In collaboration with Pam Diggle at the University of Colorado, I have investigated the origin and diversification of andromonoecy among ca. 15 species in Solanum sections Lasiocarpa and Acanthophora.

Andromonoecy is a sexual system in which all plants in populations produce two types of flowers: hermaphroditic and staminate (i.e., male).  For some species, the relative production of hermaphroditic and staminate flowers is phenotypically plastic and changes depending on the environment, whereas in other species the sexual phenotype is apparently fixed.  Moreover, in several species, floral sexual size dimorphism appears to be the result of differences in the positions of hermaphroditic and staminate flowers within inflorescences (Diggle and Miller 2004).

For all species, staminate flower production occurs within the constraints set up by whole plant architecture; staminate flowers are more likely to be produced in distal floral positions within inflorescences and also in inflorescences initiated later in development (see figure below).  The diversification of andromonoecy among species is the result of species specific differences in phenotypic plasticity and quantitative changes in total flower production and the architectural pattern of staminate flower production (Miller and Diggle 2003).

We documented that the strength of andromonoecy, defined as the proportion of staminate flowers produced within inflorescences, was significantly and positively associated with fruit mass in both naive and phylogenetically independent analyses. Our results are consistent with the hypothesis that andromonoecy functions as a mechanism to regulate allocation to female function and suggest that the strength of andromonoecy is associated with resource limitation (Miller and Diggle 2007). 

I remain interested in Solanum and, in particular, in the hypothesis that andromonoecy has arisen due to interference between male and female sexual function within individual flowers.  I plan to experimentally test this idea by manipulating style length in flowers and quantifying pollen export from the anthers.

Evolutionary diversification of andromonoecy in Solanum


Solanum palinacanthum inflorescence (@ left) with three open flowers; note that the first two flowers are hermaphroditic, whereas the third flower is staminate.  A hermaphroditic flower (left flower in picture @ right) and a functionally staminate flower (right flower in picture @ right).

photo credit: JSM