group+3

Name of group: **Polyploidy**  Group secretary plus email: **Wilson Wong** (bt09323@qmul.ac.uk)  Group members plus email: **Yasmeen Muhammad** ** < ** bt09304@qmul.ac.uk ** >, ** ** Gizzy Starr ** , **Patrycja Wawrzyniecka** ** < ** bt09415@qmul.ac.uk ** >, ** ** Edward Wilding ** 

__Task__ : Describe the advantages and disadvantages of polyploidy, 1000 words maximum, referenced.

__Definition of Polyploidy__ : An organism is considered to exhibit polyploidy if it has more than two complete sets of chromosomes.

__** Advantages **__

Inbreeding does not reduce heterozygosity as rapidly in polyploids as in diploids. That is because in diploids we can have only three genotypes: AA, Aa or aa; out of the three genotypes two are homozygous. ....NO can get more than two alleles for any gene in a population, and can gene duplication without polyploidy. The ratio of homozygosity to heterozygosity is 2:1. In polyploids, for example tetraploids, the possible genotypes are: AAAA, AAAa, AAaa, Aaaa, aaaa; out of the 5 genotypes, only two are homozygous. The ratio of homozygosity to heterozygosity is therefore 2:3. Hence, in diploid population the homozygosity will be reached sooner due to smaller genotypic diversity.

This is an advantage because in case the population size is suddenly decreased (due to a catastrophe, introduction of a new species to the habitat) it has **better chances of survival** than a diploid population of the same size, and that is because polyploids can survive longer in the small populations as inbreeding does not produce deleterious homozygotes as quickly.

Another implication of reduced homozygosity is that **inbreeding depression is** **more limited** in polyploids in comparison to diploids. Inbreeding depression is caused by recessive alleles being expressed in the phenotype. This happens less in polyploids because it is less likely that an offspring exhibits a recessive phenotype, as it would have to inherit, for example, 4 recessive alleles in the case of a tetraploid from its parents, rather than 2 in a diploid.

Also, polyploidy contributes to genetic variation, as it **increases the chance of** **new genetic mutations**. For example, if an organism has a gene that encodes for ultraviolet vision and the organism undergoes polyploidy, it will then have two copies of that gene. There is a chance that the second copy of the gene is mutated to allow infra red detection. The organism can now detect both ultraviolet and infra red light as it has retained the original copy of the gene in addition to the new mutated copy. ..... as I said last time this example is not due to polyploidy, but gene duplication in a diploid

Genetic variation lead to variable fitness in offspring, polyploids exhibit many advantageous features in comparison to the diploid counterpart. This is known as **hybrid vigour**, where beneficial traits are amplified by the increased number of chromosomes. ..... no this is caused by masking of deleterious alleles. This means that polyploidy plants are often larger, and more robust. These traits are advantageous in the food production. What is more, polyploids usually show greater adaptability which enables them to survive in harsher environment. This are the characteristics that made plant breeders interested in polyploids and artificial promotion of polyploidy by the use of drugs, like colchicine (Brooker 2004). An example of hybrid vigour is found in potatoes, where we have inbred the potato to be larger and have a higher yield and resistance to disease compared to its wild type diploid counterpart. When it comes to agriculture, sterility might be considered a desirable trait. And that is because sterility makes fruits seedless. Seedless bananas and watermelons are in fact triploids (Taylor, Green and Stout 2006).


 * __ Disadvantages __**

**High chance of death** – even though many plants are polyploids, in animals it is not as common. The increased number of chromosomes in polyploids makes normal gamete formation during meiosis much more prone to error. Polyploidy in mammals is generally a lethal condition.

Polyploid organisms also have **difficulty in cell divison and production of germ** **cells**, the principal problem is that cell division mechanisms are designed for only one pair of homologous chromosomes. When more than one pair of chromosomes are involved several problems arise. A failure to segregate properly, or another outcome could be that whole chromosomes do not become involved in the cell division, and simply finish in a daughter cell depending on where they were in the parent cell's nucleus. These cell divisions in polyploids often result in aneuploid daughter cells, and an epigenetic instability (Comai, 2006). Another problem in mitosis in polyploids is that the spindle fibres often become chaotic because of the increased chromosome number, this could mean a multipolar spindle forms, which will fail to segregate the chromosomes properly, leading to aneuploidy in the daughter cells. ..... where does this come from?

Some polyploids are **sterile due to irregular pairing of chromosomes** (triploids). However, it is possible for sterile hybrids to undergo polyploidization to become fertile, which can lead to the production of new species as observed in the case of Primila Kewensis ..... //Primula kewensis// (Comai, 2005).

The cell volume is increased greatly as there is more genetic material in polyploids. This causes nuclear enlargement and **increases the complexity** that is involved in managing and partitioning chromosomes during cell division.

**Mutation selection equilibrium frequencies** for recessive higher in polyploids than in diploids (similar for partial recessive) ..... I am unsure what this means as I said before

__** An advantage or a disadvantage ? **__

There are **higher chances of mutations** occurring in polyploids than in diploids. As there is more genetic material to duplicate, the chance of mistakes being made is greater. Mutations are usually harmful and can be fatal, however, there are chances of advantageous mutations occurring.

**Instant speciation** one may argue whether it is an advantage or disadvantage ..... or neither, just happens. Normally, it takes millions of years for a new species to evolve. However, due to polyploidy a new species may arise instantly. The very next generation may already show reproductive isolation from their parents’ species. It might be considered an advantage because new species arise straight away and improve the biodiversity of the planet. On the other hand, a species appearing in a no-time will surely influence other organisms in the habitat. If a species evolves over millions of years, other organisms have time to develop co-existence with it, but if a species arises instantly and, in addition, is aggressive (ex. Spartina anglica) it might suppress other species in the habitat.

__// References: //__ 1. Comai, L., 2005. The advantages and disadvantages of being polyploid, //Nature// [electronic], vol. 6, p. 836-846. 2. Brooker, R. J., 2004. //Genetics. Analysis & Principles//. 2nd ed. p. 210-215. 3. Taylor, D. J. Green, N. P. O. Stout, G.W., 2006. //Biological Science 2//. 3rd ed. p. 830-832. 4. Young, A. G. Clarke, G. M., 2000. //Genetics, Demography and Viability of Fragmented Populations//. p. 33-37, 238-239. 5. Frankham, R. Ballou, J. D. Briscoe, D. A., 2002. //Introduction to Conservation Genetics//. p. 60, 88-90, 373-374.