User:Hill.1770/sandbox

Topic: Evolution of Placentation in Squamate Reptiles

Article edited and commented on: https://en.wikipedia.org/wiki/Placentation

Article commented on only: https://en.wikipedia.org/wiki/Placenta

Comments


 * 1.) (In Placentation)The evolution of placentae is an important and interesting topic that could be included in this article. I believe there is a decent amount of research, enough at least for a small section (for squamata in particular). One example: (reference 1)
 * 2.) (In Placenta) I think a section discussing the evolution of the placenta would be beneficial to this article. It could even probably be separated into the evolution of placentae in mammals, live-bearing fish and in squamates (lizards and reptiles) if enough sources can be found.
 * 3.) (In Placenta) Another way to improve this article, especially if the evolution of different animal placentae is too specific, would be to include a section that goes into some amount of detail to describe placentae in animals besides just humans/mammals, such as placentae in squamates and fish. It could also reference the term placenta as it applies to plants. Along with this, it might be a good idea to link Placentation to the "See Also" section.

Sentence added


 * "This means that the occurrence of placentation in squamata is more frequent than in all other vertebrates combined,[5] making them ideal for research on the evolution of placentation and viviparity itself."

Wikipedia Article Edit (11/16)

Link: https://en.wikipedia.org/wiki/Vivipary#Evolution

Text:
 * Evolution


 * In general, viviparity and matrotrophy are believed to have evolved from an ancestral condition of oviparity and lecithotrophy (nutrients supplied through the yolk). One traditional hypothesis as to the sequence of evolutionary steps leading to viviparity is a linear model that can be restated rather simply. Initially, just an increase in the length of time that the egg remained in the reproductive tract of the mother may have gradually allowed for the evolution of egg retention, provided that fertilization was internal. Through continued generations of egg retention, viviparous lecithotrophy may have gradually appeared; in other words, the entire development of the embryo (with nutrients provided by the yolk) occurred inside the mother’s reproductive tract, after which she would give birth to the hatched young. The next evolutionary step would be incipient matrotrophy, where yolk supplies are gradually substitued with nutrients seeping in from the mother's reproductive tract.[9]


 * In many ways, viviparity can be more strenuous and more physically and energetically taxing on the mother than oviparity. However, it’s numerous evolutionary origins signify that there must be worthwhile benefits to this mode of reproduction in some scenarios, as well as selective pressures that allowed it to convergently evolve over 150 times in vertebrates.[10] One of the most profoundly advantageous features of viviparity is thermoregulation of the embryo.[11] Since the developing offspring remains within the mother’s body, she becomes, in essence, a walking incubator. This is very useful when climate change imposes cooler average temperatures on a current habitat, or when a migration event necessitates adaptation to a new, cooler environment. When considering squamate reptiles in particular, there is a correlation between high altitudes or latitudes, colder climates and the frequency of viviparity. This tendency for egg-retention, and consequently viviparity, to be selectively favored under cooler conditions due to its thermoregulatory benefits is termed "the cold climate hypothesis."[12]

FINAL DRAFT STARTS HERE (11/16)

Evolution of viviparity in squamates and other vertebrates
 * Desirée Hill
 * Recitation: Tues, 3:00 pm

The two most common modes of reproduction in the animal kingdom are oviparity and viviparity. Oviparity, commonly referred to as “egg-laying reproduction”, is the most frequent of methods seen in animals. With oviparity, the mother lays her eggs (either fertilized or unfertilized) and they develop to completion outside of her body where they will eventually hatch. Viviparity, or “live-bearing reproduction,” refers to the method in which a mother carries her developing eggs inside her reproductive tract until giving birth to live offspring. Although not the most pervasive mode of reproduction, viviparity was found to have evolved independently in various taxonomic groups. While all birds, turtles and most anamniotes are characteristically oviparous, viviparity is the reproductive mode of all mammals (excluding egg-laying monotremes), many reptiles, some amphibians, some fish and even some invertebrates. There have been at least 160 independent origins of viviparity amongst animals, with over 100 of those arising in squamate reptiles alone (Blackburn, 2005). The widespread occurrence of live birth, despite its disadvantages, makes viviparity a fascinating and ongoing topic of research amongst a number of evolutionary biologists, especially those interested in convergent evolution. In many ways, viviparity can be more strenuous and more physically and energetically taxing on the mother than oviparity. However, it’s numerous, independent, evolutionary origins signify that there must be worthwhile benefits to this mode of reproduction in some scenarios, as well as selective pressures that allowed it to convergently evolve so many times. In order to address these assumptions, one must first review two important components of these reproductive patterns: matrotrophy and lecithotrophy. Matrotrophy and lecithotrophy are both means by which embryos receive nutrients during development. Matrotrophy is the form of nutrient supply in which the mother supplies most or all of the nutrients to the embryo throughout gestation. For instance, with placental animals, the mother transfers nutrients and oxygen from her body to the developing offspring through the placenta via active and passive transport. By contrast, lecithotrophy is the term used to define nutrients supplied to the embryo through the yolk of the ovum. Since viviparity and matrotrophy are so frequently observed together, the terms are often confused. It is important to note that it would be a mistake to assume viviparity and matrotrophy are synonymous, and it is also a mistake to assume viviparity entails matrotrophy. While it is true that they are typically used in conjunction with one another, not every single viviparous animal is matrotrophic, and accordingly not all oviparous animals are lecithotrophic. For instance, many squamate reptiles are viviparous and yet still lecithotrophic (the eggs are retained within the mother, with the yolk as the main source of nutrients); in the same way, monotremes (platypus and echidna) are oviparous and also exhibit matrotrophy. Now that those terms are defined, the evolutionary hypotheses can be discussed. It is imperative to look at both “how” and “why” viviparity evolved in order to more completely understand the topic, starting by asking: how? In general, viviparity and matrotrophy are believed to have evolved from an ancestral condition of oviparity and lecithotrophy. One traditional hypothesis as to the sequence of evolutionary steps leading to viviparity is a linear model that can be restated rather simply. Initially, just an increase in the length of time that the egg remained in the reproductive tract of the mother may have gradually allowed for the evolution of egg retention, provided that fertilization was internal (the reason for retaining eggs for extended periods will be addressed later in the essay). Through continued generations of egg retention, viviparous lecithotrophy may have gradually appeared; in other words, the entire development of the embryo (with nutrients provided by the yolk) occurred inside the mother’s reproductive tract, after which she would give birth to the hatched young. As described by Blackburn, “according to the scenario, the next evolutionary stage is incipient matrotrophy; although nutrition is still relatively lecithotrophic, yolk supplies are supplemented by small quantities of nutrients from the reproductive tract of the viviparous female” (Blackburn, 1992). Over time, more and more nutrients supplied by means other than the yolk would give rise to the more concise form of matrotrophy we often see today. While this hypothesis is the simplest to follow, it is still debatably not the most parsimonious. The reason being that there has been no substantial evidence found of an animal exhibiting both strict lecithotrophy while also being viviparous; at least, not completely. Viviparous squamates do tend to be heavily reliant on lecithotrophy, but none have been found to use strict lecithotrophy exclusively (Stewart, et al, 1990). Other suggestions as to the origins of viviparity include adjustments to the traditional, linear scheme. For one, Blackburn suggests that, at least in squamate reptiles (representing the largest number of independent origins by far), viviparity and matrotrophy could have evolved simultaneously as opposed to being distinct modifications. Secondly, instead of assuming gradualism, viviparity might have evolved following a punctuated equilibria model, perhaps in conjunction with some sudden environmental, climatic or other factors (Blackburn, 1989). As a third possible adjustment, taking into account the prospect of oviparous matrotrophy of monotremes being the primitive form of reproduction in mammals (meaning that matrotrophy may have arisen prior to viviparity), it would then have to be true that the combination of viviparous matrotrophy seen in all other mammal species was a subsequent modification. Basically, there is still a lot of debate in this area. One must keep in mind that it is more than likely the case that no one setup can completely account for every evolution of viviparity that has occurred. Now that the “how” has been addressed, it is time to put together a picture of “why” viviparity evolved in the first place. As previously mentioned, there are physical and energetic strains to carrying young through gestation that make viviparity seem like an acute disadvantage to the mother. This is especially true in the case of viviparous matrotrophy, where the mother must sacrifice larger amounts of her own nutrition for a longer period of time, requiring more nutrient intake, while at the same time being heavier and partially less mobile avoiding predators or catching prey. The heaviness and poor mobility might explain why birds are the only amniotes with no living viviparous species (Dunbrack, and Ramsay, 1989). Unlike in reptiles, perhaps with birds, egg-retention was always selected against due to the nature of avian flight. However, while these disadvantages might often occur, there are certainly a range of reproductive advantages to the viviparous strategy that may be selectively favored in some environments. For example, the disadvantage of decreased mobility while internally carrying young may be balanced out by the reproductive advantage of more protected young, due to the increased safety of the embryos. Essentially, the mother slightly decreases her own survival to increase the survival of her offspring. In oviparous animals, exposed eggs are often found by a predator and eaten, or could be destroyed in one fell swoop by any number of means. However, when they are retained within the mobile mother, they are safe as long as she is. This is an example of something that might be selected for in a heavily scavenged or predated area, or an area where eggs may be easily destroyed by natural occurrences (e.g. flooding), or as discussed in the following paragraph, in areas with unfavorable temperatures. One of the most profoundly advantageous features of viviparity is thermoregulation of the embryo (Blackburn, 1999). Since the developing offspring remains within the mother’s body, she becomes, in essence, a walking incubator. This is very useful when climate change imposes cooler average temperatures on a current habitat, or when a migration event necessitates adaptation to a new, cooler environment. With oviparous animals in the same afore mentioned scenarios, the exposed, externally developing eggs may succumb to adverse temperatures. If this is the case, it is probable that when temperatures are consistently lower than ideal for embryonic development, longer egg retention could be selected for in the population. When considering squamates in particular, there is a correlation between high altitudes or latitudes, colder climates and the frequency of viviparity. This tendency for egg-retention, and consequently viviparity, to be selectively favored under cooler conditions due to its thermoregulatory benefits is termed “the cold climate hypothesis” (Lambert, 2013). Since viviparity affords the advantage in cooler temperatures, it was predicted that one would find a large quantity of the viviparous squamates occurring under these conditions, which happens to be the case (Blackburn, 1999). The fact that viviparous reptiles are frequently found in higher altitudes seems to corroborate this hypothesis. Furthermore, in an experiment monitoring the thermal biology and embryonic development in the oviparous Zootoca vivipara at both of its environment’s altitude extremes, the results indicated that “temperature is an environmental factor affecting the geographical distribution of different levels of egg retention in Z. vivipara, as predicted by the cold-climate hypothesis on the evolution of viviparity” (Rodriguez-Diaz and Brana, 2012). In conclusion, while the exact method by which viviparity evolves is still an actively studied area of research, there are aspects of its evolution that can be consistently asserted. Most notably, the safety of traveling with the mother, the thermoregulatory benefits it provides to developing embryos, and the correlation demonstrated by the cold climate hypothesis. Not only that, but viviparity (being far more widespread than the classic examples of convergent evolution, such as the evolution of flight), is certainly one of the most extraordinary cases of convergent evolution to be studied in the history of evolutionary biology.

References


 * Blackburn, D. G. (1989). A saltatory model for the evolutionary origin of viviparity and placentotro-phy in reptiles. Amer. Zool. 29:133A.
 * Blackburn, D. G. (1992), Convergent evolution of viviparity, matrotrophy, and specializations for fetal nutrition in reptiles and other vertebrates. Amer. Zool., 32:313-321
 * Blackburn, D. G. (1999). Viviparity and oviparity: evolution and reproductive strategies. Encyclopedia of Reproduction. Academic Press, New York, New York, USA, 994-1003.
 * Blackburn, Daniel G. (2005). Amniote perspectives on the evolutionary origins of viviparity and placentation. Viviparous Fishes, p 301- 322.
 * Dunbrack, Robert L. and Ramsay, Malcolm A (1989). The Evolution of Viviparity in Amniote Vertebrates: Egg Retention Versus Egg Size Reduction. The American Naturalist, Vol. 133, No. 1 (Jan., 1989), pp. 138-148
 * Lambert, S. M. and Wiens, J. J. (2013), Evolution of viviparity: a phylogenetic test of the cold-climate hypothesis in phrynosomatid lizards. Evolution, 67: 2614–2630. doi: 10.1111/evo.12130
 * Rodriguez-Diaz, T. and Brana, F. (2012). Altitudinal variation in egg retention and rates of embryonic development in oviparous Zootoca vivipara fits predictions from the cold-climate model on the evolution of viviparity. J. Evol. Biol. 25: 1877–1887
 * Stewart, J. R., D. G. Blackburn, D. C. Baxter, and L. H. Hoffman. (1990). Nutritional provision to embryos in a predominantly lecithotrophic placental reptile, Thamnophis ordinoides (Squamata: Serpentes). Physiol. Zool. 63:722-734.