Intraspecific pollen morphological variations and their importance to characterize species boundaries: A review.

Abstract

Intraspecific structural variations in pollen grains are widespread in angiosperms. The aims of the present review are to track down intraspecific variability in various pollen morphological features, and to evaluate their taxonomic significance in defining species boundaries. Shape of pollen grains, sculptures, aperture types and numbers, muri textures, presence/absence of opercula, folded ectexines and pollen corpus (in bisaccate pollen grains) were encountered to exhibit intraspecific variations in the various taxa examined. These structural characteristics were recognized to be associated with variations of evolutionary effects, in the spatial arrangements and structural diversity of the styles (heterostyly, enantiostyly, stigma-height dimorphism, inverostyly), anthers (heteranthery), stigma textures, and the reproductive (breeding) systems (androdioecy, dioecy, andromonoecy) of the flowers. Other internal (polyploidy) and external factors (environmental) linked with structural intraspecific pollen variations were also considered. In order to characterize intraspecific boundaries and to test the credibility of these characters, various statistical tools were surveyed to identify characters of diagnostic taxonomic values.