Geranium sylvaticum increases pollination probability by sexually dimorphic flowers

Area of Science:

• Ecology
• Evolutionary Biology
• Botany

Background:

• Sexual dimorphism, differing morphologies between sexes, is pronounced in gynodioecious species (females and hermaphrodites).
• Small female flowers in gynodioecious species are often attributed to resource reallocation, but pollinator attraction is vital for female fitness.
• Alternative explanations for small female flower size, beyond resource savings, are needed.

Purpose of the Study:

• To test the hypothesis that floral size dimorphism in *Geranium sylvaticum* is adaptive for pollination.
• To investigate the role of flower size and visitor behavior in pollination success within gynodioecious populations.

Main Methods:

• Video recording of small female and large hermaphrodite *G. sylvaticum* flowers.
• Parameterization of floral visitor behavior and calculation of pollination probabilities.
• Analysis of pollen and stigma display matching between flower sexes and pollinator interactions.

Main Results:

• Pollination probability varied significantly by flower sex and pollinator species, with bumblebees showing the highest probability.
• Small female flowers received pollen more effectively from several pollinator groups compared to large hermaphrodite flowers.
• Pollen presentation in hermaphrodites aligned better with female stigmas than with hermaphrodite stigmas.

Conclusions:

• Sexual dimorphism in *G. sylvaticum* flower size appears adaptive, enhancing reproductive functions for both sexes.
• Increased pollination probability in females and fathering probability in hermaphrodites may promote a shift towards dioecy.
• Flower size dimorphism plays a crucial role in reproductive success and evolutionary trajectory in gynodioecious species.