Sexual dimorphism in pollen grain size in cryptically dioecious Thalictrum macrostylum

Abstract

While several plant species are initially described as androdioecious, upon more thorough investigation, many of these are found to be cryptically dioecious with functionally male flowers and perfect flowers that produce inaperturate pollen or are otherwise functionally female on separate plants. The change in function of pollen produced by perfect flowers opens up the possibility for the evolution of sexual dimorphism in pollen grain size. We found that Thalictrum macrostylum (Ranunculaceae) is cryptically dioecious, and produces apparently inaperturate pollen in perfect flowers. In four field sites throughout North Carolina, inaperturate grains are larger than grains from staminate flowers and also show a greater variance in size. We also found substantial variation in pollen grain size among plants. The sites with lower soil nutrient content also had smaller pollen grains of both types, although local adaptation or genetic drift may also be causing among-site variation.