Resource-dependent sex-allocation in a simultaneous hermaphrodite

Area of Science:

• Evolutionary Biology
• Reproductive Ecology
• Animal Behavior

Background:

• Sex allocation theory typically examines resource investment versus fitness returns (fitness-gain curves).
• Simultaneously hermaphroditic organisms provide a unique model for studying sex allocation strategies.
• Resource availability is a critical environmental factor influencing reproductive decisions.

Purpose of the Study:

• To investigate how resource availability affects sex allocation in the hermaphroditic flatworm Macrostomum lignano.
• To determine if fitness-gain curves for male and female functions differ under varying resource conditions.
• To explore the roles of body size and direct resource effects on sex allocation.

Main Methods:

• Macrostomum lignano were maintained under controlled, differing resource levels.
• Gonad (testes and ovaries) sizes were measured over time to assess resource allocation.
• Body size was measured to account for size-dependent effects on sex allocation.

Main Results:

• Higher resource levels correlated with increased allocation to female reproduction.
• A saturating male fitness-gain curve was inferred for M. lignano.
• Both body size increase and direct resource effects contributed to observed sex allocation shifts.
• Male gonad development preceded female gonad development, aligning with the saturating male curve.

Conclusions:

• Resource availability significantly influences sex allocation strategies in Macrostomum lignano.
• The findings support the concept of saturating male fitness-gain curves in hermaphrodites.
• Sex allocation in M. lignano is influenced by both body size and direct resource availability, independent of local mate competition effects.