Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels

This is Section 7.3.4 of the Arctic Climate Impact Assessment
Lead Author: Terry V. Callaghan; Contributing Authors: Lars Olof Björn, F. Stuart Chapin III,Yuri Chernov,Torben R. Christensen, Brian Huntley, Rolf Ims, Margareta Johansson, Dyanna Jolly Riedlinger, Sven Jonasson, Nadya Matveyeva,Walter Oechel, Nicolai Panikov, Gus Shaver; Consulting Authors: Josef Elster, Heikki Henttonen, Ingibjörg S. Jónsdóttir, Kari Laine, Sibyll Schaphoff, Stephen Sitch, Erja Taulavuori, Kari Taulavuori, Christoph Zöckler

 

Many widely distributed arctic species show large ecological amplitude (broad niches) and are taxonomically complex, often with many subspecies, while species with a narrower distribution range often show a more restricted ecological amplitude. It is necessary to know the extent of genetic variation in arctic species and the underlying causes of differentiation or homogenization (biogeography, historical bottlenecks, reproductive biology, and demography) in order to assess responses of species to climate change.

Plants (7.3.4.1)

In spite of rapid development in recent years of different molecular techniques suited for population genetic studies, there are still few studies of arctic plants, most of which have focused on biogeographical and phylogeographical questions related to vascular plant species. Such studies may reveal the migratory potential of the species in response to climate change. During the Pleistocene glaciations, arctic plants were restricted to refugia within or south of the present-day Arctic, from which they could re-colonize areas as conditions improved during interglacial periods[1]. The rate of colonization by different species during the Holocene probably depended on the location of their closest refugia, their dispersal biology, and their genetic makeup. Genetic phylogeographical studies provide evidence for relatively fast migration rates in most vascular species[2] and possibly bryophytes as well[3]. However, in the modern context of rapid climate change, migration rates need to be considered on somewhat shorter timescales than thousands of years.

The level of genetic variation within and between populations indicates the potential for local adaptation to environmental change and hence population resilience to environmental change. Based on the relatively young age of populations and low recruitment of sexually reproduced offspring, it was long believed that genetic variation in arctic plants would be low. However, the number of genetic studies is limited and no such general pattern of genetic variation has been identified. Arctic plants show the same range of genetic variation as temperate plants, ranging from comparatively high levels[4] to very low levels [5] of variation. However, genetic variation among arctic plants may be of greater value in terms of biodiversity than in other biomes due to much lower species diversity. Furthermore, high levels of polyploidy in many arctic vascular plant species may promote the proportion of the genetic variation partitioned within individuals, which may be important when passing through evolutionary bottlenecks[6].

By comparing 19 different populations of three rhizomatous Carex taxa, distributed among 16 sites within arctic Eurasia, ranging from northern Scandinavia in the west to Wrangel Island in the east, Stenström et al.[7] showed that the levels of genetic variation were not related to climate, but were to a large extent explained by differences in glaciation history at the sampling sites. Populations in areas deglaciated about 10,000 years before present (BP) had significantly lower genetic variation than populations in areas deglaciated 60,000 years BP or those in areas not glaciated at all during the Weichselian. Relatively young population age may also be responsible for a low genetic variation in some other populations (e.g., [8]), while in yet others, breeding systems apparently play a large role (e.g., [9]). In general, populations of insector self-pollinated plant species have lower genetic variation than populations of wind-pollinated species[10], and this seems to apply to arctic plants as well.

Those plant species representing populations with relatively high levels of genetic variation usually have a large geographic distribution, for example purple saxifrage[11], nodding saxifrage (Saxifraga cernua[12]), moss campion (Silene acaulis [13]), Carex bigelowii sensu lato[14], and C. stans[15]. In these species, the genetic variation among populations (GST) is a relatively small proportion of the total genetic variation (i.e., they show low degrees of population differentiation). Large variation within populations, however, increases possibilities for ecotypic differentiation. In the Arctic, extremely steep environmental gradients are frequent on a microtopographical scale and ecotypic differentiation has been demonstrated over such short distances for alpine timothy (Phleum alpinum[16]), Carex aquatilis[17], mountain avens [18], and purple saxifrage [19], all widely distributed plant species in the Arctic. Ecotypic differentiation in response to this small-scale heterogeneity may preserve genetic variation and in that way contribute to resilience to change at the species rather than the population level.Thus, an initial response to climate change in such species is likely to be a change in the distribution and abundance of ecotypes within a species distribution[20]. In addition, many arctic plants show large phenotypic plasticity, which is likely to further increase their resilience[21] (Table 7.5).

If the degree of genetic variation can be used as an indication of resilience of populations to change, it is likely that this resilience will be greatest among plants in old populations of widely distributed, wind-pollinated vascular species (e.g., rhizomatous Carex populations in eastern Siberia). However, generation time and seedling recruitment may affect the adaptation rate. Many of the dominant arctic plants such as the rhizomatous Carex species are clonal, that is, they do not rely on seed production through sexual reproduction for short-term population maintenance. Genetic individuals of these plant species may live to be thousands of years old[22], which may decrease the adaptation rate. However, experiments with plants from outside of the Arctic have shown that UV-B radiation may increase the rate of genetic change. Exposure to high UV-B radiation levels can activate mutator transposons that amplify the mutation effect beyond the immediate UV-B radiation damage[23], and increased levels of UV-B radiation may lead to an increased tendency to mutations in future generations[24].

For plants with long-lived seed, further genetic variation is preserved in the seed banks. Dormant seed populations may be genetically different from the aboveground populations[25] and potentially able to better exploit a new climate.

Genetic variation has been studied in fewer moss and lichen species than in vascular plants. Boreal and antarctic bryophytes usually show high levels of variation[26], but the partitioning of genetic variation among and within populations depends on species. Scandinavian populations of the widely distributed moss Hylocomium splendens, including two subarctic alpine populations, showed high genetic variation within populations and low GST, a pattern similar to widely distributed, wind-pollinated vascular species[27]. In contrast, North American and European populations of Polytrichum commune have low within-population variation and high GST[28].

Animals (7.3.4.2)

The genetics of arctic terrestrial animals have been thoroughly studied mainly for a few well-known mammal species such as reindeer/caribou[29], lemmings[30], and Arctic fox[31]. These studies have focused on phylogeographical patterns and the relative roles of present gene flow and historic processes (especially glacial–interglacial cycles; see section 7.2) based on neutral genetic markers (especially mitochondrial DNA). The present genetic differentiation reflects to a large extent historic processes and the presence of current migration barriers. For mammals with relatively restricted mobility such as lemmings, even small-scale barriers (e.g., large rivers) can form the borders between subspecies[32], while a very mobile animal such as the Arctic fox, which readily moves between continents and islands on sea ice, appears to be relatively panmictic (i.e., shows little genetic structuring) at the circumpolar scale[33].

Current gene flow (an indication of mobility) and population history (origin and differentiation) indicate the ability of a species to track the location of its habitats through time (i.e., a species is able to relocate its distributional range according to any changes in the distribution of its habitat through the process of dispersal). A mobile species will have better prospects for survival than a relatively sedentary species. Moreover, a species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do the same in the future. It should be noted, however, that markers of genetic variation/differentiation currently used (e.g., mitochondrial DNA) may have little bearing on the genetic variation in morphology and life-history traits (see [34]). It is these latter traits that decide whether a species or a morph will be able to adapt to future changes. Currently, there are few studies of arctic animals using a quantitative genetics approach[35] that address the potential for rapid adaptations to climatic change. Elsewhere, using a quantitative genetic research protocol, Réale et al.[36] showed that northern boreal red squirrels (Tamiasciurus hudsonicus) were able to respond genetically within a decade to increased spring temperatures.

Microorganisms (7.3.4.3)

Assessment of genetic responses of microorganisms to climate change is based on laboratory models, as observations made within arctic terrestrial ecosystems are absent. Short generation times and the impressive genetic plasticity of bacteria make them a favorite topic in theoretical studies of general population genetics. Because most mutations are deleterious, mutation rates are generally thought to be low and, consequently, mutator alleles should be selected against. However, up to 1% of natural bacterial isolates have been found to be mutators. A mutator can be viewed as behaving altruistically because, although it reduces individual fitness, it increases the probability of an adaptive mutation appearing. These results may help to explain observations that associate high mutation rates with emerging pathogens that cause spontaneous epidemic outbreaks[37].

In the arctic environment, intensive mutagenic effects are likely to result from increased UV radiation levels and also from aerosols and volatile chemical mutagens transported to the cool polar atmosphere from the mid and low latitudes. The direct mutagenic effects are very likely to be weak, especially if the protective shielding effects of soil particles and impressive genetic plasticity of bacteria are taken into account. However, it is possible that mutants could lead to epidemic outbreaks that could have profound and unexpected consequences for the whole ecosystem.

Summary (7.3.4.4)

Arctic plants show the same range of genetic variation as temperate plants, ranging from comparatively high levels to very low levels. In widespread Carex taxa, levels of genetic variation were not related to climate, but were to a large extent explained by differences in glaciation history at the sampling sites: populations in areas deglaciated approximately 10,000 years BP had significantly lower genetic variation than populations in areas deglaciated 60,000 years BP.

Plant species representing populations with relatively high levels of genetic variation usually have a large geographic distribution. On a microtopographical scale, extremely steep environmental gradients are frequent and ecotypic differentiation has been demonstrated over short distances for several widespread species. This heterogeneity, together with large phenotypic plasticity, is likely to contribute to resilience to change at the population and species levels. For plants with long-lived seed, further genetic variation related to former environments is preserved in the seed banks. Thus, there are several mechanisms for widespread arctic plant species to respond to environmental change.

Experiments with plants from outside the Arctic have shown that increased levels of UV-B radiation can speed up genetic change and may lead to an increased tendency for mutations in future generations.

The present genetic differentiation of arctic terrestrial animals that have been studied thoroughly (e.g., reindeer/caribou, lemmings, Arctic fox) to a large extent reflects historic processes and the presence of current migration barriers. For mammals with relatively restricted mobility such as lemmings, even small-scale barriers (e.g., large rivers) can form the borders between subspecies, while a very mobile animal such as the Arctic fox shows little genetic structuring at the circumpolar scale. A species with high genetic/racial diversity has proved an ability to adapt to different environmental conditions in the past and is likely to do the same in the future.

There is a paucity of studies of arctic animals that have addressed the potential for rapid adaptations to climatic change. Elsewhere, it was shown that northern boreal red squirrels were able to respond genetically within a decade to increased spring temperatures.

Up to 1% of natural bacterial isolates have been found to be mutators, and high mutation rates are associated with emerging pathogens causing spontaneous epidemic outbreaks. In the Arctic, intensive mutagenic effects are likely to result from increased UV radiation levels and also from aerosols and volatile chemical mutagens. Although the effects are very likely to be weak, it is possible that mutants could lead to epidemic outbreaks that could have profound and unexpected consequences for the whole ecosystem.

Chapter 7: Arctic Tundra and Polar Desert Ecosystems

7.1 Introduction
7.2 Late-Quaternary changes in arctic terrestrial ecosystems, climate, and ultraviolet radiation levels
7.3 Species responses to changes in climate and ultraviolet-B radiation in the Arctic
                 7.3.1 Implications of current species distributions for future biotic change
                 7.3.2 General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels
                 7.3.3 Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation
                 7.3.4 Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels
                 7.3.5 Recent and projected changes in arctic species distributions and potential ranges
7.4 Effects of changes in climate and UV radiation levels on structure and function of arctic ecosystems in the short and long term
                 7.4.1 Ecosystem structure
                 7.4.2 Ecosystem function
7.5 Effects of climate change on landscape and regional processes and feedbacks to the climate system
7.6 Synthesis: Scenarios of projected changes in the four ACIA regions for 2020, 2050, and 2080
7.7 Uncertainties and recommendations

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Citation

Committee, I. (2012). Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels. Retrieved from http://www.eoearth.org/view/article/152941

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