TIME-LAPSE VIDEO OBSERVATIONS ON SEXUAL PLASMOGAMY IN THE RED ALGA BOSTRYCHIA

In the red alga Bostrychia moritziana, release of spermatia is trigger ed by slight osmotic shock; they emerge under pressure apparently gene rated by swelling of the mucilaginous sheath. Spermatia adhere tenacio usly to trichogynes of the carpogonium. Adhesion triggers spermatial m itosis, which is complete in about 30-45 min; there is no cytokinesis and the binucleate spermatium becomes vacuolated. The delicate, dynami c trichogyne cytoplasm contains complex membranous components and vacu oles. At the contact zone, the trichogyne and spermatial wall erode, f orming a pore, and cytoplasmic continuity (plasmogamy) is achieved alt er about 50-70 min. Many trichogynes rupture during these events becau se of inadequate structural connection with the spermatia. Normally, b oth spermatial nuclei enter the trichogyne in sequence; rarely, both n uclei enter together. Entrance is rapid, and the nuclei often become t hin and greatly elongated as each squeezes through the narrow pore int o the trichogyne. Once inside, each nucleus resumes its normal shape a s it starts to move steadily along the trichogyne, often with irregula r pauses. One nucleus of each pair (not necessarily the first out of t he spermatium) migrates along the trichogyne towards the carpogonium b ase to fertilize the female nucleus; the other moves in the opposite d irection, away from the entry site, and it often ends up near the tip of the trichogyne. This same scenario was observed for each of several spermatia contributing nuclei to one trichogyne. Thus, our observatio ns indicate that the two nuclei in each spermatium are differentiated so that only one is capable of fertilization, differentiation being vi sibly expressed in the direction that the nucleus moves as it enters t he trichogyne.