RENNER, SUSANNE S. Department of Biology, University of Missouri-St. Louis, St. Louis, MO 63121. - Transatlantic disjunctions in the angiosperms in relation to Gondwana break-up and ocean currents at relevant times in the past.
A review of amphi-Atlantic and amphi-Pacific disjunctions in tropical
seed plants by Thorne (1973) provides numerous examples of
disjunctions at hierarchical levels from sister populations to species
to subfamilies. These have to date back to different times, Thorne
argued, with many explainable by long distance dispersal. Here, I
concentrate on the subset of angiosperms with transatlantic
disjunctions of which Thorne presents 111 examples at the species and
genus level. I address three questions: First, how are molecular data
affecting Thorne's list? Second, for disjunct lineages with
phylogenies, what are the inferred directions and modes of
transatlantic crossing? Third, for lineages with molecular clock
estimates for the disjunction event, can we use recent models of ocean
circulation patterns to reject/corroborate inferences? The answer to Q
1 is that Thorne's list has shrunk, with many supposed relatives no
longer considered close, presumably pushing the ages of the respective
disjunctions back in time. Molecular work has added a few new cases,
but many relevant phylogenies lack key West African taxa. The answer
to Q 2 appears to be that W to E is more common than E to W. I am
still trying to find a way to assess how representative the data may
be. Although transport by wind/storm/birds has been invoked, there is
good evidence only for dispersal by ocean currents. This leads to Q 3
and to paleo-ocean circulation models for the times relevant to
molecular clock-dated disjunction events as well as to the World Ocean
Circulation Experiment, which involves compilation of large data sets
from satellite-tracked drifters. Swiftest drifter motions occur in the
westward high-velocity South Equatorial Current (Gulf of Guinea to
Brazil) and the eastward North Equatorial Counter Current (Northern
Brazil to Gulf of Guinea). Notably, even small-scale experiments, such
as studies of drift bottles, show that 1% make it from Africa to the
US.
Key words: angiosperms, biogeography, dispersal, molecular clock, ocean currents, transatlantic disjunctions