The ultrastructure of fertilization and zygote formation in the green alga Ulva mutabilis Foyn.

Fertilization and zygote formation in the multicellular green alga Ulva mutabilis Foyn were studied with the aid of transmission and scanning electron microscopes. The growth mutant 'slender' was used in this investigation. The initial contact between mating gametes is made by the flagella, whereupon cytoplasmic contact is established within a few seconds. Fusion of the cytoplasm of the 2 cells is completed in 5 min and the nuclei can be seen to fuse 30 min after the onset of copulation. The settling of the young zygotes initiates a process during which the 4 flagella are absorbed into the cell. Cell wall formation also starts when the zygotes settle. Observations indicate disintegration of one of the 2 chloroplasts in the young zygote.