Gamete plasticity in a broadcast spawning marine invertebrate

Area of Science:

• Reproductive biology
• Evolutionary ecology
• Marine biology

Background:

• Sperm competition is classically thought to maintain anisogamy (large eggs, small sperm) by favoring sperm number over size.
• Sperm quality, including size and velocity, can influence sperm competition, but studies show conflicting results.
• The role of sperm competition and environment in determining gamete size in marine broadcast spawners remains debated.

Purpose of the Study:

• To investigate whether broadcast spawners alter gamete traits in response to varying sperm competition likelihood.
• To test the hypothesis that environmental conditions, specifically adult density, influence sperm and egg phenotypes in externally fertilizing species.

Main Methods:

• Field manipulation of adult density in the broadcast spawning ascidian, Styela plicata, to alter sperm environment.
• Assessment of sperm traits (size, motility, longevity) and egg traits (size, ovicell size) under different density conditions.
• In vitro fertilization experiments to evaluate sperm performance and polyspermy rates.

Main Results:

• Sperm from high-density adults were larger, more motile, lived longer, and induced less polyspermy than sperm from low-density adults.
• Eggs from high-density adults were smaller targets for sperm but had larger ovicells.
• Adult density significantly impacted both sperm and egg phenotypes, indicating adaptive gamete trait plasticity.

Conclusions:

• Sperm competition is not a primary driver maintaining anisogamy in broadcast spawners.
• Sperm limitation, influenced by environmental density, appears to select for larger eggs and smaller, more numerous sperm.
• Broadcast spawning mothers balance pre- and postzygotic selection pressures on egg size in response to environmental cues.