Hermaphroditic origins of anisogamy

Area of Science:

• Evolutionary biology
• Game theory
• Reproductive strategies

Background:

• Anisogamy, the size dimorphism between male and female gametes, is fundamental to sexual reproduction.
• Previous game-theoretic models explaining anisogamy's origin typically assumed the existence of separate sexes from the outset.
• Empirical evidence for this assumption of pre-existing separate sexes is often lacking across taxa.

Purpose of the Study:

• To present a novel model exploring the coevolution of gamete size and sex allocation.
• To investigate the conditions under which anisogamy can emerge alongside hermaphroditism or separate sexes.
• To challenge and relax common assumptions in existing models of anisogamy evolution.

Main Methods:

• Development of a formal game-theoretic model.
• The model incorporates the coevolution of gamete size and sex allocation.
• It allows for the simultaneous emergence of anisogamy with either hermaphroditism or separate sexes.

Main Results:

• Hermaphroditic anisogamy can evolve directly from isogamous ancestors under conditions of small average spawning group size and efficient fertilization.
• Under hermaphroditism, sex allocation becomes increasingly female-biased as group size decreases and the degree of anisogamy increases.
• The model predicts the existence of complex isogamous organisms with equal allocation to two large gamete types in very small spawning groups.

Conclusions:

• Anisogamy's evolution is not exclusively tied to the initial presence of separate sexes.
• Hermaphroditism provides a viable pathway for the evolution of anisogamy, particularly in small, efficient fertilization groups.
• Future models should consider relaxing assumptions about pre-existing sexual dimorphism to better reflect empirical observations.