Fine-structural studies of the gametes and embryo of Fucus vesiculosus L. (Phaeophyta). I. Fertilization and pronuclear fusion.

In the marine brown alga, Fucus vesiculosus L., the sperm pronucleus is delimited by an envelope following penetration of the eff by the sperm. This envelope disintegrates as the pronucleus begins its migration through the cytoplasm of the egg. The highly condensed chromatin of the sperm pronucleus disperses slightly following disintegration of the envelope. Microtubules of unknown origin are associated with the sperm pronucleus during its migration. The flagellar microtubules remain in the peripheral cytoplasm but lose their tight 9 + 2 configuration. The sperm eyespot and mitochondria follow the pronucleus through the cytoplasm toward the egg pronucleus. The mitochondria of the sperm are distinguished from those of the egg by their longitudinally oriented cristae and by electron-opaque material in the intracristal space. The pronucleus of the egg becomes convoluted along the surface nearest to the advancing sperm pronucleus. Immediately prior to pronuclear fusion, many egg mitochondria aggregate in the vicinity of the sperm pronucleus. At this time, only the portion of the sperm pronucleus facing the egg pronucleus is surrounded by an envelope. The egg mitochondria disperse rapidly after pronuclear fusion. The sperm mitochondria and eyespot are still in the perinuclear region in 16-h-old embryos. At this time, the osmiophilia of the sperm eyespot has increased, and the sperm mitochondrial membranes are less distinct than in earlier stages. The fine-structural features of fertilization in Fucus are discussed in relation to the fertilization patterns in other cryptogams and marine invertebrates and to polar axis determination in the Fucaceae.