Cryptic Dioecy and Sexual Dimorphism in Thalictrum macrostylum
Prior to my work (independent and with Dr. Janet Steven), it was assumed that Thalictrum macrostylum was androdioecious, existing in populations composed of male individuals and hermaphroditic individuals. This mating system is exceptionally rare because of the stringent conditions under which a male would have a fitness advantage over a hermaphrodite. [Representative flowers from male and female plants pictured, right]
My work (Penny and Steven 2009, Penny 2014) demonstrates that pollen produced by the morphological hermaphrodites (pictured top) is without germination apertures and does not germinate. This renders these individuals female rather than hermaphroditic, and the populations are thus dioecious.
Interestingly, females produce a similar quantity of pollen per anther as do males, but females have significantly fewer stamens per flower. Males also make more flowers per plant than females, and in one population, stamens were longer in males. Populations also exhibit 1:1 sex ratios, which is characteristic of fully dioecious populations (where individuals are either male or female).
Flower from male (stamens only, left) and female (pistils and stamens, right) T. macrostylum plants
Pollen from female (inaperturate, left) and male (aperturate, right) T. macrostylum plants
Influence of Sterile Stamens in females of the Cryptically Dioecious Thalictrum macrostylum
The predominant hypothesis used to explain the presence of cryptic dioecy as seen in T. macrostylum is that of pollinator attraction. This hypothesis reasons that stamens are retained by females to maintain visitation by pollinating insects.
I find that females of T. macrostylum are not significantly insect pollinated, refuting the applicability of the pollinator-attraction hypothesis (Humphrey, in review). I further test and refute two original hypotheses: that stamens in females positively influence receipt of air-borne pollen (aerodynamic influence hypothesis) and that presence of inaperturate pollen on female stigmas increases female fitness through enhanced germination of functional pollen (from males), which may be deposited in low densities (stigmatic-priming hypothesis).
The rejection of these three compelling hypotheses suggests that another force is in play to maintain stamens in females of this functionally dioecious species. A likely possibility is that between-sex genetic correlations maintain stamens in females given positive selection on stamen production in males.
