Developmental bases for key innovations in the seed-plant microgametophyte.

The early evolution of seed plants was marked by a sequence of putative key innovations associated with the microgametophyte, including reversal of microspore polarity, loss of sperm motility and co-option of the pollen tube to a new role, siphonogamy. Data from several interfacing fields of plant development are reviewed here in the context of recent palaeobotanical discoveries and improved understanding of seed-plant relationships. The proximal-distal microspore polarity transition that occurred early in seed-plant evolution represents a fossil fingerprint for an underlying series of radical developmental shifts involving quadripartite partitioning at meiosis and an asymmetric primary mitosis. It had important downstream effects, not only on aperture location and site of germination but also on microgametophyte polarity, and, perhaps, indirectly on sperm motility.