Global biogeographic patterns in bipolar moss species.

E. M. Biersma,P. Convey,S. Koskinen,K. Linse,J. Hyvönen,J. A. Jackson,H. Griffiths

doi:10.1098/rsos.170147

Abstract

A bipolar disjunction is an extreme, yet common, biogeographic pattern in non-vascular plants, yet its underlying mechanisms (vicariance or long-distance dispersal), origin and timing remain poorly understood. Here, combining a large-scale population dataset and multiple dating analyses, we examine the biogeography of four bipolar Polytrichales mosses, common to the Holarctic (temperate and polar Northern Hemisphere regions) and the Antarctic region (Antarctic, sub-Antarctic, southern South America) and other Southern Hemisphere (SH) regions. Our data reveal contrasting patterns, for three species were of Holarctic origin, with subsequent dispersal to the SH, while one, currently a particularly common species in the Holarctic (Polytrichum juniperinum), diversified in the Antarctic region and from here colonized both the Holarctic and other SH regions. Our findings suggest long-distance dispersal as the driver of bipolar disjunctions. We find such inter-hemispheric dispersals are rare, occurring on multi-million-year timescales. High-altitude tropical populations did not act as trans-equatorial ‘stepping-stones’, but rather were derived from later dispersal events. All arrivals to the Antarctic region occurred well before the Last Glacial Maximum and previous glaciations, suggesting that, despite the harsh climate during these past glacial maxima, plants have had a much longer presence in this southern region than previously thought.

Highlights

Since the nineteenth century, scientists have been puzzled by the origin and evolution of plants with disjunct distributions, and with the most extreme pattern of all—bipolar disjunctions [1,2,3]
Even with the differences between the various dating analyses, all analyses identified similar outcomes indicating that the main inter-hemispheric movements occurred on hundred-thousand to multi-millionyear timescales, from the Pleistocene, Pliocene and/or Miocene/Late Oligocene
Even if underestimated, divergence times between populations are too young to have derived from continental vicariance

Summary

Introduction

Scientists have been puzzled by the origin and evolution of plants with disjunct distributions, and with the most extreme pattern of all—bipolar disjunctions [1,2,3]. Two recent molecular phylogeographical studies of bryophytes with disjunct distributions reaching as far south as Tierra del Fuego have suggested the distribution to be due to dispersal events, either recent (e.g. Cinclidium stygium Sw., no molecular dating but very low variation between hemispheres [9]) or in the more distant past The lack of variation in disjunct populations of C. stygium makes it difficult to distinguish whether the disjunction is natural or caused by anthropogenic vectors [9], and the dung-associated lifestyle of Tetraplodon makes this moss a likely candidate for adventitious dispersal via migrating birds [10] (e.g. becoming attached when birds forage for insects attracted to dung), which might not be a typical characteristic of the majority of bipolar moss species. In-depth investigations into global patterns of dispersal of bipolar mosses are clearly needed, including species that are more widespread and have a more typical bryophyte-representative ecology (i.e. unlike the dung-associated Tetraplodon, see above)

Methods

Results

Conclusion