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Synchrony, function, and diversification of floral scent in Hawaiian Schiedea (Caryophyllaceae)

Creative Commons 'BY-ND' version 4.0 license

Floral scent is a complex form of chemical communication between plants and pollinators that may be a part of the evolution of plant reproduction. I studied temporal, geographic, and taxonomic variation in the floral scents of species and hybrids of Schiedea (Caryophyllaceae), an endemic Hawaiian genus with diverse pollination modes. To measure genetic variation in the composition and timing of floral volatile emissions, I used dynamic headspace sampling in a common environment followed by gas chromatography – mass spectrometry or proton transfer reaction – mass spectrometry.

The individual regulation of discrete volatiles allows plants to produce scent blends with multiple functions throughout the day and night. Two sympatric species from different clades in this group, Schiedea kaalae and S. hookeri, were hypothesized to have similar scents that attract their shared moth pollinator (Pseudoschrankia brevipalpis), but instead they produce qualitatively distinct scents that diverge at night and are dominated by volatiles from different chemical classes. However, the daily timings of these scent emissions are similar, and in S. kaalae volatiles that are known to attract moths peak within the period of moth visitation in the field, while others peak during the day. The evening scent of S. kaalae flowers is sufficient to attract moths against a background of scent and visual cues from two wind-pollinated relatives from the same island, demonstrating how moths respond to species-specific scent cues.

If scent plays a role in pollinator attraction, it can maintain or erode species boundaries that contribute to adaptive radiations. Floral scent production could affect the ability of hybrids to attract pollinators and contribute to gene flow between species. Artificially produced reciprocal hybrids between these two sympatric Schiedea species have mostly intermediate scent phenotypes that combine the sets of volatiles produced by each species, at similar total rates of emission, so scent might not represent a postzygotic barrier.

Recent evolution of separate sexes from hermaphroditism and wind pollination from biotic pollination in Schiedea may affect the functions of floral scent, and emissions of attractive compounds may be lost. In S. globosa, a subdioecious species distributed over multiple islands, scent composition varies over the course of the day, between sexes, and across reproductively isolated populations, and interpopulation scent divergence increases with genetic distance in males. The nightly increase in a set of volatiles was unexpected for a wind-pollinated species, especially because this set was not shared with two moth-pollinated relatives. This work highlights the differences in floral scent in related species that have evolved similar and distinct modes of pollination, describes the genetic variation in scent among isolated populations that may lead to scent divergence at longer evolutionary timescales, and suggests that some but not all temporal changes in floral scent are related to pollination.

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