On page 112, Shubin discusses the similarities and differences between humans and jellyfish. Discuss these similarities and differences. What role do hox genes play into the similarities and differences between humans and jellyfish. Use pages 671-673 and 728-733 in Cambell to expand upon Shubin's points. What changes do we see as we travel down evolutionary history? Use the review diagram on page 665 of Cambell and Figure 34.2 on page 699 of Cambell for reference. In the discussion, include the major phyla we discussed in class. Also, page 183 of Your Inner Fish has a great diagram to help.
(Bobby Muttilainen, rmuttilainen@gmail.com)
One characteristic that distinguishes animals from other kingdoms, is the body. Cells link together to form not a colony, but a multi-cellular organism in which different cells relay different genetic information and perform different functions. The animal body, throughout evolutionary history, has maintained certain characteristics such as tissue development. These common characteristics are coded for in what has become known as an Organizer gene, or more recently, Hox genes. According to Neil Shubin, “Hox genes appear in every animal that has a body” (Shubin, 110). It is these Hox genes that provide an evolutionary link between jellyfish and humans.
ReplyDeleteHox genes play many different roles in the formation of animal bodies. For one, they mold almost all animal bodies, establishing up-down and back-front axes (Shubin, 110). These genes also establish proportions, which is why you’ll rarely, if ever, find an animal whose head is larger than its body or limbs. Also, the number of Hox genes varies by species. Because more complex animals, such as mammals, tend to have more copies of Hox genes, it is likely that the genes arose from duplicates of Hox genes in earlier species. But essentialy, from an evolutionary standpoint, the Hox genes are the reason as to why animal bodies have similar structural patterns.
At first glance, when comparing a jellyfish, or any other cnidarian, to a human, they seem to be polar opposites. The jellyfish exhibits radial symmetry while humans exhibit lateral symmetry. Jellyfish average 1-2 feet tall and live in water (http://animalhub.info/jellyfish-average-height) while humans average about 5.5 feet and live on land (http://wiki.answers.com/Q/Average_human
_height). But despite these very obvious differences, many similarities lie in the planning of their bodies. Both animals have an up-down and back-front axis. Both have a body cavity, which over time allowed for increased organ development (Campbell, 671). And in their basic body blueprint, both contain a central body from which tentacles or limbs diverge. Because one role of Hox genes is limb formation, there is little doubt that Hox genes helped maintain this structural characteristic of animal bodies throughout evolutionary history.
Some evolutionary changes we see over time are more complex body plans with organs and systems. While cnidarian organs are simple, they gave way to the more complex organs found in humans. The advent of the coelom in the phylum Annelida, allowed for a digestive, nervous, and circulatory system as well. Furthermore, this organ development allowed animals to become more independent of their environments. The development of skeletons also gave animals a selective advantage, giving body structures stability, yet flexibility. Humans, for example, have skeletons that allows for long distance movement, which in turn allowed early humans to hunt a large variety of prey (Campbell, 731). Lungs, legs, and the amniotic egg are major evolutionary developments that aided terrestrial animals (Campbell, 699). These developments allowed terrestrial animals to grow bigger and ensure successful development of offspring. Over time, animals also became more independent of their environments. They were better able to regulate their internal temperature, digest a wider variety of foods, and millions of years later, think for themselves using intelligent, skull encased brains (Shubin 183).
This all ties back to the Hox genes, however, because these genes were, and still are, the driving force in body development. While the Hox genes most likely occupy more functions in more complex animals, their basic functions remain continuous: establish axes, proportions, organs, and limbs. The organs and limbs of a jellyfish may be very different from the organs and limbs of a human, but nonetheless, they form the backbone of all animal body plans. Without the Hox genes, the animal kingdom would be in shambles.
Although Sami Kopinsky has already extensively covered how Hox genes contribute to the differences and similarities between the humans and jellyfish, she does not do a satisfactory job in explaining what these specific differences and similarities are.
ReplyDeleteThe most obvious difference between humans and jellyfish is that jellyfish live in an aquatic environment, while humans are terrestrial organisms. In addition, jellyfish are invertebrates, while humans are vertebrates. To be more specific, jellyfish are a part of the phylum known as Cnidarians, and humans are known as Chordates. As Sami briefly explains, jellyfish and humans are different in that they develop with different symmetry patterns. Cnidarians are radially symmetrical, and therefore are able to respond to stimuli from all directions. On the other hand, humans are bilaterally symmetrical and therefore are highly cephalized. Because their central nervous system develops toward the anterior side of their body, they are given the unique advantage of being able to effectively search for food and escape from its prey. Something that Sami did not mention however, is the fact that Cnidarians are dipoblastic, while humans are tripoblastic. An important idea to note is that dipoblastic organisms (dipo- meaning two) only develop two germ layers: the ectoderm and the endoderm. A tripoblastic organism would also have a mesoderm, which would eventually give rise to the notochord. Although Sami has suggested a similarity between humans and jellyfish that they “[b]oth have a body cavity”, this is NOT true. Cnidarians are actually acoelomates, which mean that they do not have a cavity within their body. In contrast, humans are coelomates, meaning that they have body cavity lined by the mesoderm. In addition, their reproductive cycles are different. While humans engage in sexual reproduction throughout their entire lifespan, Cnidarians have both an asexual stage (polyp) and a sexual stage (medusa). To expand upon this idea, Cnidarians also fertilize externally and develop externally, while humans fertilize internally and develop internally.
As Sami mentions, however, jellyfish and humans have several similarities which give us an idea of their evolutionary history. For example, both organisms have Hox genes, which determine the basic body plan of the organism. The book mentions several studies involving the importance of Hox genes on embryonic development. One of the studies that were mentioned involve the analysis of the Hox gene in the chick limb bud. The marvelous result of the experiment was that the chick embryo had developed a mirror replica of the normal set of wings, suggesting that Hox genes were somehow responsible in the process (http://dev.biologists.org/content/122/5/1449.short). These Hox genes are what enable the diversity and the similarities that are found within various species today. In addition, both are protostomes, meaning that they develop the archenteron from the mouth first, and then the anus is formed second during embryonic development. Because they are both protostomes, they both undergo radial cleavage. Radial cleavage results in the formation of parallel or perpendicular cleavage planes during the period of development. Organisms that exhibit radial cleavage often also exhibit indeterminate cleavage. The individual cells that form during indeterminate cleavage are highly versatile, and contribute to the understanding of why embryonic stem cells are so useful for scientific studies (Campbell, 660).
ReplyDeleteIn summary, Hox genes provide jellyfish and humans with their unique body plans, which contribute to their individual physical structures and adaptations. In this, we can see the theme of continuity and change; DNA molecules such as Hox genes carry biological information from its ancestors to its future generations. Because of this, the body plans of an individual species is able to remain relatively constant, while mechanisms such as mutations and cross-overs during meiosis allow for the diversity of each and individual organism of its kind.
Works Consulted:
Campbell Biology
Your Inner Fish
“Analysis of Hox gene in the chick limb bud”- http://dev.biologists.org/content/122/5/1449.short
(Keigo Tanaka; tanakarus3@hotmail.com)