Radiation following long-distance dispersal: the contributions of time, opportunity and diaspore morphology in 'Sicyos' (Cucurbitaceae)

Abstract

Aim: To infer the most plausible explanations for the presence of 14 species of the Neotropical cucurbit genus 'Sicyos' on the Hawaiian Islands, two on the Galapagos Islands, two in Australia, and one in New Zealand. Location: Neotropics, the Hawaiian and Galapagos archipelagos, Australia and New Zealand. Methods: We tested long-problematic generic boundaries in the tribe 'Sicyoeae' and reconstructed the history of Sicyos using plastid and nuclear DNA sequences from 87 species (many with multiple accessions) representing the group's generic and geographic diversity. Maximum likelihood and Bayesian approaches were used to infer relationships, divergence times, biogeographic history and ancestral traits. Results: Thirteen smaller genera, including 'Sechium', are embedded in 'Sicyos', which when re-circumscribed as a monophyletic group comprises 75 species. The 14 Hawaiian species of Sicyos descended from a single ancestor that arrived c. 3 million years ago (Ma), Galápagos was reached twice at c. 4.5 and 1 Ma, the species in Australia descended from a Neotropical ancestor (c. 2 Ma), and New Zealand was reached from Australia. Time since arrival thus does not correlate with 'Sicyos' species numbers on the two archipelagos. Main conclusions: A plausible mechanism for the four trans-Pacific dispersal events is adherence to birds of the tiny hard fruit with retrorsely barbed spines found in those lineages that underwent long-distance migrations. The Hawaiian clade has lost these spines, resulting in a lower dispersal ability compared with the Galapagos and Australian lineages, and perhaps favouring allopatric speciation.