Polyploidy At High Latitudes

The distinction between long-term and short-term residence in the Arctic makes it possible to re-examine the properties that allow certain plant species to have a long history of survival at high latitudes. One characteristic which is particularly notable in arctic species is polyploidy.

The Arctic is one the Earth's most polyploid-rich areas; it is also noted for a high incidence of recently evolved polyploids (Brochmann et al., 2004). The frequency and level of polyploidy increases markedly on moving northwards within the Arctic. A detailed examination of the levels of polyploidy throughout the Arctic could not detect any clear-cut association between polyploidy and the degree of glaciation for the arctic flora as a whole due to the number of widespread species. However, for 'arctic specialist' taxa with restricted distributions, it was claimed that the frequency of diploids was high in the Beringian area (Fig. 6.18), which remained largely unglaciated during the last ice age. Such observations support the hypothesis that polyploids are more successful than diploids in colonizing after deglaciation (Brochmann et al., 2004). It could therefore be concluded that the post-glacial evolutionary success of polyploids in the Arctic may be due to their fixed-heterozygous genomes, which buffer against inbreeding and genetic drift through periods of dramatic climate change (Brochmann et al., 2004).

Based on these arguments it is frequently suggested that polyploid species are better adapted physiologically to cold climates than diploid species. However, as noted above the exceptions to this habit are the autochthonous (ancient and indigenous) species which are usually diploid or occasionally tetraploid (Table 6.2). Even in the relatively recently deglaciated areas of Svalbard, 20% of the flowering plant species are diploid. Consequently it can be argued that a high level of polyploidy, although common at high latitudes, is not necessarily an adaptation for long-term survival in the fluctuating arctic environment. More probably, the presence of so many polyploid species in the Arctic is merely the consequence of plant migrations caused by periods of cooling and warming, with the climatic

Fig. 6.18 Frequency of polyploidy among 'arctic specialist taxa' that are either restricted to the region that was heavily glaciated during the last ice age ('Atlantic') or restricted to the region that remained mainly unglaciated ('Beringian'). The maximum extent of the late Weichselian/Wisconsian ice sheets (white) and tundra (dark grey) are shown, modified after Abbott & Brochmann (2003) and Brochmann et al. (2003). (Reproduced with permission from Brochmann et al., 2004.)

Fig. 6.18 Frequency of polyploidy among 'arctic specialist taxa' that are either restricted to the region that was heavily glaciated during the last ice age ('Atlantic') or restricted to the region that remained mainly unglaciated ('Beringian'). The maximum extent of the late Weichselian/Wisconsian ice sheets (white) and tundra (dark grey) are shown, modified after Abbott & Brochmann (2003) and Brochmann et al. (2003). (Reproduced with permission from Brochmann et al., 2004.)

disturbance bringing species into proximity in unusual combinations (Stebbins, 1971).

As in many other marginal areas, e.g. aquatic habitats, so in the Arctic there are some very successful sterile polyploid species. In terms of distribution of species and genetic exchange between populations it has to be remembered that much of the arctic flora is coastal and that coastal species in general enjoy a relative freedom from physical barriers to dispersal of ramets and other aids to vegetative reproduction. The sterile and triploid creeping salt marsh grass (Puccinellia phryga-nodes) is outstanding for the extent of its distribution throughout the Arctic as a major component of salt marsh vegetation (Jefferies & Gottlieb, 1983; Fig. 6.19). Accounts even exist of stolons of this plant, found at sea frozen into sea ice, that have been rescued, brought ashore and successfully grown (R. L. Jefferies, pers. com.). Dispersal throughout the Arctic of plant fragments frozen into pack ice must therefore be considered as an effective method of migration. In Beringia the species is found as the fertile diploid (2n — 14), while over Canada, Greenland, northern Norway, Spitsbergen and Fennoscandinavia the sterile 2n — 21 occurs, while in Finnmark, north Norway and Svalbard triploid 2n — 21, tetraploid 2n — 28, and hexaploid 2n — 42 individuals have all been recorded (Aiken et al., 2001).

As discussed above, most adaptations when examined closely have both positive and negative aspects in relation to survival potential. The question therefore has to be asked if there are similar distinctions between polyploid and diploid species in relation to survival fitness in the Arctic. It has never been demonstrated that polyploid species are more cold hardy or better adapted to short growing seasons than diploid species. The genetic advantages of polyploidy include restoring fertility to hybrids, and the ability to form new species within the home range of their parents. Polyploidy also has obvious advantages in

Fig. 6.19 Circumpolar distribution of creeping salt marsh grass (Puccinellia phryganodes). (Reproduced with permission from Hulten & Fries, 1986.)

masking the effects of harmful alleles. More questionable, however, is the assertion that polyploids benefit from having a greater store of genetic variability merely because there are more alleles at any one locus. If harmful alleles are masked by polyploidy, then the same must also be true for helpful alleles. In polyploid species mutations are less likely to be expressed and phenotypic variability is likely to be low. In diploid species neighbouring populations with different environmental preferences will have the advantage of being able to profit more readily than polyploids from gene flow as interchange of alleles from one population to another is less likely to be masked. It is therefore perhaps no coincidence that the ancient (autochthonous) arctic species (see Table 6.2) are mainly diploid (or else have only a low level of polyploidy) as they can adapt readily to climatic change by exchanging genes between different eco-types of the same species which are readily expressed and not masked by other alleles. The manner in which mutualism (see above) rather than competition between ecotypes can contribute to species survival will function only if genetic exchange leads to readily expressed adaptations. This is more likely to take place between adjacent diploid than polyploid populations.

Conflicts between adaptations at the physiological, biochemical and morphological level inevitably limit the choice of adaptations that can be expressed by any individual plant. No matter how much genetic information is present within the genome of any individual it is of little use if it cannot be expressed. It follows therefore that polyploid populations because of their genetic stability are less likely to be able to adapt to changing circumstances by exchanging small amounts of genetic material. In short, the adaptive significance ofpolyploidy, in contributing to physiological fitness in general, or in the Arctic in particular, has never been proven.

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