THE EVOLUTIONARY HISTORY OF THE SEED PLANT MALE GAMETOPHYTE

The role of the male gametophyte in the early history of seed plants remains an underappreciated but critical part of the evolution of a suite of characters that ultimately came to define seed plants. Recent paleobotanical discoveries and studies of extant primitive seed plant male gametophytes, when coupled with phylogenetic analyses of seed plants, provide insight into many key events that occurred during the early evolution of seed plants. These discoveries are changing our ideas concerning the multiple origins of the sulcus (pollen grain germinal aperture) and pollen tube, the structural and physiological relationships of the male gametophyte with the host sporophyte tissues in primitive seed plants, and the evolution of siphonogamy (conduction of non-motile sperm via a pollen tube) from a zooidogamous (swimming sperm) condition.