Heterostyly and gene-flow in Menyanthes trifoliata L. (Menyanthaceae)

NIC LUGHADHA, E. M. & PARNELL, J. A. N., 1989. Heterostyly and gene-flow in Menyanthes trifoliata L. (Menyanthaceae). Menyanthes trifoliata is shown to be a self-incompatible heterostylous species with a suite of associated pollen and gynoecium dimorphisms. These features may promote legitimate pollination. Analysis of a natural population showed substantial pollen inviability, a preponderance of illegitimate pollen on stigmas and a possible asymmetry of pollen flow. Mean pollen travel was estimated as 2.6 m by counting legitimate grains on stigmas whose distance from the nearest source of legitimate pollen was known. Direct field and laboratory measurement indicated mean seed travel of <0.2m. The density of flowering spikes varied greatly within the studied colony of Menyanthes , with a mean of 0.93 m^-2. Using suitable corrections the genetically effective density was estimated as between 0.04 and 4.4 m^-2. Crossing experiments indicated that Menyanthes is fully outcrossed (r=1). The neighbourhood area was estimated as 10–15 m² while neighbourhood size ranged from less than one to sixty-five. Some published neighbourhood estimates are revised with the results for Menyanthes being at the lower end of the range. If the genetically effective density were estimated more accurately, neighbourhood estimates would likely be lower still. It is concluded that what is generally referred to as a population of Menyanthes , may actually represent many evolutionary units.