In chapter 6, Shubin describes the importance of the Hox gene in being the "organizer" of the body plan. One type of Hox gene called Noggin determines the body axis of an organism. While humans display bilateral symmetry, other organisms such as sea anemone or jellyfish show radial symmetry. What has caused the differences between organisms to survive better with either form of body axis? How does the locomotion of each organism relate to the symmetry of the organism or the body plan in general? What is the purpose of the Hox genes to create a dorsal and ventral side on humans when these features aren't present on Cnidarians?
(Claire Yao, claire.yao521@gmail.com)
One big advantage of radial symmetry is that it allows for animals to reach out in all directions from one center, like when it feeds. This is an advantage since they don't have a high degree of controllable movement even during food collecting periods. Another advantage is that radial symmetric animals are able to receive stimuli from all directions, which have efficient defense mechanisms due to their radial symmetric distribution (Solmon 2). Maybe another advantage is that if a part of the organism gets cut off, the other parts can still function in place of the lost part. Generally, sedentary invertebrates that live in water have radial symmetry, which is generally associated with Cnidaria and Porifera. Generally, animals who have no calculated direction would have radial symmetry since they can receive stimuli from all directions, and animals that have oriented movements would have bilateral symmetry to direct stimuli.
ReplyDeleteThe secreted polypeptide Noggin, encoded by the NOG gene, binds and inactivates members of the transforming growth factor-beta (TGF-beta) superfamily signaling proteins, such as bone morphogenetic protein-4 (BMP4). By diffusing through extracellular matrices more efficiently than members of the TGF-beta superfamily, Noggin may have a principal role in creating morphogenic gradients. Noggin appears to have pleiotropic effect, both early in development as well as in later stages. The Noggin gene is incredibly similar in humans, Xenopus, rats, and mice (Rubenstein 3). This shows that the gene has developed similarly in a variety of bilateral animals, showing that animals that are bilateral have a common ancestor. Some reasons that the gene might have developed animals to become bilateral is that bilateral animals can direct their movement and respond much better to stimuli than radial symmetrical animals (Solmon 3). Being bilateral also promotes cephalization, which is a selective advantage that has developed over time from some of the first mollusks, like squids and octopi.
benitorosenberg12@comcast.net
http://docs.google.com/viewer?a=v&q=cache:t1g--FlWsSsJ:www.ndustudents.com/index.php?dir%3DCourses%252FFACULTY%2BOF%2BNATURAL%2BAND%2BAPPLIED%2BSCIENCES
http://www.ncbi.nlm.nih.gov/pubmed/7557985
As Ben pointed out, radial symmetry assists animals that exhibit it in responding to stimuli from all directions. This allows them to have better defense mechanisms against predators, since they have a higher likelihood of noticing danger. However, radial symmetry can be linked even more obviously to the locomotion of the animals! Most radial symmetrical animals are either sessile - not moving, or attached to a substrate - or planktonic - drifting around or swimming, like jellies (Campbell 659). Since they don’t move around very much, if at all, they don’t need to worry much about coordinated movement, and thus as the original species lived less and less actively underwater, radial symmetry became a selective advantage, as those that utilized more space for stimulus recognition by equal distribution of receptors were more suited to their environment and less likely to be preyed upon, and survived those that did not exhibit radial symmetry in a way that facilitated more efficient stimulus recognition. On the other hand, bilaterally symmetrical animals, for the most part, have a central nervous system to assist in coordinating complex movements such as crawling, flying, swimming, or running (Campbell 659). Even so, it has been found that internal circulation was a large factor in the development of bilateral symmetry by “affecting compartmentalization of the gut and the location of major ciliary tracts” (http://onlinelibrary.wiley.com/doi/10.1002/bies.20299/abstract). Additionally, as Ben stated, bilateral symmetry also promoted cephalization, which is a selective advantage in multiple animals (Campbell 659). In essence there are multiple reasons for why bilateral symmetry differs from radial symmetry, but they may not even be limited just to locomotion.
ReplyDeleteAs for Hox genes creating the dorsal-ventral axis in humans while not being present in Cnidarians, there is actually a purpose for them. The dorsal-ventral axis (which Shubin refers to as the “belly-to-back” axis) has no analogue in the sea anemone, which does seem to display more symmetry than usually found in Cnidarians, even though the sea anemone had basically the same Hox genes; instead, the sea anemone’s Hox genes were active along a directive axis, which separates left from right, rather than front from back (Shubin 114-115). These similarities in axis presentation between Cnidarians and members of the phylum Bilateria can actually be attributed to bilateral symmetry’s development before the split between Cnidaria and Bilateria. It has been found that the aforementioned Hox gene expression in sea anemones (particularly Nematostella vectensis) results in bilateral symmetry, and by being presented along a primary axis along with a growth factor (called dpp) implies that Hox genes creating bilateral symmetry were doing so even before Cnidaria and Bilateria became separate phyla (http://www.sciencemag.org/content/304/5675/1335.abstract). While the Hox genes may not serve exactly the same purpose in humans as they do in Cnidarians, this is actually a perfect example of evolution at work! Once again the theme of using old parts in different ways shows up, as the same type of genes is reused to serve different purposes: what were originally used for development of the directive axis in sea anemones became more complex and varied to create the multitude of axes in modern Bilateria members. Hox genes present differently in different organisms all depending on their environment, being modified depending on the selective advantage a certain method of organogenesis provided, and thus are probably one of the most flexible sets of genes present in life forms.
Eugene Bulkin (doubleaw002@gmail.com)