During development, humans are known to have gill slits for a certain period of time. The gill region of humans and sharks are shown in the diagram, on page 91. As described by Neil Shubin, the purpose of these slits is to form jaw bones, ear and throat structures, vocal cords, etc. Why is it that the gill slits in sharks do not close but the gill slits in humans close and “disappear” after a certain time in development? What signals or genes command the gill slits in sharks to stay open and for the gill slits in human embryos to close even though the gill region serves as the same purpose in both species?
Chandrika D.
Simply put, gill slits in sharks remain open for the simple reason that they use the gills to absorb oxygen form the water while humans live on land and have easier access to oxygen and don’t need gills to absorb oxygen from the water. This involves the theme of structure and function. The entire purpose of sharks to have gills is because in gills, there are gill filaments and capillaries that use countercurrent exchange for gas exchange and oxygen absorption (Campbell 917). Sharks don’t have lungs like humans that use alveoli to put oxygen in the bloodstream. That is another reason why gill slits in humans are closed. Gills in humans that live on land would have absolutely no use. However, like always, there are mutations. For example, sometimes in humans the gill slits don’t fully close causing pouches or cysts like the branchial cysts that is a “benign fluid-filled cyst that forms in an open pouch inside the neck” (Shubin 90). Technically speaking, the gills slits are only present in shark because of the third and fourth arches present in embryo development. Therefore, the genes related to the opening and closings of these gill slits are about the third and fourth arches.
ReplyDeleteFor the branchial cysts that occurs in humans, the three main genes that causes this are EYA1, SIX1, and SIX5 (http://ghr.nlm.nih.gov/condition/branchiootorenal-syndrome). It is thought that most of these cysts occur because of the mutations in the EYA1 gene rather than the other two. The proteins from the EYA1 gene interact with many other genes to regulate proper and normal embryonic development, but if one of these genes is mutated, then the protein is unable to properly interact with the other genes causing this defect in humans. This disorder is thought to be autosomal dominant which means one copy of this mutated gene will cause this syndrome. This brings in the theme of continuity and change because the chromosome that holds this gene is passed down from generation to generation from a parent. However, sometimes, the mutated genes can be present in the offspring and not in the parent which implies that some kind of mutation, like nondisjunction in meiosis, could have mutated the gene (Campbell 297). As for sharks, it is thought that PAX1 and PAX9 genes are relevant for the formation of gills in sharks (http://www.ncbi.nlm.nih.gov/pubmed/10226012). These genes encode specific transcription factors for other genes to make proteins specific for gill formation. Therefore, all of these genes interact with each other by one way to regulate the proper formation of gills in sharks and prevent the formation of gills in humans.
Benny Jeong
bennyjeong218@gmail.com
Part ONE of TWO
ReplyDeleteShubin comments on the nature of the arches, the indentations, in fish and humans. These swellings have led to the complex cranial structures inside the head, along with the advancement of the nervous system. The indentations open up to form spaces between the gills where the water flows in fish. In humans, the indentations have sealed over (unless, in rare cases of genetic mutations). Both shark and human organisms share four arches in their development.
Like Benny has said, the general reason why our gill regions have sealed over is due to the fact that we have evolved to a terrestrial environment. As oxygen diffuses in sharks through their gills to enter the bloodstream, oxygen in our body enters by the lungs through our mouths. From the nasal cavity and pharynx, inhaled air passes through the larynx and trachea. There it moves from bronchi to bronchioles. There, oxygen is able to diffuse inside the aveoli (tiny air sacs) in the lungs. Humans have millions of aveoli, with a surface area of 100 m^2, to maximize our gas exchange and efficiency.
In addition to why mammals such as humans don’t have gills, is that mammals would need to have very large ones to be functional. In fish, gills work due to their cold-bloodedness. Cold-blooded animals need a significantly smaller amount of oxygen than warm-blooded organisms do. Humans need approximately 15x more oxygen than a shark would need.
Also, sharks use their mouths and gill flaps to constantly pump water over their gills. Sharks have to swim constantly so water flows through their gills, and humans would need gills 15x larger than fish in order to achieve this. A disadvantage sharks have is that if they suddenly stop swimming, they sink. Continual swimming ensures water flows into the shark’s mouth and out through the gills for gas exchange to occur. This would not be beneficial in our anatomy and function in terrestrial living organisms like us. It would be simply redundant.
A question arises though of how we gained lungs as an evolutionary advantage. What most likely is to have happened is that fish living in oxy-poor water conditions varied by natural selection, and any fish able to absorb oxygen from air rather than water, even for a fraction of time, would have been given a head start. This can be reflecterd in modern lungfish. Today, due to a portion of fish maintaining survival through genetic advantages, terrestrial vertebrates have evolved lungs gas bladders in fish.
PART 2 RESPONSE
ReplyDeleteHuman embryos are able to develop gill slits, but in organogenesis the development stops and gills never become fully developed. These slits are merely found in the embryos of all vertebrates because we share fish as a common ancestor in which structures evolved. Even if we contain genes to encode for gills, they have deteriorated. A five-week- old human embryo has external gill slits (grooves) and these grooves instead contribute to the development of the lower jaw. In animals with functional gills, the genes are conserved due to national selection. Mutations that result in non-functional gills are eliminated.
Benny states the scientific genetic mutations that give us gill slits, yet it is unlikely humans will ever develop gills even though we may contain the specific genes necessary, due to the fact that humans have undergone so much evolution and mutation. Also, natural selection only acts upon products, not upon genes. Any nucleotide genetic change in an individual would be neither advantageous nor disadvantageous. Shubin states how the water-to-land transition happened around, 375 million years ago. We have accumulated a plethora of nucleotide substitutions that if we possess the genes for gills they would not be functional.
Sources:
Your Inner Fish
Campbell
http://express.howstuffworks.com/mb-gills.htm
http://www.globalshiksha.com/content/pharyngeal-gill-slits-in-humans
http://www.sciencemag.org/content/298/5592/381
Kyle Kim, piece847@gmail.com