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Resistance and tolerance to herbivory changes with inbreeding and ontogeny in a wild gourd (Cucurbitaceae)¹

² Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802 USA ³ Department of Biology, The Hainan Normal University, Haikou, Hainan 571158 China ⁴ Department of Biology, The Pennsylvania State University, Altoona, Pennsylvania 16601 USA ⁵ Department of Biology, The Center for Chemical Ecology and The Center for Infectious Disease Dynamics, The Pennsylvania State University, University Park, Pennsylvania 16802 USA

ABSTRACT

Herbivory is a ubiquitous component of terrestrial communities that reduces plant growth and reproduction. Consequently, a goal of evolutionary ecology is to identify the causes and consequences of variation in herbivory within plant populations. This three-year study examined the effects of inbreeding on the resistance of wild gourd plants (Cucurbita pepo subsp. texana) to herbivory by cucumber beetles and the impact of the timing of herbivory on reproduction. We grew families of inbred and outbred gourds and recorded beetle damage at three developmental stages, incidence of beetle-vectored wilt disease, survival, and reproduction. While total beetle damage significantly depressed flower and fruit production, damage until mid-July did not depress any measure of reproduction, indicating that these gourds are tolerant of moderate levels of herbivory for most of the growing season. However, beetle damage accumulating after mid-July significantly depressed reproduction, indicating that plants have reduced tolerance during peak reproduction. Early damage, however, did increase the probability of contracting a deadly wilt disease that is vectored by the beetles, suggesting that tolerance and resistance are not alternative defense strategies. Inbreeding significantly reduced resistance to herbivory and, independently of beetle damage, reproductive output. Finally, we found additive genetic variation for both resistance and tolerance that varies with ontogeny.

Key Words: bacterial wilt disease • cucumber beetles • Cucurbita • Cucurbitaceae • herbivory • inbreeding • inbreeding depression • ontogeny • resistance • tolerance

Received for publication 8 June 2007. Accepted for publication 11 November 2007.

FOOTNOTES

¹ The authors thank T. Kinney, J. Thaller, N. Myers, M. Sasuclark, G. Stephenson, and B. Miller for field and greenhouse assistance; R. Oberheim and his staff for use of the Pennsylvania State University Agriculture Experiment Station at Rock Springs, PA (Horticulture Farms); and A. Omeis for use of the Biology/Buckhout Greenhouse. This research was supported by NSF grant DEB02-35217 to A.G.S. and J.A.W. and the research of M.S. and A.D. was supported by REU supplements to NSF grant DEB02-35217. All experiments conducted in this study comply with U.S. Federal Law and the Guidelines of the Pennsylvania State University.

⁶ Author for correspondence (e-mail: as4{at}psu.edu), phone 1-814-863-1553

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