Testing the emergence of New Caledonia: fig wasp mutualism as a case study and a review of evidence

Abstract

While geologists suggest that New Caledonian main island (Grande Terre) was submerged until ca 37 Ma, biologists are struck by the presence of supposedly Gondwanan groups on the island. Among these groups are the Oreosycea fig trees (Ficus, Moraceae) and their Dolichoris pollinators (Hymenoptera, Agaonidae). These partners are distributed in the Paleotropics and Australasia, suggesting that their presence on New Caledonia could result from Gondwanan vicariance. To test this hypothesis, we obtained mitochondrial and nuclear markers (5.3 kb) from 28 species of Dolichoris, used all available sequences for Oreosycea, and conducted phylogenetic and dating analyses with several calibration strategies. All our analyses ruled out a vicariance scenario suggesting instead that New Caledonian colonization by Dolichoris and Oreosycea involved dispersal across islands from Sundaland ca 45.9-32.0 Ma. Our results show that successful long-distance dispersal of obligate mutualists may happen further suggesting that presence of intimate mutualisms on isolated islands should not be used as a priori evidence for vicariance. Comparing our results to a review of all the published age estimates for New Caledonian plant and animal taxa, we showed that support for a vicariant origin of the island biota is still lacking. Finally, as demonstrating a causal relationship between geology and biology requires independent evidence, we argue that a priori assumptions about vicariance or dispersal should not be used to constrain chronograms. This circular reasoning could lead to under or overestimation of age estimates.

Figure 1. Maximum clade credibility trees obtained for Oreosycea fig trees (a) and chronogram showing the timing of evolution of their pollinating wasps (b).

The chronogram showing the timing of evolution of Dolichoris fig wasps is derived from the sixth calibration set shown in Table 1. Grey bars around mean node ages (Ma) indicate the 95% HPD intervals. The geological time-scale is shown at the bottom. Bootstrap supports (higher than 65) and posterior probabilities (higher than 0.90) are indicated at nodes. The delineation of biogeographical regions follows Kreft Afr: Africa, Ar-Sin: Arabo-Sindic; Aus: Australia, Ind-Chi: Indo-China, Ind: India, Med: Mediterranean; NC: New Caledonia, NG: New Guinea, Phi: Philippines, Sul: Sulawesi, Sun: Sundaland, Van: Vanuatu. Stars indicate probable misidentifications of host Ficus in Genbank. Dolichoris and Oreosycea species are highlighted in bold blue.

Figure 2. Review of the divergence time estimates for 50 New Caledonian clades.

Taxon names refer to the list provided in Table S4. “Dolichoris 2” refers to the present study.