Frequency dependence in matings with water-borne sperm

Area of Science:

• Evolutionary Biology
• Reproductive Ecology
• Marine Biology

Background:

• The rare male effect (RME) is a debated evolutionary force impacting mating success.
• Previous RME studies focused on copulating species, leaving sessile organisms understudied.
• Sessile, non-copulating species may experience unequal sperm fertilization due to environmental gamete release.

Purpose of the Study:

• Investigate frequency-dependent mating success in the colonial ascidian Diplosoma listerianum.
• Determine if RME occurs in a sessile, non-copulating invertebrate.
• Explore potential mechanisms driving RME in this species.

Main Methods:

• Experimental manipulation of sperm competition in Diplosoma listerianum.
• Varying gamete concentrations and mating history to assess paternity.
• Analyzing paternity success based on sperm frequency and female mating history.

Main Results:

• Sperm at lower frequencies achieved greater than expected paternity, demonstrating the rare male effect.
• A potential mate order effect, influenced by sperm storage, was observed but not consistently.
• No RME was found when acclimation matings were limited and separated by longer intervals.

Conclusions:

• Diplosoma listerianum exhibits the rare male effect, challenging previous assumptions based on copulating species.
• Nongenetic female preferences, possibly for increasing sibship genetic diversity, are proposed as a driver for RME.
• The life history of D. listerianum, with low costs to multiple mating, supports indirect benefits as the primary explanation for RME.