Ejaculate evolution in external fertilizers: Influenced by sperm competition or sperm limitation?

Area of Science:

• Evolutionary biology
• Reproductive biology
• Comparative genomics

Background:

• Sperm competition is a primary driver of ejaculate evolution.
• Sperm limitation due to gamete dispersal is less studied, especially in external fertilizers.
• Frogs and fish exhibit distinct reproductive strategies and sperm characteristics.

Purpose of the Study:

• To investigate the macroevolutionary patterns of sperm size and number in anurans and fishes.
• To compare the relative importance of sperm competition versus spawning conditions in shaping ejaculate evolution.
• To test hypotheses regarding sperm limitation in external fertilizers.

Main Methods:

• Phylogenetic comparative analyses were employed.
• Path analyses were used to model evolutionary trajectories.
• Sperm size, sperm number, egg number, and environmental factors were analyzed across taxa.

Main Results:

• Anuran ejaculate evolution was primarily driven by sperm competition.
• Fish ejaculate evolution was more strongly influenced by egg number and water turbulence.
• Different selective pressures shape sperm traits in frogs and fish.

Conclusions:

• Sexual selection is a key factor in anuran ejaculate evolution.
• Sperm limitation, influenced by spawning conditions, is underappreciated in fish ejaculate evolution.
• Comparative studies reveal diverse evolutionary pathways for sperm traits.