized that because bats have significantly longer long bones, and mice lacking this gene showed significantly shorter long bones that this gene may be what is causing the morphological difference between bat and mice fore-limbs. To further investigate they looked at the expression of this gene in embryos of both mice and bats. They found that Prx1 is initially expressed consistently throughout the budding limb, however later on Prx1 is up-regulated (more is produced) in the end of the bat limb compared to the mouse. These differences in Prx1 gene expression correspond with the physical differences observed.
Next, researchers investigated the cis-regulation of Prx1 (the regulatory sequence of the gene that is non-coding). By deleting certain sections upstream of the Prx1 gene they found a region that acted as a transcription enhancer. A transcription enhancer is a sequence of the gene that an activating protein binds in order to turn the gene on (this helps increase the amount of Prx1 transcripts produced). This enhancer region was similar in both bats and mice. In order to examine the function of this enhancer sequence researchers replaced the mouse enhancer sequence with the bat enhancer (leaving the original Prx1 gene intact in the mice). What they found was that the bat Prx1 enhancer sequence increased the forelimb length during the development of the mice but the pattern was maintained (it still looked like a mouse forelimb). It was also interesting that when the mouse Prx1 enhancer was simply deleted (and not replaced) limb development in the mice was normal. These results show that there must be multiple enhancers that perform the same function aside from the one that was replaced or deleted in this study.
Overall, this study showed further support that changes in the non-coding region of the gene (cis-mutations) can play a role in the physical changes that have occurred and allowed for the divergence of species such as bats and mice.