Phylogeography of Marmosa robinsoni: Insights into the biogeography of dry forests in northern South America

Robinson's mouse opossum (Marmosa robinsoni) typically inhabits xeric shrublands, savannas, and deciduous forests from Panama through Colombia and Venezuela, to the islands of Trinidad, Tobago, and Grenada. We assessed its phylogeographic structure in the 1st such study based on dense geographic sampling of any vertebrate from dry habitats in this region. We sequenced the cytochrome-b gene and the X-linked intron O-linked N-acetylglucosamine transferase, largely from dried skins and residual tissue on osteological material of museum specimens. Phylogenetic analyses revealed the existence of 2 well-supported phylogroups primarily distributed to the east and west of the Cordillera de Mérida. The estimated time since divergence between these phylogroups postdates the Miocene; therefore, Andean uplifts, changes in the course of the Río Orinoco, and marine transgressions of that epoch cannot be implicated as causal vicariant agents. Instead, expansion of humid forest or marine transgressions, or both, during the Pliocene and Pleistocene more likely led to this differentiation. We encountered little structure among populations east of the Cordillera de Mérida, suggesting recent range expansion in this region. Surprisingly, isolated populations from the Península de Paraguaná (northwestern Venezuela) are not closely related to geographically proximate mainland populations, but rather to more distant populations to the west in Colombia and Panama. By contrast, populations from central and eastern Venezuela are closely related to those on the islands of Trinidad and Tobago. This genetic similarity among currently disjunct populations of M. robinsoni might have resulted from Holocene fragmentation of a more extensive Pleistocene distribution on coastal plains that were exposed during the last glacial maximum.