Recurrent short-distance dispersal explains wide distributions of hydrophytic umbellifers (Apiaceae tribe Oenantheae)
Long-distance dispersal (LDD) by migratory birds is often invoked to explain the broad and disjunct distributions of many aquatic plants. Such distributions may also be achieved by recurrent short-distance dispersal (SDD) to adjacent areas and extinctions in connecting areas. To test the relative importance of LDD and SDD in shaping the distributions of hydrophytes, we examined different dispersal models for a clade of hydrophytic umbellifers (Apiaceae tribe Oenantheae).
Worldwide, with emphasis on Eurasian–North American disjunctions.
A dated phylogeny of the group was obtained with Bayesian methods using nrDNA internal transcribed spacer (ITS) and cpDNA rps16–trnK sequences from 100 species and infraspecific taxa of Oenantheae and two outgroup species. Ancestral habits were inferred using maximum likelihood (R package ape). Six connectivity models were compared using a maximum-likelihood-based method (Lagrange), four with symmetrical dispersal rate matrices and two assuming asymmetrical exchanges between Eurasia and North America, with or without spatial and temporal constraints on LDD.
The age of the crown node of Oenantheae was estimated at 26.3 Ma, and western Europe was reconstructed as its most likely ancestral area. The divergence between Peucedanum sandwicense, an endemic of Hawaii, and its sister Oenanthe occurred 17.2 Ma, pre-dating the emergence of the Hawaiian Islands. Throughout the phylogeny, the hydrophytic habit (including helophytes and amphiphytes) dominated. Of the six connectivity models considered, the model restricting all intracontinental and transoceanic LDDs and assuming an almost unidirectional dispersal from Eurasia to North America received the highest likelihood score. This model was also characterized by the highest dispersal rate. A stratified model assuming a higher probability for dispersals between Eurasia and North America when these continents were connected with land bridges received a lower likelihood score.
The results suggest that the broad and often disjunct distribution of Oenantheae hydrophytes is mostly achieved through an increased dispersal rate and recurrent SDD rather than frequent LDD. Our data confirm the asymmetry of the floristic exchange between Eurasia and North America and do not support the increase of this exchange when the continents were connected with land bridges.