Abstract
Bipolar moss species, extreme examples of large and disjunctive geographical ranges, belong to the most important components of the vegetation cover in Antarctica. Their appropriate taxonomic interpretations are essential for fine-scale biogeographical considerations but our knowledge on their phylogenetic affinities still remains limited. Here, we address the history of the Antarctic populations of Drepanocladus longifolius, a moss species widely considered a bipolar taxon. First, based on a worldwide sampling, we verify its assumed bipolar status. Then, we ask whether its current Antarctic populations have persisted throughout at least the last major glaciation in situ or, alternatively, are they a recent, postglacial element. Phylogenetic analyses together with investigation of morphological characters were used to infer relationships among accessions representing the whole distribution range of the taxon. Morphological and phylogenetic data strictly segregated specimens from the Southern and the Northern Hemisphere, with an exception of Australian populations that were included in the latter group. The Antarctic, sub-Antarctic and South American populations belong to a single clade and are treated as D. longifolius s. str., which consequently is not a bipolar taxon. All Northern Hemisphere populations and the ones from Australia represent D. capillifolius, a lately neglected taxon, which thus displays a true bipolar distribution pattern but not comprising the Antarctic and South American populations. These results provide a sound example of key importance of a detailed taxonomical and phylogenetic treatment as a basis for biogeographical studies. In D. longifolius s. str., a homogeneity in all non-coding nuclear and plastid regions was observed throughout the whole range. It contrasted with the presence of one missense mutation in the rps4 gene in all examined Antarctic populations, which correlated with their distinct ecological situation. While identity of non-coding sequences in D. longifolius populations indicates a postglacial recruitment of extant Antarctic populations from extra-regional locations rather than a long-term in situ survival, the mutation in the rps4 gene may be hypothesised to reflect an adaptive response of plants to the Antarctic environment.
Highlights
IntroductionColonisation of the Antarctic continent presents considerable challenges
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Our observations performed on specimens spanning all major geographical regions of occurrence across the Northern and Southern Hemisphere confirmed the previously reported principal differences between Drepanocladus capillifolius and D. longifolius s. str. treated as two allopatric taxa (Ochyra and Matteri 2001; Żarnowiec 2001; Ochyra et al 2008a) and can be summarised as follows: in D. longifolius, the alar cells form a small, inflated, ovate to ovate-triangular marginal auriculate group in the leaf angles, sharply separated from the adjacent basal cells and occupying about one third or less of the leaf insertion; in contrast, in D. capillifolius, they form a very distinct, inflated, transversely triangular group, reaching the costa or nearly so (Fig. 2, Supplementary Fig. 1a–d)
Summary
Colonisation of the Antarctic continent presents considerable challenges. This is due, among others, to the influence of oceanic fronts and strong circumpolar currents which encircle the Antarctica, increase its isolation and could have prevented many taxa from dispersing to this continent. Some recent molecular studies indicated genetic isolation of Antarctic populations and supported possible long-term persistence within fragmented Antarctic habitats of such terrestrial organisms as springtails (Arthropoda: Collembola) (Stevens and Hogg 2003; McGaughran et al 2010; McGaughran et al 2011), midges (Arthropoda: Diptera) (Allegrucci et al 2006), mites (Arthropoda: Acari) (Mortimer et al 2011), green algae (De Wever et al 2009) and, more recently, bryophytes (Hills et al 2010; Pisa et al 2014; Biersma et al 2017). Knowledge on the biogeographical history of the extant Antarctic terrestrial biota remains rudimentary and reconstructing a framework of colonisation and dispersal events within a phylogeographic context is much needed for various taxonomical and biogeographical elements
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