Resolving the Northern Hemisphere source population that gave rise to the South American endemic moss Tetraplodon fuegianus

The origin of the southern South American endemic moss species Tetraplodon fuegianus represents a real mystery. It is very isolated in its worldwide distribution: it is found in the far southern parts of Southern America, yet its most closely related species is located on the Northern Hemisphere.
In addition to this disjunct bipolar distribution, it is a plant species with very specific growth preferences (growing on dung and decaying carcasses) coupled an equal specialised mode of dispersal (spores are often dispersed via insects such as flies attracted by the dung/carcasses). How such a plant has managed to get to its current distribution in the southern end of South America is therefore quite a puzzle.

A previous study (Lewis et al 2014) estimated that a direct long-distance dispersal from the Northern to the Southern Hemisphere was responsible for the current distribution, which would likely have happened during the Miocene–Pleistocene. However, the specific location of the Northern Hemisphere population which was the source of T. fuegianus remained unclear. Improved knowledge on the source population could be informative of the likely mechanism (biotic or abiotic vectors) behind the distribution event that gave rise to the species in the Southern Hemisphere - a result both informative for T. fuegianus as well as to the origin of bipolar species in general.

Using restriction-site-associated DNA (RADseq) combined with Bayesian and maximum likelihood phylogenetic approaches we studied the spatial genetic structure and phylogeographic relationships within the bipolar lineage of the genus, includes T. fuegianus. These analyses revealed that the source population of T. fuegianus is likely located in northwestern North America. The study showed that the species likely originated from a single long-distance dispersal event from a population, that is now rare and potentially restricted to the Pacific Northwest of North America. 

The areas of this northwestern location in North America and the southern tip of South America are potentially linked via migrating birds, which are known to migrate between both areas. It could be that the sticky moss spores, which are meant to hitch-hike along with insects, in fact took a hitch-hike across the equator to the other side of the planet via a migrating bird. This is even more likely if the bird was attracted to the insects feeding on the dung or carcass this particular moss was growing on.

Although of course the mechanism behind the real dispersal event originating in T. fuegianus still remains a mystery, we’ve come one step closer to solving the secrets behind the origin of this bipolar lineage.

Read the paper here. 

Reference:
​Lewis, L. R., Biersma, E. M., Carey, S. B., Holsinger, K., McDaniel, S. F., Rozzi, R., & Goffinet, B. (2017). Resolving the northern hemisphere source region for the long-distance dispersal event that gave rise to the South American endemic dung moss Tetraplodon fuegianus. American Journal of Botany. 104(11), 1651-1659.

Lewis, L. R., Rozzi, R., & Goffinet, B. (2014). Direct long‐distance dispersal shapes a New World amphitropical disjunction in the dispersal‐limited dung moss Tetraplodon (Bryopsida: Splachnaceae). Journal of biogeography, 41(12), 2385-2395.