Evolution of Mimosoid Legume Pollen

Most legume taxa release pollen as single grains (monads), but pollen of the legume subfamily Mimosoideae is known to largely occur in coherent groups of pollen grains (polyads). This study examines structure, development, systematic significance, ecology and evolution of pollen polyads in the legume subfamily Mimosoideae.

Collection locations of mimosoid legume taxa studied for pollen type in Madagascar, showing the tendency for taxa releasing pollen in monads to occur in wetter regions.

The pollen of the legume subfamily Mimosoideae is known to largely occur in polyads, meaning that mature pollen is released in coherent groups of a variable number of pollen grains. Research is being carried out to find out how and why this pollen has evolved from species with single grains of pollen (monads) which occur in the closest relatives (subfamily Caesalpinioideae). This study is focussing on the micro-structure and development of polyads, systematic significance, ecology and evolution. Research into some of the first branching mimosoid taxa with polyads has shown that the number of pollen units within each polyad matches the number of ovules within each ovary. Also the stigmatic pore size matches the size of the polyad. One pollination event could therefore result in all the ovules being fertilised, providing an efficient pollination system in environments where perhaps pollination events are less common. It appears that the structure of polyads also could protect the pollen grains from dehydrating too quickly in dry habitats. In Madagascar, there are some mimosoid species in which pollen is released as individual grains, these occur in early-diverging mimosoid genera alongside other species that release their pollen in polyads. Our research examines whether there are differences in the ecological distributions of these endemic Malagasy mimosoid taxa; preliminary findings suggest that species with polyads inhabit drier habitats than those with monads. RBG Kew has particular expertise in producing and analysing GIS data for Madagascar, and this will be utilised in this study. Our findings so far suggest that polyads may have an evolutionary advantage in drier habitats.

Key publications 2006-2011

• Banks, H. & Lewis, G.P. (2009). Pollen morphology of the Dimorphandra group (Leguminosae, Caesalpinioideae). Grana 48: 19-26.
• Banks, H. Himanen, I. & Lewis, G.P. (2010). Pollen, stigmas and ovule numbers at the caesalpinioid-mimosoid interface (Fabaceae). Botanical Journal of the Linnean Society 162: 594-615.