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Oceanic islands have long been considered ‘natural laboratories’ and have provided many key insights into ecological and evolutionary processes. Because the flora of islands originates largely from the nearest mainland source populations, immigration and extinction rates are dependent on both the distance to the mainland and the size of the island. Plant communities on increasingly remote islands should have greater capacity for long-distance dispersal. They must also possess reproductive traits that allow establishment from single, rather than multiple concurrent, introductions. Community-level studies of plant reproductive biology have provided insights into colonization history on a number of oceanic island systems, but studies on extremely isolated oceanic islands are still relatively rare.
In their new work published in AoBP, Yomai & Williams characterized the breeding systems of 28 flowering species on Pohnpei, the largest (335 km2) and highest (~800 m) island in the Federated States of Micronesia, a group of remote Pacific islands that are considered a global biodiversity hotspot. The authors hypothesized that recently ‘naturalized’ colonists would be able to both self-pollinate and self-fertilize, whereas older ‘indigenous’ species may evolve outcrossing mechanisms. Three families present on the island with both naturalized and indigenous species were selected – Fabaceae, Malvaceae and Melastomataceae. Measurements included field observations of dichogamy/herkogamy and floral attraction traits, pollen:ovule ratios and experimental hand-pollinations for assement of self-compatibility and pollen limitation.
In this, the first study of plant breeding systems on Pohnpei island, flowers of all 28 studied species exhibited overlapping male and female phases and short anther–stigma distances. Low pollen:ovule ratios, ranging from 9 to 557, suggested self-fertilization is common. Contrary to the hypothesis of Yomai & Williams, the breeding systems of indigenous species were no different than those naturalized species. Therefore the results provide evidence of a strong establishment filter and selection for reproductive assurance on oceanic islands. The authors concluded that on Pohnpei, high ovule numbers, and the inaccessibility of wind pollination and obligate outcrossing strategies, reflect the importance of retaining reproductive assurance mechanisms in the face of pollinator uncertainty.