The Evidence-Animal Development
In a footnote from page 24, Shenk refers to Lawrence Harper’s idea that “most genes cannot be counted on to directly produce specific traits” and “they are active participants in the development process and are built for flexibility” (187). We once discussed how many of the genes in a fruit fly are incredibly similar to us humans, yet we do not have compound eyes and wings and fruit flies are nothing compared to a size of a human. Consider our unit on animal development and how the amount of yolk for a sea urchin, frog, chicken, and human accounts for a different type of cleavage in the organism. How do animals that share a similar developmental lineage differ in cleavage type and how does this disprove gene determinism? Why are humans able to develop a central nervous system (brain and spinal cord) via a neural tube during organogenesis while a clam only develops a nerve ring and could such differences account for habitats in which they live in? Discuss the significance of many terrestrial animals having adapted to land with an amniotic egg or extraembryonic membranes and how Harper’s statement is supported by such an adaption. How can we relate Harper’s assertion to evolution?
-Trish Chari (trishtennis@gmail.com)
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ReplyDeleteThe amount of yolk in animal eggs determine the type of cleavage the zygote undergoes. Since sea urchin, frog, and human eggs have little yolk, they undergo holoblastic cleavage, which is a pattern in which the cleavage furrow passes all the way through the cells (Campbell 1027). Unlike humans, sea urchins and frogs, eggs of birds (chicken), reptiles, and insects contain a lot of yolk, and therefore undergo meroblastic cleavage, an incomplete division of a yolk-rich egg (p.1027) because cleavage furrows cannot pass through yolk.
ReplyDeleteGenetic determinism by definition means the theory that human character and behavior are shaped by the genes that comprise the individual’s genotype rather than by culture, environment, and individual choice (http://www.reference.md/files/D033/mD033141.html). While cleavage types in these animals disprove gene determinism because of each animal’s specific pattern formation, as Kirk says, cleavage types in these animals disprove gene determinism also because of each animal’s specific function or niche to its environment. For example, humans are able to develop a central nervous system (brain and spinal cord) during organogenesis while a clam only develops a nerve ring because of its function or niche to its environment. I agree with Kirk on the facts that humans are able to develop a central nervous system since we are vertebrates, which allows us to develop a neural tube, and that clams are unable to because they are invertebrates. However, this is also largely due to where they live, and their niche in order to survive and reproduce. Clams are filter feeders, which means that they physically do not need to use up a whole lot of energy in order to obtain food. Clams draw in water through the incurrent siphon, and filter the food through their gills, in which the food then is digested. Humans, on the other hand, are predators and need the use of a backbone in order to catch their prey, as well as prey to larger animals. The central nervous system allows humans to react more quickly to prey/predator; it allows them to run faster and more strategically in order to capture prey or escape a predator.
The adaption of the amniotic egg supports Harper’s statement that “cellular gene expression can be stably altered in response to conditions outside the organism to permit it to adapt to its environment” (p. 187) in the way that the creation of the amniotic egg introduced vertebrates to terrestrial life. As Campbell says, “the movement of vertebrates onto land could only occur only after the evolution of structures that would allow reproduction in dry environments”(Campbell 1033), with a hard shell to protect the embryo and the fluids around it from harsh, dry environments, as well as keep in the: amnion, which prevents dehydration in the embryo; allantois, which is the disposal sac for metabolic wastes from the embryo; the chorion, which exchange gases between the embryo and the air around it; and the yolk sac and albumen, which is full of food for the embryo. With so many predators in a marine or freshwater environment, it was probably favorable for species to adapt to terrestrial life, with less competition for food and living space.
Tracy Lai (tracymlai@hotmail.com)
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ReplyDeleteCleavage differs in each organism based on a specific pattern relative to the animal and vegetal poles of the zygote (Campbell 1045). Holoblastic cleavage, division of the whole egg, takes place for species whose eggs have smaller amounts of yolk (frogs, mammals, etc) and meroblastic cleavage takes place for species with yolk-rich eggs (birds and reptiles). Gene determinism is disproven through fate mapping and how specific regions of the blastula or zygote develop into specific parts. Although there may be a similar developmental lineage, each species has a specific pattern formation, which involves inductive signals for organ and tissue arrangement in their own characteristic place. As animals undergo morphogenesis, there are specific changes in cell shape, position, and adhesion (Campbell 1038). Ultimately, genes do not form specific phenotypes as the developmental fate of each cell is based on inductive signals which will not exactly give a specific morphological phenotype.
ReplyDeleteHumans are able to develop a central nervous system unlike clams because humans are chordates. The notochord, or the skeletal rod characteristic of all chordates, allows humans to form the spinal part of the central nervous system (Campbell 1031). Since humans are vertebrates, that also allows the development of the neural tube, which will be the central nervous system. Clams on the other hand do undergo cleavage and gastrulation, but they do not develop a central nervous system because they are not vertebrates (http://www.pac.dfo-mpo.gc.ca/science/species-especes/shellfish-coquillages/geopath/develop-eng.htm). Clams do not have the genes to make a central nervous system. Possibly because the clam never had a central nervous system and they were not able to develop one is the reason why the difference is in the environment between humans and clams. Humans are able to develop a central nervous system (brain and spinal cord) during organogenesis while a clam only develops a nerve ring because of its function to its environment. Most invertebrates tend to live in environments with water while many vertebrates can live on land where it is favorable for species to adapt to terrestrial life, with less competition for food and living space.
Lastly, terrestrial animals develop with an amniotic egg and extraembryonic membranes to be “flexible” in their environment as the environment plays a role in their developmental process. By surrounding the embryo in a fluid within a sac, terrestrial animals were able to evolve and survive based on being “active participants in developmental process and are built for flexibility” (187). The theme of evolution is exhibited as organisms were able to adapt to a new environment (water to land) through evolving structures including an amniotic egg and extraembyronic membranes which allows them to develop and survive on land (Campbell 1033). “The movement of vertebrates onto land would occur only after the evolution of structures which can allow the organisms to survive and reproduce in dry environments” (Campbell 1033). These specific structures create and preserve traits that are seemingly fitted for the functional roles they perform in the theory of natural selection to adapt to the environment. Over time, evolution took place as the theory of natural selection was applied to the favorable traits developed over time with the change across successive generations in the heritable characteristics of the population. The central nervous system allows humans to react more quickly to the environment and it allows us to run faster and more strategically in order to capture prey or escape a predator. Meanwhile clams are filter feeders and they are able to survive with just a nerve ring and not a central nervous system as they draw in water through the incurrent siphon, and filter the food through their gills, in which the food then is digested.
(Kirk Chiu- krkpchiu@gmail.com)