Multiple colonization and dispersal events hide the early origin and induce a lack of genetic structure of the moss Bryum argenteum in Antarctica.

Serena Zaccara,Peter Convey,Nicoletta Cannone,Jairo Patiño,Isabella Vanetti

doi:10.1002/ece3.6601

Abstract

The dispersal routes of taxa with transoceanic disjunctions remain poorly understood, with the potential roles of Antarctica not yet demonstrated. Mosses are suitable organisms to test direct intra‐Antarctic dispersal, as major component of the extant Antarctic flora, with the cosmopolitan moss Bryum argenteum as ideal target species. We analyzed the genetic structure of B. argenteum to provide an evolutionary time frame for its radiation and shed light into its historical biogeography in the Antarctic region. We tested two alternative scenarios: (a) intra‐Antarctic panmixia and (b) intra‐Antarctic genetic differentiation. Furthermore, we tested for evidence of the existence of specific intra‐Antarctic dispersal routes. Sixty‐seven new samples (40 collected in Antarctica) were sequenced for ITS nrDNA and rps4 cpDNA regions, and phylogenetic trees of B. argenteum were constructed, with a focus on its Southern Hemisphere. Combining our new nrDNA dataset with previously published datasets, we estimated time‐calibrated phylogenies based on two different substitution rates (derived from angiosperms and bryophytes) along with ancestral area estimations. Minimum spanning network and pairwise genetic distances were also calculated. B. argenteum was potentially distributed across Africa and Antarctica soon after its origin. Its earliest intra‐Antarctic dispersal and diversification occurred during a warming period in the Pliocene. On the same timescale, a radiation took place involving a dispersal event from Antarctica to the sub‐Antarctic islands. A more recent event of dispersal and diversification within Antarctica occurred during a warm period in the Pleistocene, creating favorable conditions also for its colonization outside the Antarctic continent worldwide. We provide evidence supporting the hypothesis that contemporary populations of B. argenteum in Antarctica integrate a history of both multiple long‐range dispersal events and local persistence combined with in situ diversification. Our data support the hypothesis that B. argenteum has been characterized by strong connectivity within Antarctica, suggesting the existence of intra‐Antarctic dispersal routes.

Highlights

Some plant species exhibit transoceanic disjunctions and, for species with disjunct distributions, their underlying dispersal routes and diversification patterns across space and time remain to be clarified
The ancestral area analyses using the two BEAST chronograms selected DIVALIKE as the best-fit model (Table S3). Both analyses indicated that the most recent common ancestor of B. argenteum was potentially distributed across Africa and Antarctica, the calibration I analysis indicated its presence in Asia and North America (See Table 2, Figure 3; Fig. S1)
The phylogenetic tree and biogeographic evidence, achieved by comparing dating analyses using two different calibration schemes based on angiosperm and moss substitution rates, in combination with ancestral area estimations, show genetic signatures supporting the hypothesis that B. argenteum was present in both Antarctica and Africa soon after the species’ origin

Summary

| INTRODUCTION

Some plant species exhibit transoceanic disjunctions and, for species with disjunct distributions, their underlying dispersal routes and diversification patterns across space and time remain to be clarified. Bryophytes are generally assumed to have considerable dispersal power (Patiño & Vanderpoorten, 2018), facilitated by the production of small sexual spores and asexual propagules, sexual reproduction is observed frequently only at Antarctic locations where climatic conditions are less extreme, such as in Maritime Antarctica in which includes the Antarctic Peninsula and Scotia Arc archipelagos (Convey & Smith, 1993; Ochyra et al, 2008; Smith & Convey, 2002) Such life history traits promote widespread distribution and presence in extremely diverse terrestrial habitats throughout the world, from the tropics to the poles (Biersma et al, 2017; Huttunen, Kuznetsova, Li, Wang, & Ignatov, 2015; Lewis, Rozzi, & Goffinet, 2014; Patiño, Goffinet, Sim-Sim, & Vanderpoorten, 2016). We hypothesized that patterns of genetic structuring would correspond to specific intra-Antarctic dispersal routes, as suggested by Winkworth et al (2015), in particular along the Transantarctic Mountains and/or the coast of continental Antarctica, with dispersal events occurring during periods of reduced ice cover (Cannone, Convey, & Guglielmin, 2013)

| MATERIALS AND METHODS

F Africa A Asia T Antarctica E Europe N North America S South America

Findings

| DISCUSSION