In the red alga Bostrychia moritziana, release of
spermatia is triggered by slight osmotic shock; they emerge under pressure
apparently
generated by swelling of the mucilaginous sheath. Spermatia adhere
tenaciously to trichogynes of the carpogonium. Adhesion triggers
spermatial mitosis, which is complete in about 30–45 min; there is
no cytokinesis and the binucleate spermatium becomes vacuolated. The
delicate, dynamic trichogyne cytoplasm contains complex membranous components
and vacuoles. At the contact zone, the trichogyne and
spermatial wall erode, forming a pore, and cytoplasmic continuity (plasmogamy)
is achieved after about 50–70 min. Many trichogynes
rupture during these events because of inadequate structural connection
with the spermatia. Normally, both spermatial nuclei enter the
trichogyne in sequence; rarely, both nuclei enter together. Entrance is
rapid, and the nuclei often become thin and greatly elongated as
each squeezes through the narrow pore into the trichogyne. Once inside,
each nucleus resumes its normal shape as it starts to move
steadily along the trichogyne, often with irregular pauses. One nucleus
of each pair (not necessarily the first out of the spermatium)
migrates along the trichogyne towards the carpogonium base to fertilize
the female nucleus; the other moves in the opposite direction,
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 observations
indicate that the two nuclei in each spermatium are differentiated
so that only one is capable of fertilization, differentiation being
visibly expressed in the direction that the nucleus moves as it enters
the trichogyne.