American Scientist

I found Katrina Claw’s article “Rapid Evolution in Eggs and Sperm” (May–June) to be very interesting, and I especially appreciated her effort in its introduction to present the extraordinary variation in the morphology and functional strategies of gametes within the sexual systems in which they are involved across the Eukarya. However, she is badly mistaken if she thinks pollen is plant sperm. There is no sperm present in a pollen grain. A pollen grain is a tiny male plant, a male gametophyte (gamete-forming plant). He is a multicellular organism (usually two cells in flowering plants, but more in the case of gymnosperm pollen) capable of producing sperm if he gets an opportunity. That is to say, if a pollen grain makes a successful “connection” with a flower or female cone (the process called pollination), it is then and only then that he will produce sperm (usually more than one) and deliver them to the vicinity of an egg, often by means of a pollen tube.

Likewise, the cactus ovules depicted in Figure 1 are not eggs. A plant ovule is a container for a tiny female gametophyte—a multicellular organism genetically and physically distinct from the cactus plant that produced the flower that contains her—that will produce an egg when she is mature.

This is all a consequence of the often misunderstood sporic sexual reproductive system used by almost all true plants (embryophytes), also known as the alternation of generations. Sporic reproductive cycles offer many opportunities for evolutionary influences. For example, when growing those tubes, pollen that produce tubes to deliver sperm race one another for the opportunity to fertilize an egg within an ovule. The alternate sporophyte generation (individuals we usually recognize as plants) has often evolved selective mechanisms that either encourage or minimize outcrossing by inhibiting or killing pollen of certain genotypes or by specializing the morphology or timing of reproductive structure development.

Dan Gladish
Department of Botany
Miami University
Hamilton, OH