Hermaphroditic sex allocation evolves when mating opportunities change.

Area of Science:

• Evolutionary Biology
• Reproductive Strategies
• Sex Allocation Theory

Background:

• Hermaphroditic organisms must balance resource allocation between male and female functions.
• Mating opportunities with unisexuals (males or females) are predicted to influence this sex allocation.
• Previous models suggest selection favors male-biased allocation when hermaphrodites encounter females, and female-biased when encountering males.

Purpose of the Study:

• To experimentally test the evolutionary response of hermaphroditic sex allocation to the presence or absence of males.
• To investigate if Mercurialis annua hermaphrodites adapt their reproductive resource deployment based on mating environment.

Main Methods:

• Evolved populations of Mercurialis annua hermaphrodites in controlled environments with and without males.
• Assessed changes in sex allocation (male vs. female reproductive investment) over evolutionary time.
• Compared evolutionary trajectories in the presence versus absence of unisexual males.

Main Results:

• Hermaphrodites evolved greater male allocation when mating in the absence of males.
• No significant evolution in sex allocation was observed when hermaphrodites were present with males, likely due to pre-existing female bias.
• Results confirm predictions regarding evolutionary shifts in sex allocation driven by the frequency of unisexuals.

Conclusions:

• This study provides the first empirical evidence of an evolutionary adaptation in hermaphroditic sex allocation in response to the frequency of unisexuals.
• The findings support theoretical models predicting the transition from hermaphroditism towards dioecy (separate sexes) under specific ecological conditions.
• Demonstrates the plasticity and adaptive potential of reproductive strategies in hermaphrodites.