e-Learning portal for Arctic Biology

1.2.3 Diversification of arctic flora and fauna during the Pleistocene

On an evolutionary time-scale, the arctic flora and fauna have had very short time to develop. This partly explains the low number of truly endemic arctic species compared to lower latitude biomes. The Arctic has even been described as an ”evolutionary freezer”, because besides being a young biome, the low temperatures and the short vegetation period negatively impact genetic divergence and speciation (Rohde, 1992, Allen et al., 2006, Wright et al., 2006, Allen et al., 2007, Wright et al., 2011). Metabolic processes are usually slower when it is cold, leading to lower mutation rate and less genetic diversity for evolution to act upon. Further, short growing season often give prolonged generation time.

Although there is an overall trend towards less richness and diversity when moving northwards, this trend vary among taxonomic groups; some genera contain species that have remained almost unchanged the last 3 million years, while others show amazingly diversification. How can some groups have diversified so fast? Acquisition of novel genetic diversity and speciation are not always dependent on slow mechanisms like mutation rate and natural selection; processes like hybridisation and polyploidisation can be efficient short-cuts.

Range-cycles induced by glaciation cycles have played a key role in promoting hybrid- and polyploid speciation (Box polyploidy), as seen in many arctic plants and some invertebrate complexes (e.g. Daphnia). The number of polyploidy plant species (> 4x) are significantly higher in glaciated areas (Brochmann et al., 2004). One explanation is the secondary contact model (Stebbins, 1984). The recurrent retreats to isolated glacial refugia resulted in reduced gene flow, leading to divergence. This divergence was further enhanced through successive founder events during postglacial range expansions (Hewitt, 1996, Ibrahim et al., 1996, Hewitt, 1999, Hewitt, 2004). Upon glacial retreat, when former fragmented and now differentiated populations came into contact again, they would occasionally give origin to hybrids that were then stabilized by polyploidization. Further to this, it has been shown that environmental stress, such as extreme climatic conditions, will enhance the production of unreduced gametes, which is usually a prerequisite for polyploidisation to occur.

The recurrent retreats to isolated glacial refugia leading to divergence. Upon glacial retreat, when former fragmented and now differentiated populations came into contact again, they would occasionally give origin to hybrids that were then stabilized by polyploidization

Polyploidy, i.e. whole-genome duplication, is a main driver of plant evolution (Otto, 2007; Soltis & Soltis, 2009; Soltis et al., 2014), and thought to have occurred at least once during the evolutionary history of all angiosperms (Jiao et al., 2011). Suggested advantages with polyploidy are for instance increased number of alleles per loci, as this should allow masking of deleterious recessive mutations and thus insure against the loss of fitness (Gu et al., 2003). Further, the duplicated gene copies can evolve into new or slightly changed functions, potentially allowing for ecological niche expansion or increased flexibility in how the organism can react to environmental change (Adams & Wendel, 2005; Moore & Purugganan, 2005).

In order to understand how polyploidy arise; you have to remember what is happening during cell division, in particular meiosis – the type of cell division giving rise to sexual gametes. A diploid cell contains two sets of chromosomes (2n). During meiosis, a diploid cell usually gives rise to four haploid cells (gametes) with only one set of chromosomes (n) each. However, sometimes errors occur, and the number of chromosomes is not reduced, leading to the formation of unreduced gametes. Meiotic errors and the formation of unreduced gametes are assumed to be the driving force in the formation of polyploids.

Two types of polyploidy are generally recognised: allopolyploidy and autopolyploidy; where autopolyploids arise from a single species, and allopolyploids are the results of hybridisation between two different species, followed by chromosome doubling (Stebbins, 1947; Ramsey & Schemske, 1998; Levin, 2002).

Allopolyploidy generally results in instantaneous speciation because any backcrossing to the diploid parents produces a high proportion of unviable or sterile triploid offspring.

One example of autopolyploidysation Autopolyploid through unreduced gametes and self-fertilization.

Autopolyploidy has received comparably little attention, but is more common than traditionally assumed (Soltis et al., 2007; Parisod et al., 2010; Soltis et al., 2014). Their occurrence is often overlooked, as many autopolyploids often lack morphological distinction from its parental diploid species. This may have led to a vast underestimation of species numbers (Soltis et al., 2007), and obscuring research results based on the assumption that a species ploidy level is uniform (Eidesen et al., 2013).

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Brochmann C, Brysting AK, Alsos IG, Borgen L, Grundt HH, Scheen AC & Elven R (2004) Polyploidy in arctic plants. Biological Journal Of The Linnean Society 82: 521-536.

Eidesen PB, Müller E, Lettner C, Alsos IG, Bender M, Kristiansen M, Peeters B, Postma F & Verweij KF (2013) Tetraploids do not form cushions: association of ploidy level, growth form and ecology in the High Arctic Saxifraga oppositifolia L. s. lat. (Saxifragaceae) in Svalbard. Polar Research 32.

Gu Z, Steinmetz LM, Gu X, Scharfe C, Davis RW & Li W-H (2003) Role of duplicate genes in genetic robustness against null mutations. Nature 421: 63-66.

Hewitt GM (1996) Some genetic consequences of ice ages, and their role in divergence and speciation. Biological Journal of the Linnean Society 58: 247-276.

Hewitt GM (1999) Post-glacial re-colonization of European biota. Biological Journal of the Linnean Society 68: 87-112.

Hewitt GM (2004) Genetic consequences of climatic oscillations in the Quaternary. Philosophical Transactions of the Royal Society B: Biological Sciences 359: 183-195.

Ibrahim KM, Nichols RA & Hewitt GM (1996) Spatial patterns of genetic variation generated by different forms of dispersal during range expansion. Heredity 77: 282-291.

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Levin DA (2002) The role of chromosomal change in plant evolution. Oxford University Press, New York.

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Rohde K (1992) Latitudinal Gradients in Species Diversity: The Search for the Primary Cause. Oikos 65: 514-527.

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Soltis DE, Soltis PS, Schemske DW, Hancock JF, Thompson JN, Husband BC & Judd WS (2007) Autopolyploidy in angiosperms: have we grossly underestimated the number of species? Taxon 56: 13-30.

Soltis PS & Soltis DE (2009) The role of hybridization in plant speciation. Annual Review of Plant Biology 60: 561-588.

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Stebbins GL (1984) Polyploidy and the distribution of arctic-alpine flora: new evidence and a new approach. Botanica Helvetica 94: 1-13.

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Wright SD, Ross HA, Jeanette Keeling D, McBride P & Gillman LN (2011) Thermal energy and the rate of genetic evolution in marine fishes. Evolutionary Ecology 25: 525-530.