User:Collins.1116/sandbox

Topic: Evolutionary significance of egg laying in Echidnas

https://en.wikipedia.org/wiki/Echidna

Penile Spines
I believe the article should explain the purpose of the penile spines in regards to echidna mating. It seem strange to include them in the discussion without explaining what they are and what they do.

Threats
When concerning how to help echidnas, it seems that teaching people how to help them seems a bit much. It would probably be best to not mention handling echidnas at all to prevent people from trying to help them directly, which would probably do more harm then good.

Penis Use
For clarity's sake, it may need to be established what is meant by "During mating, the heads on one side "shut down" and do not grow in size; the other two are used to release semen into the female's two-branched reproductive tract. Each time it has sex, it alternates heads in sets of two." Whether this mean it switches during each mating season or instance of mating is a little unclear.

These may be used to induce ovulation in the female. [18] Virginia Douglass Hayssen; Ari Van Tienhoven (1993). Asdell's Patterns of Mammalian Reproduction: A Compendium of Species-specific Data. Cornell University Press. ISBN 978-0-8014-1753-5.

Edits Start Here
https://en.wikipedia.org/wiki/Monotreme

General Characteristics
The presence of vitellogenin genes(a protein necessary for egg shell formation) is shared with birds, suggesting that when the common ancestor from 166 million years ago split into birds, reptiles, and mammals, egg laying was retained in monotremes and lost in all other mammals. DNA suggests that while this trait is shared and is synapopmorphic with birds, platypuses are still mammals and they evolved lactation with other mammals. This venom is derived frome b-defensins, proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of b-defensins, another trait shared with reptiles. [16]

Reproductive system
Another trait of note deals not so much with the physical characteristics of the animals, but rather their developmental characteristics. One such developmental characteristic deals with the zygotic development of platypuses. Most mammal zygotes go though holoblastic cleavage, meaning that following fertilization the ovum is split due to cell divisions into multiple, divisible daughter cells. This is in comparison to meroplastic division in birds, which causes the ovum to spate but not completely. This causes the cells at the edge of the yolk to be cytoplasmically continuous with the eggs cytoplasm. This allows the yolk, which contains the embryo, to exchange waste and nutrients with the cytoplasm.

https://en.wikipedia.org/wiki/Platypus

Venom
The function of defensins is to blow holes in pathogenic bacteria and viruses, but in platypuses they also are formed into venom for defense.

Reproduction
Most mammal zygotes go though holoblastic cleavage, meaning that following fertilization the ovum is split due to cell divisions into multiple, divisible daughter cells. This is in comparison to meroplastic division in birds and platypuses, which causes the ovum to split but not completely. This causes the cells at the edge of the yolk to be cytoplasmically continuous with the egg’s cytoplasm. This allows the yolk, which contains the embryo, to exchange waste and nutrients with the cytoplasm.[57]

FINAL DRAFT STARTS HERE
Monotreme’s Defining Characteristics in the Context of Synapomorphy vs. Homplasy When trying to answer the question of how evolution has lead to the many different subsets of creatures that make up our world, one must first agree upon how this division of unique organisms into groupings will take place. Thanks to evolutionary forces both random and non-random, a plethora of characteristics has arisen in our world that allow us to divide creatures into separate and distinct groupings. Thanks to this system, one knows easily that a living creature with scales and perhaps claws is a reptile, while a creature with fur and horns is probably a mammal. However, like most any model, this system is not absolute as will soon be demonstrated. If one hears that a creature bears live young, the first assumption would probably be that said creature is a mammal. Contrary to this tacit understanding, there is an order of mammals that actually lay eggs as opposed to having a life birth. This is the order Monotreme, and it is this as well as other unique traits of the order which shall be used to better understand its unique taxonomy as well as the distinction between homoplasic and synapomorphic traits. The order Monotreme (Latin for “one-holed”) includes the platypus (Ornithorhynchus anatinus) and four separate species of echidnas: Tachyglossus aculeatus, Zaglossus. attenboroughi, Zaglossus bruijnii, Zaglossus barton. The name “one-holed” arises from the fact that monotremes, like birds and lizards, have one opening that their bodies use for both excretion and reproduction in the case of females of the species. This structure is referred to as a cloaca. Also like lizards, the legs of monotremes are at the sides of their bodies rather than underneath them and they lack the claustrum in their brain (Butler, Molnár, and Manger, 2002). They also have beaks they use as the analog to a normal mammalian mouth or snout, but monotremes exhibit many similar characteristics to other mammals, including hair, high metabolic rates, three middle ear bones, one bone in their lower jaw, and the production of milk to nourish their young (Mervyn, 1978). It is interesting to notes how monotremes share many traits between both mammals and other taxonomic classes, especially as there is some discussion as to which traits they possess are synapomorphic and which are homoplastic. Traits found in platypuses can also be found in birds and reptiles. The first of these similar traits was the presence of a cloaca in females as previously stated. Rather than having a separate tract for egg laying, the tract for both solid and liquid waste doubles as the birth canal. When it comes time to lay an egg, the cloaca descends over the waste tract and a series of muscular contractions pushes the egg out. It should be noted however that while platypuses lay eggs, compared to chickens one outstanding difference is the time spent in internal incubation versus external incubation. Chicken eggs as a comparison spend one day in the reproductive tract and twenty-one days in external incubation under the mother, while for platypuses the egg remains in the uterus for twenty-eight days with only ten days of external incubation (Myers, 2008) (Fitzpatrick, Benjamin, and Nachman, 2004). This particular trait both shows how similar traits have arisen in separate orders of animals. Another trait of note deals not so much with the physical characteristics of the animals, but rather their developmental characteristics. One such developmental characteristic deals with the zygotic development of platypuses. Most mammal zygotes go though holoblastic cleavage, meaning that following fertilization the ovum is split due to cell divisions into multiple, divisible daughter cells. This is in comparison to meroplastic division in birds, which causes the ovum to spate but not completely. This causes the cells at the edge of the yolk to be cytoplasmically continuous with the eggs cytoplasm. This allows the yolk, which contains the embryo, to exchange waste and nutrients with the cytoplasm. The question still remains, however, whether these traits mean that monotremes are closely related to egg laying animals such as reptiles and birds. According to research done at the University of Minnesota, the fact that monotremes lay eggs but are still classified as mammals is less of a evolutionary mystery and more of a way to view how ancestral states pass on traits and how these traits, or lack thereof, can shed light on a common ancestral state (Myers, 2008). The research team looked at varying traits amongst monotremes, reptiles, and birds to try to create a picture of what traits evolved from a common ancestor, which traits are due to convergent evolution, and which are inconclusive tools for assigning relatedness. To begin, the team uses the example of egg lying between platypuses and chickens. It would be easy to presume that since chickens and platypuses lay eggs that each are closely related to one another. It is a discrete trait that can be used for taxonomic purposes, but from a genetic basis there is more to be said for a possible ancient ancestor versus a more common one. For any organism, the traits they attain are a combination of traits passed more or less directly down the line, derived traits, and traits that are lost. Using genomic data, platypuses are concluded to share a common ancestor that split into reptiles, birds, and mammals 166 million years ago. This could mean one of two things. Either that common ancestor passed on this trait but only monotremes retained it in the mammalian trajectory (synapomorphy) or this trait was lost until it appeared again due to evolutionary forces such as natural selection, mutation, etc., rather than it being “awoken” in the genetic code (homoplasy) (Myers, 2008). One protein necessary for using eggs for reproduction as monotremes do is the gene that codes for vitellogenin. This protein is necessary for the proper formation and upkeep of egg yolks, something that humans have no use for as they bear live young. Instead, most mammals use placentation to provide nutrients to embryos. The presence of vitellogenin genes is however shared with birds, suggesting that when the common ancestor from 166 million years ago split into birds, reptiles, and mammals, egg laying was retained in monotremes and lost in all other mammals. DNA suggests that while this trait is shared and is synapopmorphic with birds, platypuses are still mammals and they evolved lactation with other mammals. The theory behind this is that lactation provided nutrition for young, with placentation later evolving to facilitate embryonic nutrition. Eventually placentation and lactation took over as the primary source of nutrition for embryos and infants respectively for mammals, but monotremes instead gained lactation at the same time as other mammals while retaining the egg-laying trait instead of evolving placentation (Fitzpatrick, Benjamin, and Nachman, 2004) (Brawand, Wahli, and Kaessmann, 2008). This could also explain the inviability of hybrids between monotremes and other mammals (Nicol, Stewart, and Andersen, 2007). Another trait that monotremes have is the ability to develop venoms. Like some reptiles, male platypuses (not females or echidnas) can produce powerful venom that they can inject into a target using a spur on the ankle of the back legs (Butler, Molnár, and, Manger, 2002). This, combined with a few extra bones in their shoulders along with their reptile-like way of walking, suggests a relation between the two (Myers, 2008). As previously stated, these traits could very easily be attributed to synapomorphy. For the shoulders, the unique structure allows monotremes to burrow efficiently, as well as swim in the case of the platypus. This trait could easily be assumed to be retained from the ancient ancestor shared with reptiles. However, the fossil record is largely inconclusive for monotremes, so while this trait can be speculated upon, a definitive answer has yet to be found (Nicol, Stewart, and Andersen, 2007) (Cromer, 2004). Saying this about similar physiology, the production of venom can actually be traced back along the monotremes lineage. B-defensins are proteins in mammals that are used as part of the innate immune system to kill pathogens by blowing holes in viral and bacterial pathogens. Platypuses use these b-defensins in their venom, only they select for a form that is more toxic to other animals once injected into the body of their target. Some venomous snakes also use b-defensins in their venoms, suggesting synapomorphy for venom production as a possible reason. However, platypuses and snakes have different duplications for these b-defensins. This suggests that this trait evolved independently and is rather a case of homoplasy. The ability for b-defensins to be used as toxins can be assumed to have been used by monotremes and snakes more so due to the ease of its conversion a toxin versus a common ancestor necessarily using it. DNA data further supports this idea in that the common ancestor did not seem to pass on this trait when it split into mammals, birds, and reptiles. As such, venom production acts as further evidence of a lack of a close relationship between monotremes and other taxonomic classes other than Mammalia (Myers, 2008) (Choi, 2008). As previously stated, the complex relationship between the expression and repression of traits between organisms can be used to distribute them into unique taxa and draw conclusions on the origins of their similarities based on phenotypic and genetic data. Using monotremes as an example it can be seen that while traits can be used to assume and identify similarities between animals, assumptions must be clarified in the context of genetic data. Similar traits may be an example of synapomorphy, though even if this is so the common ancestor may be very ancient and thus these traits do not necessarily point to a common ancestor or a close genetic relationship between organisms. Similar traits may also be due to homoplasy caused by convergent evolution, causing similar traits not due to a common ancestor but rather outside evolutionary forces creating an opportunity for analogous traits to arise. In understanding how such traits arise and the genetic and evolutionary basis behind them, a greater understanding for taxonomy and the mechanisms behind evolution in general can be attained. References 1.	Butler, Ann B., Zoltán Molnár, and Paul R. Manger. "Apparent Absence of Claustrum in Monotremes: Implications for Forebrain Evolution in Amniotes." Brain, Behavior and Evolution 60.4 (2002): 230-40. Web. 2.	Griffiths, Mervyn. The Biology of the Monotremes. New York: Academic, 1978. Print. 3.	Myers, Pz. "Interpreting Shared Characteristics: The Platypus Genome." Nature.com. Nature Publishing Group, n.d. (2008) Web. 28 Oct. 2014. 4.	Fitzpatrick, Benjamin M, and M Nachman. "Rates of Evolution of Hybrid Inviability in Birds and Mammals." Evolution. 58.8 (2004): 1865-1870. Print. 5.	Brawand, D, W Wahli, and H Kaessmann. "Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and Placentation." Plos Biology. 6.3 (2008). Print. 6.	Nicol, Stewart, and Niels A. Andersen. "The Life History of an Egg-Laying Mammal, the Echidna ( Tachyglossus Aculeatus )." Ecoscience. 14.3 (2007): 275-285. Print. 7.	Cromer, Erica. "Monotreme Reproductive Physiology and Behavior." Monotreme Reproductive Physiology and Behavior (2004). Iowa State University, n.d. Web. 26 Oct. 2014 8.	Choi, Charles Q. "Extreme Monotremes: Why Do Egg-Laying Mammals Still Exist?" Scientific American Global RSS. Scientific American (2009), n.d. Web. 23 Sept. 2014.