A Whale of a Discovery: 5’HoxD Genes and the Evolution of Cetacean Flippers

You’ve all been through intro organismal biology or at least high school biology, and you have probably heard of a term biologists like to use to describe relationships among groups of organisms – homology. You were probably also shown a picture like this:

http://www.mun.ca/biology/scarr/139393_forelimb_homology.jpg

Forelimb Homology

(http://www.mun.ca/biology/scarr/139393_forelimb_homology.jpg)

In case you don’t remember, homology is the likeness between two structures in different organisms that evolved from the same structure in the common ancestor of both. (Not to be confused with “analogy”, which is the similarity of function in two structures with no common ancestry.) In the picture above, we see the homology of forelimb bones.

Several million years ago, we shared an ancestor with today’s cetaceans (whales and dolphins). That ancestor had forelimbs with a bone structure that eventually evolved into all the varied forms of mammalian forelimbs we see today. The ancestral mammal was pentadactyl, meaning it had five digits in the “hand”. Today’s mammals still only have a maximum of five digits, but a few groups have toyed with the design, so to speak. These mammals are broken into two main groups, Cetartiodactyla, and Perissodactyla. The former is comprised of even-toed ungulates, such as cattle, reindeer, camels, and sheep, (and cetaceans) and the latter is comprised of odd-toed ungulates, such as horses, zebras, rhinos, and tapirs.

Mostly, we see an evolutionary reduction in the number of digits to increase mobility, mainly for speed, but cetaceans have done something entirely different with their forelimbs. They added phalanges. In the second through fourth digits, some species of cetaceans have as many as 13 phalanges! For comparison purposes, the human hand has three phalanges on each digit, except for the thumb, which has two. Recently, curiosity about the strange pattern of cetacean limbs has again come to the forefront. So, the big question researchers Zhe Wang, et al. asked is, what is the molecular mechanism for creating extra phalanges and is there evidence of a possible mechanism in the key evolutionary transitions in mammalian history? The answer they found lies in Hox genes.

Digit pattern control is regulated by a few primary Hox genes, mainly Hoxd12, and Hoxd13. These are expressed strongly in the posterior sections of limb buds and their genetic sequences are highly conserved across vertebrates.  The 5’-end of the Hoxd13 sequences is especially important in cases where things go wrong in the forming of the digits, such as in polydactyly (extra digits), brachydactyly (shortened digits) and syndactyly (fused digits). In general, it has been assumed that both of these Hox genes regulate the number of phalanges. Wang and colleagues set out to find conserved Hoxd12 and Hoxd13 gene sequences in DNA samples taken from as many various mammals representing different limb patterns as possible. With DNA evidence, they were able to construct a phylogeny that matched the evolution of repeated segments of the 5’- end of the Hoxd13 gene to the mammalian “family tree”.

They noticed four major transitions in digit number. Reductions occurred in the ancestral lineages of both perissodactyls and cetartiodactyls. Interestingly, the cetacean lineage shows an increase in digit number along with a loss of a digit. Cetaceans have a wide variety of phalanx numbers in both ancestral and modern groups, but in general, it was found that digits I and V generally show a reduced number of phalanges, while digits II, III, and IV have an increased number.

Another interesting bit of evidence was uncovered when researchers took a closer look at the 5’-end repetitions of the Hoxd13 gene. They found duplicate alanine residues in the genetic sequence. Cetaceans and their common ancestor had two more alanines in this sequence than all the other mammals examined. Other mammals have 15 on average, and 14 if one is lost (rabbits, dogs, and badgers), but cetaceans have 17 or 18. Statistical analysis of this revealed that selection pressure on Hoxd12 and Hoxd13 in cetaceans was three times higher than pressure in all other mammals. A specific analysis of key genetic sites in the Hoxd12 gene found that selection in cetaceans has experienced accelerated and adaptive evolution.

For a very long time, it has been assumed that Hox genes are so important to development that they are highly conserved and do not change without drastic consequences for the organism. What this study has revealed is that Hox genes are not so constrained as we once thought. Cetacean evolution is perhaps, one of the best, yet underappreciated occurrences of Hox evolution in existence. However, the question we must then ask is how did the cetaceans manage to change their Hox genes to their advantage without causing serious problems for themselves? After all, mutations in Hoxd12 and Hoxd13 are linked to developmental disorders in humans, and presumably in other mammals as well. Not only that, but within cetaceans, numbers of phalanges and carpals vary considerably. How are polymorphisms like these viable? Wang et al. suggest that the homeodomain (the critical portion of the Hox gene) is not what is under selection in cetaceans, and is, in fact, highly constrained. It is likely that the less critical portions of these genes are able to mutate, resulting in new morphology without causing fatal or crippling problems for the organism. There is also evidence that the development of the digits in the forelimbs and hind limbs are linked. So when cetaceans lost their hind limbs, it is possible that the hind limb genes took on a role in regulating forelimb development and these could adapt to selection pressure on cetacean limb morphology, since they are “extras” in a sense. The next step is to conduct further research to determine the exact roles of Hoxd12 and Hoxs13 in limb development, and maybe we will be able to answer some of these questions.

Cetaceans are a great example of rapid evolution involving Hox genes. What other organisms might have evidence of Hox manipulation in their ancestry?

For further reading on this topic, visit: http://mbe.oxfordjournals.org/cgi/content/full/26/3/613

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~ by enewell on November 20, 2009.

4 Responses to “A Whale of a Discovery: 5’HoxD Genes and the Evolution of Cetacean Flippers”

  1. Liz,

    I’m not sure if you know this, but Chet Fornari does research on Hox genes in the Bio DePartment here, if you are interested in this sort of research. Also, Amanda spent a good portion of her research time with Chet working with Hox genes and can probably say all of this better than I can, but I’m going to attempt it anyway:

    I was very surprised to learn about the evolution of Hox genes when you first mentioned it in your post. However, as it went on, it made more sense. You state that the homeodomain is not under selection and is highly constrained. I thought I might comment on why it is that one would wonder at any mutations in this area of the Hox gene product.

    The Hox gene encodes sequence-specific DNA binding proteins, which, like you said, are involved in development. The homeobox portion of this gene encodes what is called the homeodomain of the protein product of the Hox gene. The homeodomain portion consists of a helix-turn-helix motif containing three alpha helices. One of these helices recognizes the target DNA and contains conserved positively-charged residues which form hydrogen bonds to the negatively-charged phosphate backbone. The other helices stabilize the motif through the interaction of conserved hydrophobic amino acid residues. Thus, it would be surprising if any changes could occur in the nucleotide sequence that codes for this domain of the protein. Thus, the homeobox, a sequence of approximately 180 nucleotide bases, remains highly conserved among species. Hence, when I first read your post, I was rather skeptical. But now, your selected article seems to make quite a bit of sense.

    It does seem that variation in other portions of the Hox gene could be responsible for limb evolution. I am curious as to how this might work. Does it affect the times that Hox binds to DNA during development? Does Hox play other roles? Any ideas?

    Source: Many things for Chet Fornari’s Molecular Biology class, including papers, class lectures, and lab assignments.

  2. Liz!
    I was so excited when I read your post, because, after the molecular mechanisms of cancer, homeobox genes are my favorite research topic! Actually, the work I did (which was actually a collaboration between Chet Fornari’s and Janet Vaglia’s lab), was along similar lines. We were studying salamander posterior Hox genes (and actually I looked at HoxC13 and HoxD13) in the hopes that we would find unique properties of the genes as Dr. Vaglia completes studies to see if Hox genes are activated during limb regeneration.
    The reason we think these genes may be involved in regeneration is that the genes are clearly involved in body patterning. Also, similar studies completed in the newt suggest different roles for different Hox genes in limb regeneration, although more studies need to be completed to fully characterize these roles.
    http://dev.biologists.org/cgi/reprint/117/4/1397

  3. The molecular mechanism of this growth is interesting, but I’m also interested in the evolutionary/ecological role of extra phalanges. What is the advantage to having extra joints? Obviously flexibility is increased, but isn’t there a cost to stability and strength? What kind of creatures have 13 phalanges and how does that help them survive (or at least not interfere)?

  4. I am taking molecular biology with Chet Fornari as well, and I was taken aback to find out that there is mutations in the hox gene. However, it makes sense that mutations that cause changes in the phalanges would not be fatal to the organism, and considering the phenotype, it does not create a drastic effect. This allows the organism to have viable off springs. However, I wonder if mutations in the hox gene cause also other phenotypic mutations. If so since hox genes are important in developing important body parts, I wonder if they can be genetically manipulated to make changes in the organism.

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