The evolution of dioecy and the maintenance of monoecy in the flora of Madagascar

• Main Author: Vary, Laura B
• Format: Dissertation
• Language: English
• Subjects: biology, botany; biology, ecology; biology, evolution and development
• ISBN: 9781124677767; 1124677763

Plant mating systems are highly diverse and the separation of male and female function in space (unisexual flowers) and time (dichogamy) can have important consequences for levels of outcrossing in plant populations. Most flowering plant species are hermaphroditic (∼80%), with individual plants having both male and female function. Dioecy, separate male and female individuals, and monoecy, individuals with separate male and female flowers, are rare worldwide (dioecy: ∼6%; monoecy: ∼4%), but with an uneven geographic distribution and a higher incidence of dioecious and monoecious species in tropical island floras. Dioecy has frequently been associated with many ecological traits, including woody habit, fleshy fruits, small flowers and endemism, though the ecological associations of monoecy are not well known. In this dissertation, I used a multi-level approach to examine the maintenance of monoecy and the evolution of dioecy in the flora of Madagascar. To examine the relationship of dioecy and monoecy with their ecological traits at the community level, I analyzed the breeding systems and their ecological traits in the littoral forest community of Madagascar. Dioecy has the strongest association with woody habit and fleshy fruit, and monoecious species have a strong association with small flowers, although this association does not hold at higher taxonomic levels. Understanding the floral sex expression of individuals is a key factor in interpreting the breeding system of populations and potential explanations for levels of self-fertilization. In two populations of the Malagasy endemic Tina striata (Sapindaceae), I determined the functional sex expression of individuals, through floral morphological measurements, experimental pollinations and field observations. Morphologically hermaphroditic flowers are functionally female and individuals are monoecious. Populations of T. striata are heterodichogamous, where some individuals open male flowers before female flowers (protandrous) and other individuals open female flowers before male flowers (protogynous). Despite high overlap of male and female flowering within individuals, levels of self-fertilization were low in both populations (< 10%). Heterodichogamy and monoecy are associated with high levels of outcrossing in these two populations of Tina striata. High outcrossing in these populations of Tina striata may maintain monoecy and prevent selection to evolve dioecy.