Resource-dependent sex allocation in andromonoecy

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Kallol Mozumdar and Vinita Gowda, Biological Sciences, Indian Institute of Science Education and Research Bhopal, Bhopal, India, Priya Iyer, Centre for Ecological Sciences, Indian Institute of Science, Bengaluru, India

Background/Question/Methods:
Sexual systems in plants have been suggested to have evolved as a dynamic response to environmental and genetic cues, representing a classic GxE interaction in nature. The role of stress-inducing abiotic factors such as soil nutrients and water availability in influencing the sex of flowers, especially among plants that have a flexible sexual system (andromonoecy, gynodioecy, dioecy, protandry, protogyny), is known but underexplored. Since abiotic stresses are often different in organisms that are spread across heterogeneous habitats, modelling its role in influencing sex allocation presents a very important problem, the solution to which will allow us to understand the evolution of sexual strategies in plants as a response to environmental conditions.
The prevalence of various sexual strategies in plants is explained by the sex allocation theory which proposes that strategies will be selected based on trade-offs between allocation to male and female reproductive functions. Andromonoecy is a unique sexual system where a plant produces both male and hermaphrodite flowers. The optimal resource allocation theory suggests that under resource limitation, a plant will invest more into male function as it is energetically cheaper in comparison to female function. To understand allocation strategies, we designed mathematical models for sex allocation in andromonoecious plants. We used evolutionarily stable strategies (ESS) to optimize sex-specific allocation in our models. Our models address the following questions: (1) How does resource availability influence the sex ratio and allocation pattern in an andromonoecious plant? (2) Under resource limitation, what is the sex-specific allocation strategy when pollinator visitation is affected by display size?
Results/Conclusions:
Our models predict that with increasing resource availability, the plant will show an increase in hermaphrodite-to-male flower ratio and a female-biased allocation. We observed that if female fitness is limited by pollinator visitation, producing more male flowers will increase the plant’s female fitness. We also corroborate the predictions from our mathematical model with empirical observations from an andromonoecious plant Murdannia simplex (Commelinaceae), which was studied at Kaas plateau, Maharashtra, India. Our study is the first to show resource-dependent sex allocation in andromonoecious plants and allows us to understand parameters that may affect at least one of the sexual strategies in plants. We consider our mathematical model an important step towards our understanding of evolutionary trade-offs when two sexes are present on the same plant, and highlight the need for empirical studies in the same.