Expression and functional significance of andromonoecy in Solanum houstonii Martyn

Abstract

Andromonoecy, the production of both hermaphrodite and female-sterile (staminate) flowers in the same plant, is a sexual system that has evolved independently numerous times and is found in 33 families and ~4000 species of flowering plants. Over the last three decades, andromonoecy has been used as a model to study resource allocation in plants, and to investigate the evolution of unisexual flowers. However, large gaps remain in our knowledge of the mechanisms that promote the production of staminate flowers. In this thesis, I investigated the expression and functional significance of andromonoecy in Solanum houstonii, a Mexican endemic perennial shrub, by 1) assessing sex determination of staminate flowers, 2) examining the functional role of staminate flowers and 3) evaluating the reproductive consequences of andromonoecy in natural populations. In this thesis, first, I characterised the andromonoecy of S. houstonii. I achieved this by determining the main morphological differences among flower types (hermaphrodite and staminate) and establishing how these differences arise through floral development (Chapter 2). Second, I assessed the lability in the production of hermaphrodite and staminate flowers within an individual in order to determine whether sex expression is a plastic response to environmental changes in resource availability (Chapter 3). Third, I investigated the functional role of staminate flowers on their efficiency at pollen donation, pollinator attraction and in siring seeds in a laboratory experiment using commercial bumblebees as pollinators (Chapter 4). Finally, I conducted field surveys in natural populations to evaluate the reproductive success of S. houstonii and the ecological factors that may maintain andromonoecy in this species (Chapter 5). Overall, my results demonstrated that in S. houstonii the suppression of female organs on staminate flowers occurs at early stages of development and is influenced by inflorescence architecture. Staminate flowers do not promote pollen donation or pollinator attraction in laboratory experiments more than hermaphrodite flowers. However, in natural populations, staminate flowers may increase pollen export and deposition as I found a relatively high incidence of pollinators and that fruit and seed set were not limited by pollen receipt. These findings provide new insights into the mechanisms involved in the production of staminate flowers, the functional significance of staminate flowers, and the reproductive success of an, andromonoecious species.