Anther-Stigma Separation in Desmodium Species (Papilionoideae-Fabaceae) from Northwestern Argentina

The repeated evolutionary transition from outcrossing to self-fertilization in angiosperms has been suggested to occur because selfing provides reproductive assurance. This transition is commonly associated with modifications of floral traits, among them herkogamy (the spatial separation between dehiscing anthers and the receptive stigma). Here, we investigate variation in levels of herkogamy in Desmodium incanum, D. pachyrrhizum, D. subsericeum and D. uncinatum, to determine the relationship between variation in this floral trait and self-pollen deposition. These species are self-compatible and bee-pollinated, with ornamental potential and may be used for forage given their high nutritional value. We studied natural populations located in Lerma Valley, Salta Province, Argentina. In January and February 2009, we marked and bagged 10 floral buds from 10 individual plants per species. The flowers were collected before their senescence; anther-stigma separation was measured immediately after collection with digital calipers (0.01 mm) as the distance between the surface of the stigma and the nearest anther. The stigmas were preserved in 70% ethanol. Pollen grains were counted under a microscope (10x) in the laboratory. There was significant variation in herkogamy levels among species. There was a negative correlation between anther-stigma distance and the number of self-pollen grains deposited on the stigma in isolated flowers. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of pollinators.