THE DISTRIBUTION AND PHYLOGENETIC SIGNIFICANCE OF BINUCLEATE AND TRINUCLEATE POLLEN GRAINS IN THE ANGIOSPERMS

Studies of the cytology of angiosperm pollen which extend our knowledge of the distribution of binucleate and trinucleate pollen to almost 2,000 species of flowering plants are summarized. Approximately 70% of the species studied release pollen in a binucleate stage, and none of them shed both types of pollen as a constant varietal trait. All phylogenetically primitive taxa are binucleate. The trinucleate trait evidently has originated independently at many times during angiosperm evolution. In no instance must one infer the origin of binucleate taxa from trinucleate ancestors. Most genera are monotypic with respect to pollen cytology, and only 10 genera are known to include both binucleate and trinucleate species. Among the 265 families studied, 179 include only binucleate genera and 54 include only trinucleate genera, while 32 include both types. Nearly all aquatic species with submersed flowers shed pollen in a trinucleate stage. A group of physiological differences which distinguish binucleate and trinucleate pollen is reviewed in relation to the survival and possible selective advantage of trinucleate mutants, and the significance of the binucleate pollen grain in the origin and evolution of flowering plants is discussed.