Phylogeographical break and limited connectivity between multiple refugia in panantarctic moss species

Aim: Historical biogeography of the Antarctic terrestrial biota remains poorly studied and understood. We aim to advance this through a range-wide, multilocus analysis of a pan-Antarctic moss species, in the context of its age, range dynamics, phylogeographical structure, and location of possible long-term refugia.

Location: Continental and maritime Antarctic, South America (Patagonia), Australasia.

Taxon: Syntrichia sarconeurum Ochyra & R.H. Zander (Pottiaceae).

Methods: We used a comprehensive, range-wide and taxonomically-informed sampling, and multilocus sequencing of nuclear and plastid DNA regions. Temporal evolutionary framework, regional genetic diversification and diversity were assessed with phylogenetic and phylogeographic reconstructions, molecular dating, haplotype networks, mismatch analysis, and S-DIVA reconstruction of past events and ancestral areas.

Results: Intercontinental disjunction between Australia and Antarctica/S. America was dated to 3.77 Ma, while diversification of extant Syntrichia sarconeurum lineages had taken place since roughly 1.36 Ma. Antarctic populations contained two high-frequency, allopatric cpDNA haplotypes, which highlighted separation of the continental populations. ITS data showed higher diversification and revealed three main lineages with a main genetic break mostly concordant with plastid data. ITS also showed contrasting diversity between the Antarctic continent and maritime Antarctic/Patagonia.

Main conclusions: Age of the Antarctic range of Syntrichia sarconeurum potentially reaches back to mid-Pliocene, while diversification of extant genetic lineages was linked with recurrent macroenvironmental changes of the Pleistocene. Significant phylogeographical structure displays a major genetic break, which coincides with a known boundary in the terrestrial biota and suggests constrained population connectivity. S. sarconeurum persisted in situ over several glacial periods in refugia both in the peripheral maritime Antarctic and within the Antarctic continent. Disparity in genetic diversity among these areas hypothetically reflects asymmetric changes of reproduction mode in the past. Finally, nested topology of Patagonian subclades and ancestry analysis indicates that South American populations were secondary and originated through multiple dispersal from the Antarctic region.

Methods
Plant sampling was based on collections gathered during Antarctic expeditions between 1940 and 2016, with specimens deposited in the bryological herbarium of the Institute of Botany, Polish Academy of Sciences (KRAM).

One nuclear ribosomal marker, ITS1-5.8S-ITS2, and three plastid (cpDNA) markers, rps4, tRNAGly, and trnL-F, were selected for the multilocus analysis. Forward and reverse sequences were automatically assembled using Geneious 10.1.3. Sequences of the four DNA regions were individually aligned using the algorithm mafft-auto with the remaining default settings in Mafft (https://mafft.cbrc.jp/alignment/server//). Selected DNA regions were concatenated using SequenceMatrix. For all molecular analyses, indels in DNA sequences were coded and calculated in SeqState v1.4.1 using a simple indel coding. All programs are listed in the ReadMe text file.