Floral biology, pollination and mating
Many plant species possess floral polymorphisms and understanding their evolution and functional significance has been of considerable interest to evolutionary biologists since Darwin’s pioneering studies on the topic summarized in “Forms of Flowers” (1877). We have investigated a range of problems associated with floral polymorphisms including heterostyly (39, 92, 103), stigma-height dimorphism (187, 196, 215), enantiostyly or mirror image flowers (190, 200, 201), heteranthery (254, 270) and various sexual polymorphisms (168, 229, 247, 262, 269) using diverse approaches. These have included comparative and phylogenetic methods (136, 199, 214, 241), floral manipulations and marker genes (97, 104, 130), and experiments focusing on the interaction of flowers and their pollinators (114, 119, 254). My reviews on plant sexual diversity (188, 262) identify many unanswered questions that would repay detailed investigation.
-
- Floral diversity in Wachendorfia (Haemodoraceae). The species exhibit dimorphic enantiostyly in which populations are often composed of plants with styles that are deflected to either the right or the left. This condition promotes cross-pollination.
-
- Flowering plant species with heteranthery; the occurrence of two distinct anthers within bee-pollinated flowers that differ in form and function. Heteranthery is commonly associated with enantiostyly, buzz pollination and nectarless flowers.
-
- Floral diversity in Narcissus (Amaryllidaceae). The genus exhibits four distinct forms of stylar variation – stylar monomorphism, stigma-height dimorphism, distyly and tristyly.
Bird-pollinated Babiana (Iridacaeae) species endemic to the Western Cape, South Africa. a) B. ringens subsp. ringens; b) B. ringens subsp. australis; c) B. hirsuta; d) B. carminea; e) B. avicularis. Both B. ringens and B. avicularis exhibit a specialized bird perch composed of the naked inflorescence axis. All species are pollinated by sunbirds.
In collaboration with Bruce Anderson ( University of Stellenbosch, South Africa) we have investigated the origin and function of the specialized bird perch in Babiana, a genus of geophytes native to the Western Cape (220). Caroli de Waal completed her M.Sc. thesis in my lab on the reproductive ecology of four bird pollinated Babiana species, two of which possess specialized bird perches. She investigated the pollination biology, mating systems and patterns genetic diversity of these species and found that in B. ringens there was evidence for the loss of perch function associated with increased self-pollination in populations at the periphery of the eastern range of the species.
Malachite sunbird (male) visiting flower of Babiana ringens with an intact perch and a plant with the perch removed. Perch removal resulted in a striking reduction in fertility and an increase in the selfing rate.
A selection of wind-pollinated species with diverse morphologies: A) Carex pedunculata (Cyperaceae); B) Gynomonoecious Gunnera peltata (Gunneraceae); C) Leucadendron rubrum (Proteaceae). This species is dioecious with extreme sexual dimorphism (female left; male right); D) Pennisetum clandestinum (Poaceae); note the anthers extended on very long filaments to aid in pollen dispersal. Image by L.D. Harder
The evolution of wind pollination is poorly understood despite the fact that it represents one of the most important evolutionary transitions in pollination systems.
Jannice Friedman, now on the faculty at Queens University, investigated a range of questions concerning mating and pollination in anemophilous plants using diverse approaches including comparative methods, theoretical models, field experiments, marker gene studies of mating patterns, and glasshouse quantitative genetic analyses. Her work has helped to explain a number of long standing questions including why anemophilous species commonly have unisexual flowers with only a single ovule.
David Timerman recently completed his PhD thesis on the biomechanics and ecology of wind pollination. He investigated the floral trait modifications promoting wind pollination, their fitness effects in contrasting environments, and the evolutionary significance of ambophily (insect and wind pollination) in Thalictrum, a genus in which this transition has occurred repeatedly. David also explored the causes of male-biased sex ratios and the demographic context for pollination in an ambophilous species.