Pollination of thermogenic inflorescence of Pandanus odorifer by a specialist Amystrops sap beetle that reproduces on the male inflorescence

Abstract

Pandanus is a morphologically unique plant group of the Paleotropical flora, with approximately 450 known species occupying diverse habitats. Because the male inflorescences of this genus are pendent and nectarless, they are commonly assumed to be wind-pollinated; however, their floral scent coupled with the observation that female inflorescences mature while still covered by bracts suggest the possibility of an alternative pollination mechanism. We studied the pollination system of Pandanus odorifer in subtropical Japan. A peroxidase assay was conducted to determine the timing of stigma receptivity, which showed that the stigmas were sensitive to peroxidase while the female inflorescences were covered by bracts. We found numerous pollen-bearing adult beetles of the genus Amystrops on both male and female inflorescences. Nitidulid larvae were also found on male inflorescences, and the larvae became adults of the same Amystrops species as found on the inflorescences by feeding only on Pandanus pollen. Last, floral temperature was monitored to test for floral thermogenesis. Both male and female inflorescences were thermogenic at night when insect visitation occurred. This study provides the first examples of insect pollination and floral thermogenesis in Pandanaceae. Most known Amystrops species are associated with Pandanus inflorescences, suggesting a widespread, specialized Pandanus–Amystrops association.