Although flowering plants are famous for their diversity of life histories and reproductive systems, this diversity has considerable structure. Particular features have evolved many times independently, in distantly related plants, and some traits are much more common than others. For example, most species of flowering plants are hermaphroditic, with each individual capable of reproducing as both a male and a female parent. The other reasonably common sexual systems are dioecy (separate males + females) and gynodioecy (females + hermaphrodites). Why is hermaphroditism the most common system, and why are some systems (e.g., androdioecy: males + hermaphrodites) vanishingly rare?

One of my main research foci has been the analysis of the role of resource allocation trade-offs in the evolution of plant sexual systems. Studies of plant sexual systems traditionally invoked the disadvantages of inbreeding as the dominant factors favoring the evolution of separate sexes, which cannot self-pollinate. A portion of my dissertation work showed that sexual-system evolution can be driven by the fitness consequences of resource allocation, even in the absence of selection against inbreeding.

In more recent work I have focused on a idea developed independently by me and by a few researchers elsewhere. Namely, the fact that resource allocation to male reproductive function in seed plants precedes the bulk of allocation to female function has unexpected consequences. This timing implies that apparently small, early investments in male function can incur large opportunity costs to later female function. Because such opportunity costs appear to be especially severe in low-nutrient environments, this line of research has implications for the regulation of seed yield in economic species as well as for the evolution of sex expression in wild ones.

• Eckhart, V.M. and J. Seger 1999. Phenological and developmental costs of male sex function in hermaphroditic plants. pp. 195-214 in: Vuorisalo, T., and P. Mutakainen (eds.) Life history evolution in plants. Kluwer Academic, Doordrecht.
• Eckhart, V.M. 1999. Sexual dimorphism in flowers and inflorescences. pp. 123-148 in: Geber, M.A., T.E. Dawson, and L.F. Delph (eds.). Gender and sexual dimorphism in flowering plants. Springer, Berlin.
• Eckhart, V.M., and F.S. Chapin, III. 1997. Nutrient sensitivity of the cost of male function in gynodioecious Phacelia linearis (Hydrophyllaceae). American Journal of Botany 84:1092-1098.
• Seger, J., and V.M. Eckhart. 1996. The evolution of sexual systems and sex allocation in hermaphroditic plants when growth and reproduction overlap. Proceedings of the Royal Society of London B 263:833-841.
• Eckhart, V.M. 1993. Do hermaphrodites of gynodioecious Phacelia linearis (Hydrophyllaceae) trade-off seed production to attract pollinators? Biological Journal of the Linnean Society 50:47-63.
• Eckhart, V.M. 1992a. Resource compensation and the evolution of gynodioecy in Phacelia linearis (Hydrophyllaceae). Evolution 46:1313-1328.
• Eckhart, V.M. 1992b. Spatio-temporal variation in abundance and variation in foraging behavior of the pollinators of gynodioecious Phacelia linearis (Hydrophyllaceae). Oikos 64:573-586.
• Eckhart, V.M. 1992c The genetics of gender and the effects of gender on floral characters in gynodioecious Phacelia linearis (Hydrophyllaceae). American Journal of Botany 79:792-800.
• Bierxychudek, P., and V. Eckhart. 1988. Spatial segregation of the sexes of dioecious plants. American Naturalist.