Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Area of Science:

• Evolutionary biology
• Sexual selection
• Reproductive biology

Background:

• Sexual traits diversify rapidly, often attributed to sexual selection.
• Sperm morphology varies greatly and correlates with reproductive factors.
• The impact of postcopulatory processes on sperm form evolution is unclear.

Purpose of the Study:

• To investigate if postcopulatory sexual selection influences the rate of sperm form evolution.
• To quantify evolutionary rates of sperm length divergence in passerine birds.
• To assess the relationship between sperm competition levels and sperm evolution speed.

Main Methods:

• Quantified evolutionary rates of sperm length divergence in passerine bird lineages.
• Estimated sperm competition risk using relative testes mass.
• Analyzed associations between relative testes mass and rates of divergence in sperm dimensions.

Main Results:

• Relative testes mass positively correlated with faster divergence in midpiece, flagellum, and total sperm length.
• No association found between relative testes mass and sperm head size divergence.
• Sperm head length appears to be evolutionarily constrained.

Conclusions:

• This study provides the first evidence linking sperm competition strength to the speed of sperm evolution.
• Postcopulatory sexual selection significantly promotes rapid evolutionary diversification of sperm morphology.
• Sperm head size may be subject to different evolutionary pressures or constraints.