Required for male fertility. Plays a role in pollen tube guidance and successful gamete attachment. Essential for the fusion of gametes during double fertilization, where one male gamete fuses with the egg to produce a zygote, and another male gamete fuses with the central cell to produce the endosperm. Mediates the fusion of cell membranes. Not required for pollen tube outgrowth. HAP2/GCS1 family members mediate membrane fusion between gametes in a broad range of eukaryotes, ranging from algae and higher plants to protozoans and cnidaria, suggesting they are derived from an ancestral gamete fusogen. They function similar to viral fusogens, by inserting part of their extracellular domain into the lipid bilayer of an adjoining cell. Belongs to the HAP2/GCS1 family.