MILLER, JILL S.* and PAMELA K. DIGGLE. Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309. - Evolutionary Diversification of Plastic Sex Expression in Andromonoecious Solanum section Lasiocarpa.
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