The age of amphitropic plant disjuncts coincides with the Great American Biotic Interchange

Background/Question/Methods

Long distance plant disjunctions are mysterious. Over 130 closely related plants are distributed in California and western South America, yet nowhere in between. Charles Darwin posited that avian dispersal created these 5,000-10,000 km gaps in distributions, known as amphitropic disjunctions. Many of these plants lack obvious characteristics that would facilitate bird dispersal (e.g., edible berries, barbed fruits, or sticky seeds) and do not currently occupy habitats conducive to dispersal by the few coastal bird species known to make long migrations. Although the avian dispersal hypothesis is widely accepted, it may not be the only mechanism explaining the dispersal of plants from California to South America. Approximately 3 million years ago, the final closing of the Isthmus of Panama afforded an unprecedented migration of large, flocculent animals from North to South America. This primarily mammalian migration, known as the Great American Biotic Interchange (GABI), lasted until approximately 1.4 million years ago and provides an alternative hypothesis explaining amphitropic plant disjunctions. We examined the directionality and age of 34 amphitropic dispersal events representing 31 disjunct plant genera using molecular phylogenies. We built phylogenetic trees for these lineages using sequences from the commonly utilized nuclear ribosomal ITS region available in Genbank. We used parsimony and maximum-likelihood ancestral-state-reconstructions to infer the directionality of the dispersal events. We then dated these events using an approximate molecular clock by applying previously reported ITS substitution rates in BEAST to generate time-calibrated phylogenies for each amphitropic disjunction. From these trees, we extracted the range of ages of the South American lineages using stem dates (maximum dispersal age) and crown dates (minimum dispersal age). This approach prompted the following questions: 1. Did most plants disperse in the same direction as the GABI migration? 2. Do the dates of these dispersal events coincide with the timing of the GABI migration?

Results/Conclusions

The results strongly support GABI as an alternative mechanism for amphitropic disjunct plants. First, ancestral-state-reconstructions indicate 31 of the 34 events proceeded from North to South America – reflecting the primary direction of the GABI migration. Second, amphitropic dispersal dates ranged from 0.27 to 16.41 million years ago. The median crown and stem ages were 2.09 and 3.15 million years, respectively. Furthermore, 85.3% of sampled disjuncts have median dispersal dates that coincide with GABI. Clearly, GABI provides a plausible, if not likely, alternative mechanism to avian dispersal that may help explain the mystery of long distance amphitropic plant disjunctions.