Secondary Metabolites of Ripe Fleshy Fruits: Ecology and Phylogeny in the Genus Solanum

Why do ripe fleshy fruits contain secondary metabolites, sometimes in concentrations that make the fruits toxic to vertebrates? And how do the chemical and physical characteristics of fruits influence choice by frugivores? These questions have drawn the attention of those interested in understanding the adaptive roles of fruit traits (see Cipollini and Levey, 1997a, b,c; Cipollini, 2000). However, a criticism of such studies is that few have addressed the possible influence of physiological and phylogenetic constraints on fruit traits (see Cipollini and Levey, 1998; Eriksson and Ehrlen, 1998). We are addressing this concern by studying fruit traits of the genus Solanum, while assessing and accounting for phylogenetic effects. Here we provide a preliminary analysis of our data, which we hope will stimulate others to conduct similar analyses. Theoretically, predictable differences in the quality of seed dispersal among frugivores should influence the evolution of fruit traits (see Janson, 1983; Wheelwright, 1985; Jordano 1987a; Debussche and Isenmann, 1989; Willson et al., 1989; Gautier-Hion, 1990; Stiles and Rosselli, 1992). Nevertheless, studies using phylogenetic null models have not supported this theory (see Bremer and Eriksson, 1992; Herrera, 1992; Jordano, 1995). This lack of support is assumed to result, in part, because of extensive decoupling of interactions between particular frugivores and the plants whose seeds they disperse (see Howe, 1984; Jordano, 1987b; Herrera, 1998). However, most studies on the evolution of fruit traits have overlooked the effects of secondary metabolites, which are probably important mediators of fruit– frugivore interactions (Cipollini and Levey, 1997c, 1998). We have presented several adaptive hypotheses on the adaptive significance of secondary metabolites in ripe fleshy fruits (Cipollini, 2000). In general, these hypotheses assume a selective advantage to plants bearing fruits containing secondary metabolites and thus predict that patterns of secondary chemistry in fruits can be explained, at least in part, by interactions with frugivores. Likewise, the hypotheses