The fast block against polyspermy in fucoid algae is an electrical block

Area of Science:

• Marine Biology
• Reproductive Biology
• Algal Physiology

Background:

• Polyspermy, the fertilization of an egg by multiple sperm, is a common challenge in external fertilization systems.
• Understanding the mechanisms that prevent polyspermy is crucial for reproductive success in many species.
• Fucoid algae, with their external fertilization, provide a model system to study these mechanisms.

Purpose of the Study:

• To investigate the presence and nature of an electrical block against polyspermy in the eggs of fucoid algae (Pelvetia fastigiata, Fucus vesiculosus, and Fucus ceranoides).
• To determine if the electrical block is sufficient to prevent polyspermy or if other mechanisms are involved.
• To examine the role of the fertilization potential and subsequent cell wall formation in preventing polyspermy.

Main Methods:

• Electrophysiological recordings of fertilization potentials in Pelvetia fastigiata, Fucus vesiculosus, and Fucus ceranoides eggs.
• Manipulation of external sodium concentration (Na+) and membrane potential using steady current to assess the electrical block.
• Observation of sperm-egg interactions and timing of cell wall formation post-fertilization.

Main Results:

• Fucoid eggs exhibit a rapid electrical depolarization (fertilization potential) upon sperm binding, serving as an electrical fast block against polyspermy.
• This electrical block is dependent on external sodium and can be overcome under specific conditions (e.g., altered membrane potential, low Na+).
• Fucus ceranoides shows a faster cell wall formation compared to P. fastigiata and F. vesiculosus, suggesting a complementary role in polyspermy prevention. Non-fertilizing sperm also actively move away from the egg surface.

Conclusions:

• Fucoid eggs possess an effective electrical fast block against polyspermy, mediated by a transient depolarization upon fertilization.
• The timing of cell wall formation varies among species, with F. ceranoides having a potentially more significant role for this secondary block.
• An additional 'intermediate block,' possibly related to sperm behavior, may also contribute to preventing polyspermy in fucoid algae.