BIOGEOGRAPHY paper modification

Introduction

There is a striking genetic differences between plants and animals, and one that is highly striking is the fact that plant show more cases of polyploidy than animals. Polyploids are defined simply as individual organisms where there are more than double sets of chromosomes. This can be represented as 3n, 4n, 5n and so forth. Around 30% to 50% of the angiosperms are polyploidy yet there are few polyploidy series that are found in animals (Margulis et al., 2009) Individuals that are polyploidy in nature are genetically isolated from their ancestors who in most cases are non polyploidy. What most intriguing is why up to date we do not have one explanation that can effectively explain why polyploidy is rare among most animals. There are around three flawed explanations that have been given as the reason for such a situation.

It is generally accepted knowledge that polyploidy plays an important role in plants, but its effect is less modest when it comes to animal kingdom. The significance of polyploidy in animals is heavily underestimated. The interest of polyploidy towards animals is inclined towards two important elements of areas of concern. The first area of concern is the role of polyploidization in autogenesis in animals; this is particularly significant when it comes to tissue and cell differentiation. This does take place during the process of embryogenesis and the process of pathological development. Such differentiation does take place during processes such as growth of cancerous cells and it is generally referred to as malignant differentiation

The second key area is the evolutionary importance of animal polyploidy for their phylogenesis. It does play a role in specification which is the possibility that polyploidization will take place under different modes of reproduction in animals, and example with is the most common reproduction that is bisexual, hermaphroditism, vegetative and asexual reproduction.

There are experimental studies which have been carried out in bid to determine how polyploidy can be of importance to animals. Majority of the work is centered on the second element or area of concern where there could be some animals which have bisexual characteristics due to the effect of polyploidy and no significance research has been done to determine how such strain can affect the animal species that are existent.

Literature Review

The first argument that has been provided state that animal development is highly and easily perturbed by changes in the genotype which are tetraploidy and diploidy. This is different when it comes to development in plants (Orr, 1990). Polyploidy is rare among animals due to the fact that animal development is a concept that is very complex and closed and cannot easily tolerate any dramatic change in genotype. This argument is seen as quite simple and to the point, but it has received enough criticism in the sense that there is a finding which state that hermaphroditic and parthenogenetic animals do show some elements of polyploidy

The second and most widely accepted explanations was provided by Muller (1999) where he argued that tetraploidy is not easily established in animals due to its ability to disrupt sexual determination among the animals. Muller used the model of a Drosophila, the species living in most part of the world, but less in Canada, Russia, northern Africa and Costal area of China, to illustrate that any cross of tetraploid animals with a non diploid fly, appears in coastal area of China, would lead to intersexes that are sterile, supermales and superfemales. But it was also determined that this would lead to creation of a fertile tetraploids. It is therefore illustrative that sex determination is the reason that prevents the shift from diploidy into tetraploidy.

The above explanation by Muller has received its share of criticism in the sense that it has two significant problems. The first one is the fact that his explanation only seems to be in line with animals that show sex determination that is Drosophila related. In Drosophila it is known that sex determination is influenced by ratio of the X chromosomes to the autosomes. The Y chromosomes in Drosophila have no or little effect in the gender determination.

The third explanation that has been offered states that plants are highly tolerant compared to animals when it comes to major alterations on the structure of the gene (Orr, 1990). Polyploidy is not the only change in genetic structure in which plants are more tolerant to, there are also cases of translocation, duplication and aneuploidy. In plants new species can easily arise due to changes in chromosomal changes that are hereditary (Orr, 1990). This is the reason as to why agriculture has decided to exploit the genetic change ease in plants to come up with plant species that are very important examples of which include cotton, wheat and tobacco. They are all products of hybrid polyploidy crops. The evolution did take place due to conscious efforts by early agricultural experts who aimed at incorporating characteristics that are desirable from the wild species.

The hybrid that initially comes out from an interspecies crossing is usually very infertile due to the fact that chromosomes that are inter specific are not homologous during the prophase I in meiosis. In any case the hybrids can be polyploidy; pairing from the homologous can take place effectively well. In most cases, tetraploid and hexaploid descendants are usually fertile. It means that they do possess traits that are desired from two or even more progenitor species. This is what Muller was able to establish when he tried to study the Drosophila species. Polyploidy does render plants more vigorous and large and can multiply the attractive body parts of the plant; this is what is done by most florists when they bread roses of different attractive colors.

In animals, unlike plants, polyploidy is rare and is only found in certain species of fishes (salmon, found in northern hemisphere), insects(drosophila) and amphibians (Salamander, found in most places of Europe, north America, exclude extreme cold area in Canada, Russia and part of China). It has taken several years until recently when polyploid mammal was discovered. There was also a case of tetraploid rat which was discovered in Argentina. Cells of polyploid nature are larger compared to diploid ones. This is the reason as to why there is increased amount of DNA located in its nucleus. It has been determined that the liver cell found in the Argentina rat is larger than that found in its relatives that are diploid. It sperm has also been determined to be huge in comparison. In a normal mammal, including normal rats that could be found in most part of the world, the sperm head is known to contain 3.3 picograms of DNA and sperm of a rat contains around 9.2 pictograms.

Polyploidy is a phenomenon that is usually limited to certain tissues in animals such as the liver as has been shown the case of the rat above. It is also evident in the megakaryocytes through which the platelets are made. The megakaryocytes in most cases go through several S phases which end up producing one giant cell that has a single nucleus that contains around chromosome 128n (Margulis et al., 2009).

It is the fragmentation of the chromosomes that leads to the creation of the platelets. In the placenta there is also case of giant trophoblast in the placenta. From the three areas of the animal cells, it has shown that certain cells in animals can manage to withstand the effect of polyploidy and not the entire animal during the process of fertilization.

According to Aegerter (2010) polyploid animals are less common compared to plants; however, there is a belief that polyploidy could have been very important when it comes to the evolution of certain lower vertebrates. There are certain polyploidy species among the chickens, lizards, salamanders, frogs and fish. Thorgaard (2011) does state that there are several reasons which explain the reason as to why this is not common among animals. Animals do have sex chromosomes yet plants do not have. In order to for a fertilized egg to exist, there must be a proper ratio. At times there are more genes which are shown from either of the parental sex chromosomes. It therefore means a proper ratio of chromosomes originating from each sex will be very crucial. One fact that supports Thorgaad’s idea is the observation which states that animal species which are polyploidy usually will have chromosomes which are highly similar than for those animals that do not.

There is also an argument which states that the difference in the manner of growth in plants and animals can also have a role when it comes to explaining why polyploidy is common in plants and not in animals. Plants do have a growth pattern that is very open, which is different to animals which have more of a deterministic growth pattern. Animals do have multitudes of systems that are very complex. The nervous and the skeletal system must always be integrated. When there is an extra gene from any extra chromosome there will be an imbalance when it comes to the actual integration.

Generally, researchers contend that same mistakes that take place in plants do take place in animals leading to sperms and eggs that have the potential to create polyploidy, the only difference is that in animals they don’t survive. This is attributed to failure in integration of the complex skeletal and nervous system composition in animals. The cells do not survive only because they usually cannot manage to integrate when one important chromosome is challenged.

Organisms that are polyploidy have two sets or more of chromosomes. This is inclined to plants where up to 70% are polyploidy. This does occur due to failure in spindle fibers to disjunction. This does give rise to double production of the chromosome which is complement of the cells gamete. A plant that does undergo non disjunction to completeness will have gamete that is diploid. In any case this self-fertilize there will be a tetraploid plant. This is a situation that could actually take place in animals, but the resultant diploid gamete will never survive to fertilize the tetraploid plant.

Research in animal evolution has indicated that changes in animals usually takes several years or several generations where one group of individual from a species are detached and gain genetic distinction from those other one whom they separated from several decades later. Evolution is therefore studied on organisms that are considered normal, and plants are not considered as such due to the fact that their changes can take place without actual long time separation from one group of a species. When animals stay apart and become distinct enough, they become members of new species. Plants do live on an open form of life, this could be the reason as to why the end up having various age differences after a short period of time when the diploid chromosome survives and fertilize another to form a diploid.

All plants species which have managed to evolve in more than one instance have done so through the polyploidy process. There are fewer animals that have evolved in more than once through polyploidy, and one such animal exception is the Salmon (Thorgaard, 2011). Evolution through polyploidy is different from normal evolution since it is considered to be instant. The mistake in the egg or pollen of the plant can lead to many copies of the chromosome, and when it unites with the opposite, the outcome is a plant that has an extra chromosome.

For animals to evolve more than one time, it will take millions and millions of years, plant do not have such longevity and therefore can evolve more than once through the genetic mistake that is polyploidy (Thorgaard, 2011). Most of the polyploids that have been examined and researched on have shown that they have managed to evolve more than once. This is a concept that is unique to plants, as generational changes among animals have to separate one from the original ancestor for millions of years before they are able to turn into something different. This in most cases does not take place easily since animal species do live a much closed life where one member of a species is not easily separated from the others.

According to Margulis et al., (2009) polyploidy among plants can happen from individuals from two species that are distinct which is known as allopolyploidy or it can as well take place between individuals from individuals in the same species, a concept that is generally known as autopolyploidy (Thorgaard, 2011). Autopolyploidy is considered very rare as well as maladaptive; this is so because it was thought that autopolyploidy would create a high proportion of pollen or eggs that are extremely sterile.

Conclusion

Over the years, research has indicated that there is high level of polyploidy in plants than in animals. This has been placed at around 70% of all animals. This is a biological phenomenon that is rare among animals and it is only found to be common among lower vertebrates. The reason which has been given as an explanation does ring around the fact that most animals are difficult to adapt to abrupt changes in their genetic composition. When such changes do occur, the animals rarely survive.

Another reason which has been provided to explain the disparity is the fact that most of the animals relies on the chromosome to determine their gender. When such chromosomes are mixed up, the end result is sterile offspring which cannot reproduce to continue a generation of the new species. This is not the case when it comes to plants since they do not rely on the chromosome to determine the gender of the plant that comes out of it.

Plants, unlike animals do not have a complex skeletal system similar to that in animals. Whenever there is a genetic mistake that leads to polyploidy in animals, the resultant outcome does not survive in most cases. This is due to failure to integrate the complex nervous systems that are existent in animals. This is not the case with plants, and therefore they mostly survive. Cases of hermaphroditism in animals does indicate the difficult nature of animal genetic structure which cannot be compatible with other any other chromosome which arises from a mixed up sexual activity. When such offspring are generated, they are not in a position to effectively mate with any offspring of their kind and be able to come up with a completely new species.

References

Aegeter, W. H. (2010). Polyploidy: Biological relevance. New York: Plenum Press.

Margulis, L., Schwartz, K. V., & Margulis, L. (2009). Kingdoms and domains: Illustrated phyla of life. London: Academic.

Muller, H. (1999). The American naturalist. Chicago, IL: Published for the American Society of Naturalists by the University of Chicago Press.

Orr, A. (1990). Why is Polyploidy Rare in Animals than in Plants: Revisited. American Naturalist, 136(6). Retrieved from http://www.jstor.org/stable/2462166?seq=1#page_scan_tab_contents

Thorgaard, G. H. (2011). Chromosome rearrangements and sex chromosomes in the rainbow trout and sockeye salmon.