Sexual Dimorphism in Cryptically Dioecious Thalictrum macrostylum

Abstract

Premise of research. Among dioecious angiosperms, occasional species exist in which one sex produces sterile reproductive structures of the opposite sex. These cryptically dioecious systems have the potential to provide significant insights into the selective factors and constraints involved in the evolution to and from dioecy. Thalictrum macrostylum is a morphologically androdioecious species in which the apparently hermaphroditic morph is functionally female.Methodology. I quantified sex ratios, the presence of sexual dimorphism, trait variation, and correlations of floral and vegetative traits in three or four natural populations. Lack of function of pollen from females was also tested. Morphological data were collected in field populations over two or three seasons to calculate the sex ratio and between-sex differences in trait means and to compare traits among populations and between and within morphs. Pollen from greenhouse-grown plants was used for in vitro pollen germination assays.Pivotal results. Thalictrum macrostylum populations consistently have 1∶1 sex ratios and are significantly sexually dimorphic, with males having more stamens per flower and flowers per plant. Pollen collected from females does not germinate. Other vegetative characters generally did not differ consistently between the sexes, though among-population differences were seen for some floral and vegetative characters.Conclusions. Lack of germination in pollen from females and 1∶1 sex ratios strongly suggest that this species is cryptically dioecious. Greater per-flower stamen number and per-plant flower number in males suggest that stamen and pollen production are important to male fitness. Among-population differences in floral and vegetative trait means may indicate differences among environments. Further studies should aim to test hypotheses for the maintenance of stamens in females of T. macrostylum.