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Few studies have examined the potential for pathogens with complex life cycles to cause selection on their required alternate (=intermediate) hosts. Here we examine the effects of two fungal pathogens on an herbaceous mustard, Arabis holboellii. One pathogen species uses A. holboellii as a primary host, the other uses it as an alternate host. This plant-pathogen system is especially interesting because the host, A. holboellii, is apomictic; thus individuals reproduce exact copies of themselves. Despite this mode of reproduction, A. holboellii populations are surprisingly genetically diverse. Could frequency dependent selection by pathogens be maintaining clonal diversity? This study assesses the potential for selection by pathogens. In a controlled greehouse experiment we show that there is heritable variation in A. holboellii's resistance to the rust, Puccinia monoica, and that host fitness is severely reduced by P. monoica infection in both the greenhouse and under natural conditions. Field observations indicate that host clones are also differentially susceptible to the short-cycled rust, P. thlaspeos, and that host fitness is reduced by infection to this pathogen as well. Although the preconditions for pathogen-mediated selection are present, frequency-dependent selection by pathogens is unlikely to be important in structuring populations of Arabis holboellii because multiple host genotypes are susceptible to the same inoculum and the pathogen has a long generation time.

Jack-in-the-pulpit, Arisaema triphyllum, is a perennial forest herb with the ability to change sex. At two sites in upstate New York, plant sex was correlated with plant size: males were smaller than females, nonflowering plants were smaller than males. Changes in plant size were accompanied by changes in sex. Sex change occurred quite frequently; at one site, 8% of nonflowering plants, 64% of males, and 63% of females changed their sex from one season to the next. The probability that a plant will change size and sex between years was altered by artificial defoliation and by the production of seeds, but was not affected by supplementing plants with nutrient fertilizer. Discriminant analysis indicated that several historical factors significantly affected plant sex: a model including the variables of current plant size, previous year's plant size, and previous year's sex was significantly better at predicting the current sex of individuals than was a model containing only current plant size. However, even the consideration of these three variables left up to a third of the plants misclassified with respect to gender. This analysis explains in part why plants at the two sites changed sex at different sizes, but it is likely that other factors-e.g. genetic differences-are involved.