The effects of pheromones on fertilization in brown algae. Pheromone induced orientation movements of the male gametes of ectocarpus,

normal speed, slow and quick motion. The filamentous brown alga ectocarpus siliculosis releases its gametes into the surrounding seawater.

Like many brown algae, it uses chemotaxis caused by a so-called pheromone to ensure that gametes locate and meet each other. The female gametes swim freely in the water by means of their two flagella.

After a short swimming period, the female gametes settle down on the substrate. The sessile female gametes secrete a pheromone with a juniper-like smell into the surrounding water and

attract the actively swimming male gametes chemotactically. This substance has been named ectocarpine. Traces of synthetic ectocarpine contained in the right droplet evoke, in contrast to the control droplet left, a positive chemotactic response from the male gametes.

The density of the male gamete population increases in the neighborhood of the droplet containing ectocarpine. The style and speed of movement of the male gametes are also changed by the pheromone.

The flagella movements of the male gametes are shown in slow motion. Without ectocarpine, the gamete swims straight ahead or in wide curves by meandering beats of its anterior flagellum.

The shorter backward directed flagellum generally remains inactive. The male gametes react to a tactile stimuli and that is why they prefer to swim along the surface of the coverslip.

Female gametes have settled on the coverslip. A female cell secreting ectocarpine functions like a point source of chemical stimulation.

Its effect on male gametes is to increase the frequency of sideward beats performed by the hind flagellum.

They move in U-turns which bring them closer to the female cell. The beating of the posterior steering flagellum makes them move in narrow circular paths.

This represents the track of a male gamete. Outside the female's sphere of influence, it changes its direction only occasionally by beating with its posterior flagellum.

When it senses the pheromone secreted by a female gamete, the posterior flagellum beats more frequently. The resulting U-turns and narrow circular paths bring it closer to the diffusion source and entrap it there.

Under normal conditions, the male gamete contacts the surface of the female with the tip of its anterior flagellum.

Shortly afterwards, the two protoplasts fuse to form the zygote. Pheromone induced release and attraction of male gametes of laminaria, normal speed and quick motion.

The brown alga laminaria is distributed throughout northerly seas. Its microscopic gametophytes are attached to the substratum.

The larger cells develop to female, the smaller ones to male gametophytes. Compared with ectocarpus, laminaria develops an additional mechanism to increase the chances for fertilization.

The female gametophytes extrude a single egg spontaneously from the urgonia. The egg remains at the tip of the urgonium.

The smaller male gametophytes are more densely branched. On tiny lateral branches, they produce minute antheridia, which each release a single male gamete.

In contrast to ectocarpus, liberation of male gametes in laminaria is affected by the pheromone produced by the ripe eggs. Mass liberation of male gametes is due to the effects of a multitude of ripe eggs.

This pheromone they produce can be isolated, and in the following experiment, it is introduced into a culture of mature male gametophytes. The bright line is the water-air interface, onto which a drop of water containing egg pheromones is placed.

Mass liberation of male gametes occurs within a few seconds. This increases the probability of successful fertilization.

The experiment is repeated. The actual time between introduction of the substance and liberation of gametes is about 12 seconds.

The chemical structure of the laminaria substance is closely related to ectocarpine. The liberated gametes are chemotactically attracted to the eggs, just as in ectocarpus. Thus, in laminaria, the pheromone affects both liberation and attraction of the male gametes.