Plant biodiversity in an extreme environment genetic studies of origins, diversity and evolution in the antarctic

Abstract

Plants in Antarctica survive in one of the harshest environments on Earth. Less than 2% of the 14 million km2 that make up continental Antarctica is free of permanent ice and snow and therefore available for colonisation by plants. Vegetation is sparse and low-growing, and is dominated by mosses and lichens. Two species of flowering plants occur on the Antarctic Peninsula (Edwards and Lewis Smith 1988, Lewis Smith 2003), but none in continental Antarctica. The flora of continental Antarctica comprises 15 species of mosses (Lewis Smith 1984), one species of liverwort (Bednarek-Ochyra et al. 2000) and at least 88 taxa of lichens (Øvstedal and Lewis Smith 2001). Eight species of moss have been recorded from southern Victoria Land and Ross Island (Seppelt and Green 1998). Continental Antarctic moss species usually grow as small colonies, but some coalesce to form turfs up to several square metres in extent and with up to 70% ground cover (Lewis Smith 2003, 2005). In a few locations, turfs can cover almost 100% of the ground over 25m2 or more. Clumps and turfs of moss are found in areas sheltered from the strong winds common in this region: in depressions and cracks in the ground surface, drainage lines and near rocks. Mosses only grow in niches where some moisture is available in summer, such as melt water from glaciers and persistent snow banks, and melting snow accumulated amongst rocks, cracks and depressions in the ground surface. These mosses are subjected to extremes of cold, drought, wind and light, with plants south of 67°S existing for weeks or months each year in a freeze-dried state in complete darkness. In the harsh continental Antarctic environment with its short summer growing period, mosses do not reproduce sexually. Of the moss species recorded from southern Victoria Land and Ross Island, only Hennediella heimii is known to produce sporophytes (Seppelt et al. 1992), although mature sporophytes and shedding spores have not been recorded. Colonies originate either from immigrant propagules from other lands (Marshall 1996, Marshall and Convey 1997) or from vegetative propagules dispersed locally. Colonisation of new locations seems to be difficult and immigration from other land masses appears infrequent. With the annual expansion of the 'ozone hole' (Farman et al. 1985, Kennedy 1995) continental Antarctic mosses are subjected to increasing exposure to UV-B irradiation. With their haploid genomes, lack of sexual reproduction, perennial growth, and extreme isolation from colonies elsewhere around the world, these mosses appear to provide an ideal model system with the potential to reveal significant insights into colonisation, mutation and speciation. Such a combination of characteristics is not available in other plants, or even in mosses in most other parts of the world (Wyatt 1994). On the Antarctic Peninsula, there is a wider diversity of plants, with more moss, liverwort and lichen species, and two vascular plant species, Deschampsia antarctica and Colobanthus quitensis (Lewis Smith 1984). On the subantarctic islands, vegetation is yet more diverse, with numerous vascular plants in addition to a higher number of bryophyte and lichen species. For example, from Heard Island 12 species of vascular plants (George 1993, Turner et al. 2006) 37 species of mosses (Bergstrom and Selkirk 1997), 19 species of liverworts (Vana and Gremmen 2005) and 71 species of lichens (D. Øvestdal and N. Gremmen, unpubl. data) have been found to date. On Macquarie Island, there are 44 native species of vascular plants (George 1993), 88 species of mosses (Seppelt 2004), 51 species of liverworts and many algae and lichens (Selkirk et al. 1990). Other subantarctic islands have similarly diverse floras, especially those islands that have been subjected to human occupation and introductions of alien species (Frenot et al. 2005). Under predicted scenarios of global climate change, the climatic constraints of the Antarctic environment are likely to be reduced and the level of biodiversity is likely to increase (Kennedy 1995). With climate change already well underway on subantarctic islands such as Macquarie and Marion Islands, Iles Kerguelen and Heard Island, on-going research can analyse the spread of species and monitor colonisation of newly deglaciated ground. Two examples of biodiversity increasing are the recent arrivals of Poa annua (Scott 1989) and Leptinella plumosa on Heard Island (Turner et al. 2006). Uniquely in continental Antarctica, many of the short-term consequences of climate change will affect bryophyte ecosystems and colonisation; these communities may be particularly vulnerable to global change. Research into the genetic diversity and susceptibility to increased UV radiation of endemic Antarctic species may make prediction of the consequences of ozone depletion more feasible (Adamson and Adamson 1992, Kennedy 1995, Robinson et al. 2003, 2005). An interesting question is whether the mosses found today have been present in Antarctica for a very long time, surviving periods of extensive ice cover in refugia, or whether moss populations on the Antarctic continent face regular extinction and become re-established by colonisation from outside Antarctica when conditions are favourable (Walton 1990, Marshall 1996). A promising indirect approach is to make inferences on colonisation history and processes from the current spatial (geographic and microgeographic) distribution of natural variation in particular moss genes. In addition, as bryophytes constitute one of very few successful groups with a functional haploid phase, the evolutionary processes operating in these plants are of great general interest and fundamental scientific importance (Longton 1988, 1994). We have used techniques of molecular genetics to investigate the genetic diversity of these plants, their origins and dispersal mechanisms and their potential to respond genetically to climate change (Skotnicki et al. 2000, 2004). We have also used molecular genetics to resolve some taxonomic uncertainties (Skotnicki et al. 2001, 2002), since the extreme environment can lead to phenotypic plasticity where morphological characters can vary in response to different environmental conditions rather than being due to genetic changes (Seppelt and Selkirk 1984, Lewis Smith 1999). In some Antarctic mosses, such as Ceratodon purpureus, it has been suggested that the taxon is so variable that extreme phenotypes could be distinct species (Ochyra 1998). When morphological identification has been difficult or impossible, molecular techniques have proved valuable (Skotnicki et al. 1997, 2001, Bargagli et al. 2004). Molecular genetics can also assist in the taxonomic identification of potential hybrid plant species found on subantarctic islands. Techniques of molecular genetics have the potential to reveal a wide variety of characteristics in Antarctic plants that cannot be determined by traditional morphological microscopic examination. For example, by comparing highly conserved gene sequences, it is possible to analyse the colonisation patterns of mosses in Antarctica, or dispersal of vascular plants among subantarctic islands. © 2006 Springer.