Implications of Pollen/Ovule Ratios and Pollen Size for the Reproductive Biology of Potamogeton and Autogamy in Aquatic Angiosperms

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Abstract
Several pollination systems including anemophily, ephydrophily, and hyphydrophily have been reported for the aerial and/or submerged flowers of Potamogeton. Anemophily is generally believed to characterize the aerial-flowered taxa. The supposition that hyphydrophily characterizes submerged-flowered taxa derives from observations of underwater seed production. Hyphydrophily is often associated with larger pollen and, like anemophily, generally implies xenogamy and stochastic pollen delivery.sbd.features predicting high pollen/ovule (P/O) ratios. Relative to this, we present data from limited breeding studies and extensive analyses of pollen size and P/O ratios of North American species. The species tested were partly autogamous. Aerial-flowered species bear larger pollen and show significantly higher P/O ratios than those with submerged flowers. Thus, we suggest autogamy as an alternative to hyphydrophily for the submerged-flowered species of Potamogeton. Hydroautogamy is proposed to define a special class of autogamy; e.g., in Potamogeton, wherein self pollination is effected by pollen moving from anthers to stigmas over the surface of air bubbles. Autogamy is posited as a more likely first hypothesis in studies of breeding systems of submerged-flowering angiosperms with bisexual flowers. We also suggest that in Potamogeton P/O ratios reflect both breeding and pollination systems. A model is presented that shows how higher P/O ratios, than the breeding system of a taxon would imply, could be generated by the added effect of the pollination system.