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Polyploidy has played an integral role in the evolutionary history of plants, as well as vertebrates and other eukaryotes. Polyploidy is the condition where organisms that undergo meiosis have more than the usual two complete sets of chromosomes (Comai, 2005). Generally the polyploid organism has an even set of chromosomes, for example four is the most common number of chromosome sets and this is referred to as tetraploidy (Comai, 2005). Polyploidy occurs through several means including genomic doubling, gametic non-reduction and polyspermy and results in a gamete having more than one set of chromosomes (diploid) (Otto & Whitton, 2000). These diploid gametes can fuse with haploid ones producing a triploid zygote which are unstable and in some species sterile, or can contribute to further polyploid gametes in others (Comai, 2005). If two diploid gametes fuse it produces a tetraploid zygote, these zygotes are more stable than their triploid counterparts (Comai, 2005). The route to polyploidy is reliant on the species in question, as different routes are common in different species. Types of polyploids include autopolyploids which are created by chromosome duplication within a species and allopolyploid which are polyploids created by hybridization between different species (Wendel, 2000). As polyploidy generates duplicated genes it encourages diversification and can have evolutionary benefits to the polyploidy species. Its occurrence is widespread in plant species with an estimated frequency of 30% to 80%, and occurs sporadically among eukaryotes (Otto & Whitton, 2000). Polyploidy occurs in both natural environments and as well as in agriculture settings, as it assists in introducing new genes to plants grown by humans. As polyploidy plays such a major role in the evolution of plants and eukaryotes it is important to understand the effects of polyploidy and why it occurs. This article will discuss polyploidy introducing new genes, polyploidy occurrence in Arctic plants and will end with conclusions about polyploidy's role in evolution in the past and predictions of the future.
Polyploidy introducing new genes
Polyploidy's advantages in evolution have been known for some time. In 1951 S.G. Stephens made the statement "one might expect that a mechanism which new functions could be added and the old ones retained would have a considerable selective advantage" (Stephens, 1951). As polyploidy creates an entire set of chromosomes (sometimes many entire sets of chromosomes) in an individual, the genome of that individual is increased. Simply put, polyploidy occurring in a species can lead to its evolution. Normally for a new species to evolve it must be spatially separated, and generally occurs over a long time span (many generation) (Aegerter, n.d.). Evolution via polyploidy is different; it occurs immediately (Adams, 2007), and there is no need for spatial separation within the species (Aegerter, n.d.). Evolution in polyploid species occurs because the duplicated genes are open for selection, mutation and gene evolution (Adams, 2007). The divergences that occur from duplication are apparent in a myriad of ways, including novel phenotypes, greater physiological and biochemical flexibility and superior environmental adaptability (including new niche invasion) (Wendel, 2000). The way in which polyploidy contributes to genome evolution is through chromosomal rearrangement, interlocus concerted evolution of ribosomal DNA repeats, unequal rates of sequences and changes in DNA methylation (Adams, 2007). Polyploid species whose genes have undergone divergences and have therefore undergone evolution are more successful than original diploid species they evolved from (Adams, 2007).
Polyploidy does not only encourage evolution through the divergence of genes but also through gene silencing (Wendel, 2000). Gene silencing is the "switching off of genes" by the cell. As some genes are not in use due to gene silencing the gene expression of the individual differs from its ancestors (Wendel, 2000). An advantage of gene silencing is its ability to diversify gene function over time, this means that the extra copies of genes that are not integral for general function of an individual may end up being used in new different ways (Adams & Wendel, 2005). This process can lead to novel opportunities in evolutionary selection (Adams & Wendel, 2005). Gene silencing can be advantageous given it has the potential to silence a "bad gene"; the gene would still be carried but not exhibited, therefore it can be selected upon in later generations if environmental conditions change. Another advantage of gene silencing is that the polyploid individuals will be protected from the harmful effects of recessive mutations; this can occur because the polyploid has double (or more) copies of any particular gene (Adams & Wendel, 2005). Gene silencing which accompanies polyploidy leads to the loss of duplicated gene expression, as this changes the gene expression of the individual affected it has caused evolution (Wendel, 2000).
The evolutionary success of polyploid organisms is due to the extra set or sets of chromosomes that they have acquired. The extra set of chromosomes encourages processes such as genome evolution and gene silencing to occur through various means. Evolution of most plant species and a variety of animal ones is due to polyploidy occurrence, and it is difficult to understate polyploidy importance in evolutionary history (Wendel, 2000).
Occurrence of polyploidy in Arctic plants
Research has been vast relating to polyploidy occurrence in the Arctic. The Arctic is today known to be one of the most polyploid-rich areas in the world (Brochmann et al., 2004).The numerous polyploid plants which have developed over the last 2-3 million years make the arctic an idyllic place to study the consequences of resent gene duplications and their expressions, the method and incidence of formation of polyploids and the evolutionary importance of polyploidy (Brochmann et al., 2004).The frequency of polyploids increases with latitude in the Northern hemisphere; the reason as to why this occurs has been debated over many decades and many theories have been put forward (Brochmann et al., 2004). Some scientists have suggested frequency of polyploids increases as latitude does (in Northern hemisphere) because polyploids have a higher resistance to the unforgiving Arctic climate than their diploid counterparts (Brochmann et al., 2004). Another theory brought forward by G.L. Stebbins suggested that there is a correlation between polyploid frequencies and degree of glaciations, rather than with polyploid frequencies and latitude (Brochmann et al., 2004). Although the theories as to why polyploid species are so common in the Arctic differ, the fact remains that polyploids in the arctic are evolutionary successful.
As polyploidy in the Arctic is so pronounced many examples are available for review on the evolution of polyploid species. One such example is the Arctic polyploids of Saxifraga (Saxifragaceae), where the same direct ancestors have hybridized to produce diverse polyploid species (Soltis et al., 2003). The two allopolyploids Saxifraga opdalensis and Saxifraga svalbardensis are offspring of the two diploid species Saxifraga cernua and Saxifraga rivularis and are genetically and morphologically distinct (Soltis et al., 2003).
Svalbard is an archipelago in the Arctic Ocean, and because it contains a high frequency of polyploid species (78.3%) it makes the area a model place to study the genetic consequences of polyploidy (Brochmann et al., 2004). Many of the polyploid species residing in Svalbard have high ploidal levels, with the average ploidal level of all species (inclusive of diploids) being near to hexaploid (Brochmann et al., 2004). This means that many of the polyploid species have six chromosome sets and some even more. The ploidal level strongly links with the level of heterozygous (as a positive correlation) (Brochmann et al., 2004). The polyploid species in Svalbard are mostly fixed heterozyogtes meaning they are genetically allopolyploid with normal chromosome inheritance; the few present diploid species are highly homozygous this is due to recurrent self fertilization (Brochmann et al., 2004). In polyploid species much of the genetic variation is found as fixed heterozygosity and as deviation amid different populations (Brochmann et al., 2004). It has been suggested that elevated levels of heterozygosity have been a driving force for polyploids to expand into habitats that their direct ancestors were unable to inhabit (Brochmann et al., 2004). Studies made on the genus Draba which contains arctic ployploids and diploids suggest that there is a connection between the evolutionary successes of arctic polyploids, high heterozygosity and ecological amplitude ((Stedje & Borgen, 1992)). It was found that as ploidy level increased so did their levels of heterozygosity and ecological amplitude ((Stedje & Borgen, 1992)). This study as well as evidence incurred in the past about the numerous origins of polyploids and gene flow through various ploidy levels led to the hypothesis that, allopolyploidy may serve as an escape from genetic and ecological depauperation caused by drift and uniparental inbreeding at the diploid level ((Stedje & Borgen, 1992)). This means that polyploidy in the form of allopolyploidy allows for evolutionary progress to occur in a species which may be stunted in its growth or development caused by regular inbreeding.
Comparatively diploids and tetraploids occur in similar frequencies in boreal (subarctic) and arctic areas, whereas cases of higher ploidy level increases significantly in the arctic region compared to boreal areas (Soltis et al., 2003). The relationship between ploidy level and the Arctic is obvious and the complexity of much of the Artic flora can be accredited to polyploidy (Soltis et al., 2003).
Conclusion and future prospects
In this review I have attempted to summarise our basic understandings of the role of polyploidy in evolution. Using the Arctic polyploid system as a model I used the example of the Saxifraga to illustrate how speciation can occur via polyploidy. Of course polyploidy is not limited to just the Arctic, it has occurred all over the world in various frequencies. It has been established that polyploidy induces evolution; the question is, is polyploidys role in evolution advantageous? One view is the process of polyploidy disrupts regulatory patterns that were previously developed by selection (Comai, 2005). Another way to look at the effects of polyploidy is that it contributes to the adaptive potential of polyploids by increasing diversity and heterosis (Comai, 2005). Any speciation can be looked upon as being a positive process as evolution has shaped the world to be the diverse place it is today. Speciation via polyploidy can lead to new niches being occupied and also species extending their ranges, this only increases their chances of survival. For the advantages of speciation which are conferred by polyploidy to be felt, a fertility barrier must be conquered (Comai, 2005). Polyploids of a genus may have exceedingly variable phenotypes but if they have poor fertility the benefits of polyploidy may never be felt. Further research into what causes sterility at the molecular basis may be helpful for additional insight into polyploidy occurrence. Our understanding of polyploidy is not yet complete; which adaptations might assist the change from diploid to polyploid, the various regulatory consequences of allopolyploidy versus autopolyploid and the effects of an abnormal number of chromosomes (aneuploidy) on polyploids are all unclear and additional study on these subjects is needed (Comai, 2005). What is clear is that polyploidy is an important evolutionary process that leads to speciation. If not for polyploidy plant species may not be as developed as they are today and the natural world may be a very different. Future polyploidy events in plants and eukaryotes will hopefully lead to even more diversification and will benefit new species developed through the process.