New paper on the movements of small plants between both hemispheres

One small sweep for a spore   …one giant leap for mosskind!


Mosses are the dominant flora in Antarctica, yet we know little of when and how they got there. The majority of Antarctica’s moss flora (~45% of species) has a curious ‘bipolar’ distribution pattern - a pattern with species occupying regions in the high latitudes of both hemispheres, with no or very small intermediate populations at higher elevations in tropical regions. This non-continuous distribution pattern has puzzled scientists, including biologists such as Darwin and Wallace, since the 19th century.

In this large-scale genetic study we investigated the origin of the curious ‘bipolar’ distribution pattern in several common moss species of the family Polytrichaceae. We found that the driver of these bipolar disjunctions is long-distance dispersal. This could have happened when a spore or other reproductive plant propagule has crossed the equator, either via winds or birds. Nevertheless, we found that such inter-hemispheric dispersals are rare events, occurring on multi-million year timescales, also illustrating the difficulties for plants to make such long journeys.

Sampling locations and bipolar distributions (shown in green) of the different moss study species. 

North to south, or south to north?

Three out of the four species studied have moved from the north to the south. However, one species, currently a particularly common species in the Holarctic (Polytrichum juniperinum), showed the opposite pattern (see picture on the right). Even though the main distribution of P. juniperinum is currently in the Northern Hemisphere, the population genetic analyses presented here suggest the moss originated in the Antarctic region (Antarctic, sub-Antarctic and southern South America), and from here colonised both the entire boreal regions of the Northern Hemisphere, as well as other Southern Hemisphere regions such as Australasia and South Africa. ​

Past distribution patterns of Polytrichum juniperinum between both hemispheres

No 'stepping stone' dispersal across the equator
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Polytrichum juniperinum also has some small populations in mountainous regions of northern South America, which could have acted as cold habitat “stepping-stones” to get from one hemisphere to the other across the equator. However, the study found this was not the case, and that these populations were in fact derived from a separate dispersal event from the south.

Persistence in Southern Hemisphere glaciated regions 

The study also found that all mosses had been present in the far south (Antarctic or southern South America) well before the Last Glacial Maximum and previous glaciations. These regions are thought to have experienced extensive glaciations throughout these glacial cycles, complicating survival for these plants. The findings of this study suggest that plants must have been present here, despite the harsh conditions throughout these glacial periods.

Polytrichaceae mosses in the Scottish countryside - these mosses occur across the temperate regions of the Northern Hemisphere, but are also present in the outer ends of the Southern Hemisphere. Photos by Peter Convey

Read the paper here: http://rsos.royalsocietypublishing.org/content/4/7/170147