Abstract
Ecological research on ant/plant symbioses in Fiji, combined with molecular phylogenetics, has brought to light four new species of Squamellaria in the subtribe Hydnophytinae of the Rubiaceae tribe Psychotrieae and revealed that four other species, previously in Hydnophytum, need to be transferred to Squamellaria. The diagnoses of the new species are based on morphological and DNA traits, with further insights from microCT scanning of flowers and leaf δ13C ratios (associated with Crassulacean acid metabolism). Our field and phylogenetic work results in a new circumscription of the genus Squamellaria, which now contains 12 species (to which we also provide a taxonomic key), not 3 as in the last revision. A clock-dated phylogeny and a model-testing biogeographic framework were used to infer the broader geographic history of rubiaceous ant plants in the Pacific, specifically the successive expansion of Squamellaria to Vanuatu, the Solomon Islands, and Fiji. The colonization of Vanuatu may have occurred from Fiji, when these islands were still in the same insular arc, while the colonization of the Solomon islands may have occurred after the separation of this island from the Fiji/Vanuatu arc. Some of these ant-housing epiphytes must have dispersed with their specialized ants, for instance attached to floating timber. Others acquired new ant symbionts on different islands.
Highlights
The angiosperm family with the highest diversity of ant-plant is the Rubiaceae [1]
Based on molecular-phylogenetic data, the four new species belong in the genus Squamellaria in the Hydnophytinae, a subtribe erected by Huxley and Jebb [2] to set apart a group of epiphytic ant plants from the rest of the Psychotrieae, which contain over 2000
The electronic version of this article in Portable Document Format (PDF) in a work with an ISSN or ISBN will represent a published work according to the International Code of Nomenclature for algae, fungi, and plants, and the new names contained in the electronic publication of a PLOS article are effectively published under that Code from the electronic edition alone, so there is no longer any need to provide printed copies
Summary
The angiosperm family with the highest diversity of ant-plant is the Rubiaceae [1]. In Southeast Asia, it is the tribe Psychotrieae that is especially rich in epiphytic species occupied by ants living in specialized domatia. During ecological research on ant/plant symbioses in the Psychotrieae of Fiji, we discovered several new species that we are here placing in a phylogenetic and biogeographic context. Based on molecular-phylogenetic data, the four new species belong in the genus Squamellaria in the Hydnophytinae, a subtribe erected by Huxley and Jebb [2] to set apart a group of epiphytic ant plants from the rest of the Psychotrieae, which contain over 2000. Four New Ant-Plant Species Illuminate the Biogeography of Squamellaria in the South Pacific
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