Abstracts (first author)
The evolution of sexual segregation within inflorescences: the role of resource competition by flowers
Spatial segregation of sexual functions within inflorescences has been explained as a mechanism of avoiding self-fertilization reducing the negative effects of inbreeding. However, many species with sexual functions separated in different flowers also have self-incompatibility systems making difficult generalizations of this hypothesis. An alternative hypothesis for this pattern is that flowers within inflorescences compete by resources, and that the sequential development of flowers and architectural constraints lead to a gradient on resource availability. For instance, fruit size also follows a positional pattern within inflorescences supporting this resource gradient. This variation on resource availability produces distinct optima for sex allocation accordingly with the position of the flower within an inflorescence, potentially driving the evolution of sexual specialization on separated flowers. Using as a model the largest family of flowering plants, Asteraceae, we explored whether floral specialization in sex functions (either male or female unisexual flowers) within inflorescences was related with higher levels of floral aggregation and therefore with higher resource competition between different flowers within the inflorescence. We measured number of flowers and capitulum size to estimate flower density in 101 species with different levels of sexual specialization: hermaphroditic (only bisexual flowers), gynomonoecious (bisexual and female flowers) and monoecious species (female and male flowers), using herbarium specimens from Swedish Natural History Museum and Coimbra University Herbarium. Statistical differences were assessed with phylogenetic generalized least squared models. Flower number and flower density at inflorescence level was correlated with the specialization degree on floral gender, suggesting that the increase of floral competition might favor sex specialization of male and female functions on different flowers within inflorescences.