Experimentally evolved and phenotypically plastic responses to enforced monogamy in a hermaphroditic flatworm

Area of Science:

• Evolutionary biology
• Animal behavior
• Reproductive biology

Background:

• Sexual selection is a key evolutionary driver, yet experimental evidence in simultaneous hermaphrodites is limited.
• Understanding evolutionary responses to mating systems is crucial for diverse organisms.

Purpose of the Study:

• To investigate the evolutionary impact of enforced monogamy on sexual traits in the hermaphroditic flatworm Macrostomum lignano.
• To assess phenotypic plasticity in reproductive traits in response to varying group sizes.

Main Methods:

• 32 independent selection lines of Macrostomum lignano evolved under monogamy or polygamy for 20 generations.
• Evaluated changes in testis area, sex allocation, genital morphology, sperm morphology, and mating behavior.
• Assessed phenotypic plasticity of traits in response to group size.

Main Results:

• Enforced monogamy led to evolutionary changes in male copulatory organ shape and sperm appendage length.
• No significant evolutionary shift in sex allocation was observed between monogamy and polygamy regimes.
• Phenotypic plasticity was substantial; larger group sizes resulted in male-biased sex allocation and longer sperm.

Conclusions:

• Enforced monogamy induced moderate evolutionary changes in specific male sexual traits in M. lignano.
• Phenotypic plasticity in response to group size played a significant role in trait variation.
• M. lignano exhibits adaptive flexibility through both evolutionary and plastic responses to mating systems and social environment.