ORIGINAL ARTICLE: Freshwater paths across the ocean: molecular phylogeny of the frog Ptychadena newtoni gives insights into amphibian colonization of oceanic islands

Abstract

Aim Amphibians are a model group for studies of the biogeographical origins of salt‐intolerant taxa on oceanic islands. We used the Gulf of Guinea islands to explore the biogeographical origins of island endemism of one species of frog, and used this to gain insights into potential colonization mechanisms. Location São Tomé and Príncipe, two of the four major islands in the Gulf of Guinea, West Africa, are truly oceanic and have an exceptionally high biodiversity. Methods Mitochondrial DNA is used to test the endemic status of a frog from São Tomé and compare it with congeneric taxa from tropical Africa. Existing data on surface currents, surface salinity, atmospheric circulation and bird migration in the Gulf of Guinea are summarized to address hypotheses concerning colonization mechanisms. Results The endemic status of Ptychadena newtoni (Bocage) is supported here by mitochondrial DNA sequences, and analysis of this and other molecular data indicates that an East African species close to Ptychadena mascareniensis (Duméril and Bibron) is its nearest relative. We refute the possibility that this population was anthropogenically introduced, in favour of a natural dispersal mechanism. Main conclusions With six endemic frogs and one caecilian, the Gulf of Guinea islands harbour a diverse amphibian fauna. Five of these species appear to have their closest relatives in East Africa. Insufficient evidence exists for transportation by storms, birds or rafts alone. However, we propose a synergy of rafting, favourable surface currents and a reduction in salinity of surface waters. Catastrophic events, or wet periods in climatic history, could allow freshwater paths to open far enough to enable continental flora and fauna to reach these and other isolated oceanic islands.

Large-scale phylogeny of chameleons suggests African origins and Eocene diversification

“…Oceanic dispersal has become fundamental to many biogeographic and diversification hypotheses [28,[73][74][75][76][77][78] even for taxa which were traditionally considered poor dispersers, such as amphibians [79][80][81]. Consideration of oceanic palaeo-currents and the timing of diversification events are both necessary components of many biogeographic scenarios.…”

How do seemingly non-vagile clades accomplish trans-marine dispersal? Trait and dispersal evolution in the landfowl (Aves: Galliformes)

“…The chance dispersal hypothesis relies on rare, stochastic events to explain colonization by poor dispersers over evolutionary time [9][10][11]. Examples include New World monkeys, rodents and salt-intolerant amphibians that rafted between continents or islands on floating debris [12][13][14][15]. Alternatively, the vagile ancestor hypothesis posits that nondispersive lineages are descended from mobile ancestors that reached new islands and continents by colonizing across geographical barriers.…”

Genetic drift and rapid evolution of viviparity in insular fire salamanders (Salamandra salamandra)

“…In general, the presence of amphibians on islands can be explained by (1) population isolation before island formation and (2) overseas colonization or intentional/accidental anthropogenic introductions from adjacent mainland populations. Amphibian tolerance to seawater varies among species, and long-distance movements by rafting have been proposed as a possible mechanisms of colonization for continental islands (Hedges et al, 1992;Vences et al, 2003;Measey et al, 2007), suggesting that seawater is not necessarily an absolute barrier to gene flow. In this study we found high levels of genetic differentiation and absence of gene flow between island and coastal populations (F ST ¼0.181-0.287, Tables 2 and 3; Figure 2).…”