Research in the Miller Lab

Breeding systems in North American Lycium (Solanaceae)

Explanations for the transition from cosexuality to gender dimorphism have concentrated on overcoming the inherent 50% fitness loss of single-sexed nuclear gene mutants arising in cosexual populations. These mechanisms generally fall into two broad, non-exclusive categories: elimination of inbreeding depression by male-sterile mutants (i.e., selection for outcrossing) and compensatory resource reallocation following loss of one sexual function. My past research has focused on the relative importance of these mechanisms in the evolution of gender dimorphism in Lycium (Solanaceae).

Plants of North American Lycium californicum, L. exsertum, and L. fremontii are either male-sterile or morphologically hermaphroditic, and populations are morphologically gynodioecious. Flowers on hermaphrodites are larger and have broader calyces and corollas than those on females (Miller & Venable 2003). Controlled pollinations and allozyme estimates of mating systems in the cosexual relatives of dimorphic species indicate that gender dimorphism has evolved on a background of self-incompatibility (Miller & Venable 2002). In contrast, studies of pollen tube growth indicate that hermaphrodites in the dimorphic species are self-compatible.

To determine if females compensate for loss of male function, I estimated components of female reproduction for all three dimorphic and three cosexual species. I also investigated reallocation within flowers by quantifying the portion of total floral biomass allocated to each floral whorl. Despite substantial savings due to loss of male function, females do not produce increased numbers of seeds, fruits, or flowers, nor do they allocate additional biomass to gynoecia compared to cosexual relatives.

Within the genus, gender dimorphism is strongly associated with the presence of polyploidy (Miller & Venable 2000). Indeed, all the dimorphic taxa within Lycium are polyploids, whereas most cosexual species are diploids. We have suggested a novel hypothesis for the evolution of separate sexes involving polyploidy. In this scenario, polyploidy disrupts self-incompatibility leading to inbreeding depression. Subsequently, male-sterile mutants invade and increase because they are fully outcrossed.