Evol Ecol Res 9: 1319-1328 (2007)     Full PDF if your library subscribes.

Herbivory as an agent of natural selection for floral-sex ratio in horsenettle (Solanum carolinense)

Michael J. Wise1* and Jeremiah J. Cummins2

1Department of Biology, Duke University, Durham, NC 27708 and  2Department of Biology, University of North Carolina, Chapel Hill, NC 27599, USA

Address all correspondence to M.J. Wise, Department of Biology, Bucknell University, Lewisburg, PA 17837, USA.
e-mail: mwise@bucknell.edu


Hypothesis: Floral herbivory can affect the strength or pattern of natural selection for floral-sex ratio in a host plant with sexually dimorphic flowers.

Organisms: The andromonoecious herb horsenettle (Solanum carolinense) and its main floral herbivore, the potato bud weevil (Anthonomus nigrinus).

Methods: In a controlled experiment with potted plants, we exposed a group of 27 plants to simulated-weevil herbivory on 50% of their flower buds, the florivory rate found in a field study. A control group of 27 plants received no florivory. We used phenotypic-selection analyses to look for directional, disruptive, and stabilizing selection on floral-sex ratio in the two groups and to determine whether florivory changed the optimum ratio.

Results: In the absence of florivory, directional selection acted to increase the percentage of male flowers, at least to a ratio of 67% males and 33% perfect flowers. Florivory changed the pattern to stabilizing selection, with an optimum of 9% male and 91% perfect flowers.

Keywords: andromonoecy, floral-sex ratio, florivory, phenotypic-selection analysis, plant-breeding system.

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