The genus Solanum is predominately hermaphroditic in sexual expression, but andromonoecy has evolved numerous times within the genus. Andromonoecy shows a single origin in the subgenus Leptostemonum, and within section Lasiocarpa there has apparently been diversification in the expression of andromonoecy; the number and proportion of staminate flowers per inflorescence varies among species. Experimental analysis of sexual expression in one species of Lasiocarpa, shows that the production of staminate flowers is phenotypically plastic. Therefore, the observed variation in the expression of andromonoecy among species may be the result of plasticity. We assessed both plasticity and andromonoecy for four species in section Lasiocarpa: Solanum candidum, S. hirtum, S. pseudolulo, and S. quitoense. Replicates of eight genotypes of each species were grown under two treatments: high fruit set and no fruit set. When andromonoecy is defined as the mean proportion of staminate flowers produced by fruit-bearing plants, then andromonoecy varies from 25% staminate flowers in S. hirtum to nearly 65% staminate flowers in S. quitoense. The species also differ in plasticity, measured as the difference between the mean proportion of staminate flowers per inflorescence on fruit bearing and fruit-less plants. S. hirtum, S. candidum, and S. pseudolulo are all plastic, that is, more staminate flowers are produced by fruit bearing plants, but the degree of staminate flower production in the absence of fruit, as well as the magnitude of change in sexual expression between treatments, differ among the species. Solanum quitoense, the species with the "strongest" andromonoecy is not plastic; a large proportion of flowers are staminate regardless of fruiting status. Analysis of these characters within the phylogeny of Bruneau et al. 1995, shows that plasticity of sex expression is likely ancestral within Lasiocarpa and that the fixed production of large numbers of staminate flowers by S. quitoense is derived.

Key words: andromonoecy, phenotypic plasticity, Solanaceae, Solanum