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(American Journal of Botany. 2008;95:720-730.)
doi: 10.3732/ajb.2007407
© 2008 Botanical Society of America, Inc.
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Systematics and Phytogeography

Origin and evolution of the northern hemisphere disjunction in the moss genus Homalothecium (Brachytheciaceae)1

Sanna Huttunen2,5, Lars Hedenäs2, Michael S. Ignatov3, Nicolas Devos4 and Alain Vanderpoorten4

2 Swedish Museum of Natural History, Department of Cryptogamic Botany, Box 50007 SE-104 05 Stockholm, Sweden 3 Main Botanical Garden of Russian Academy of Sciences, Botanicheskaya 4, Moscow 127276, Russia 4 University of Liège, Institute of Botany, B22 Sart Tilman, B-4000 Liège, Belgium

ABSTRACT

Competing hypotheses that rely either on a stepping-stone dispersal via the North Atlantic or the Bering land bridges, or more recent transoceanic dispersal, have been proposed to explain the disjunct distribution of Mediterranean flora in southern Europe and western North America. These hypotheses were tested with molecular dating using a phylogeny of the moss genus Homalothecium based on ITS, atpB-rbcL, and rpl16 sequence data. The monophyly of two main lineages in Western Palearctic (Europe, central Asia and north Africa) and North America is consistent with the ancient vicariance hypothesis. The monophyly of Madeiran H. sericeum accessions supports the recognition of the Macaronesian endemic H. mandonii. A range of absolute rates of molecular evolution documented in land plants was used as probabilistic calibration prior by a Bayesian inference implementing a relaxed-clock model to derive ages for the nodes of interest. Our age estimates for the divergence of the American and Western Palearctic Homalothecium clade (5.7 Ma, IC 3.52–8.26) and the origin of H. mandonii (2.52 Myr IC 0.86–8.25) are not compatible with the ancient vicariance hypothesis. Age estimates suggests that species distributions result from rare instances of dispersal and subsequent sympatric diversification. The calibrated phylogeny indicates that Homalothecium has undergone a fast radiation during the last 4 Myr, which is consistent with the low levels of morphological divergence among sibling species.

Key Words: biogeography • Brachytheciaceae • cryptic species • Macaronesia • Mediterranean biota • northern hemisphere disjunctions • phylogeny • rapid radiation

Received for publication 11 December 2007. Accepted for publication 4 March 2008.

FOOTNOTES

1 The authors thank the curators of the herbarium of the University of British Columbia (UBC) and the Natural History Museum of Stuttgart (STK) for loans of herbarium material. This research was supported by a Marie Curie Intra-European Fellowship (MEIF-CT-2005-009452) within the 6th European Community Framework Program for S.H. and by the Swedish Foundation for International Cooperation in Research and Higher Education (STINT, Dnr. 05/063). The Swedish Research Council (Vetenskapsrådet project no. 621-2003-3338) supported the participation of L.H. in this project. N.D. and A.V are grateful for financial support from the Belgian Funds for Scientific Research (FNRS). M.I. was partly supported by the Russian Foundation for Basic Research (RFBR 07-04-00013).

5 Author for correspondence (e-mail: sanna.huttunen{at}nrm.se)


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