Trade-offs among anti-herbivore resistance traits: insights from Gossypieae (Malvaceae)

Plant defense theories commonly predict negative correlations among anti-herbivore resistance traits. Although this prediction has been widely accepted, the majority of empirical studies have failed to account for similarities among species due to common ancestry, thus risking pseudoreplication. Wild cotton plants possess traits conferring both direct resistance (toxic leaf glands and trichomes) and indirect resistance (extrafloral nectaries that reward enemies of herbivores). The evidence for negative phenotypic correlations among these resistance traits was examined at two levels: within Gossypium thurberi (wild cotton) and across species in the cotton clade (Gossypieae). A phylogenetic analysis controlled for shared ancestry among species. Across the Gossypieae, a strong negative correlation emerged between the direct resistance traits, leaf gland and trichomes. This correlation may reflect costs of these traits, a negative genetic correlation, or redundancy in their actions against herbivores. In contrast, the direct resistance traits (glands and trichomes) were not correlated with the indirect resistance trait of extrafloral nectar, either within or across species. The robust lack of correlation suggests that these direct and indirect resistance mechanisms evolve independently over evolutionary time scales. This conclusion conflicts with both predictions of plant defense theory and the majority of prior comparisons of direct and indirect resistance traits and may reflect the facultative nature of indirect resistance in Gossypieae.