With the bursting of spring, pollen is in the air. Most of the pollen that is likely tickling your nose and making your eyes water is being dispersed in a sexually immature state consisting of only two cells (a body cell and a reproductive cell) and is not yet fertile. While the majority of angiosperm species disperse their pollen in this early, bicellular, stage of sexual maturity, about 30% of flowering plants disperse their pollen in a more mature fertile stage, consisting of three cells (a body and two sperm cells). And then there are plants that do both.
So which is the ancestral state, why did the earlier onset of maturity (the tricellular state) evolve so often, and is the tricellular state an evolutionary "dead end"? These questions, and others, were tackled in the classic work by James L. Brewbaker in 1967 and have been revisited in a new study, drawing upon an impressive database of over 2000 species, to determine which came first, tri- or bicellular pollen, and which leads to greater species diversity.
In the 1920s it was proposed that tricellular pollen had evolved independently within angiosperms numerous times and was an irreversible state. These predictions were supported by a classic, elegant, and notable study conducted by Brewbaker and published in the American Journal of Botany in 1967. Brewbaker used data from 1,908 species in one of the first large-scale tests of an evolutionary developmental hypothesis. He mapped pollen state (bi- vs tri-) onto a phylogenetic tree and found that tricellular families always appeared to be nested within bicellular families. He thus concluded that bicellular pollen was ancestral and had given rise to tricellular pollen multiple times. He also concluded that tricellular pollen never seemed to give rise to bicellular pollen.
Joseph Williams, an associate professor at the University of Tennessee, has had a long-standing interest in the reproductive biology of flowering
|Contact: Richard Hund|
American Journal of Botany