Intraspecific Variation in Pollen-Ovule Ratios and Nectar Secretion- Preliminary Evidence of Ecotypic Adaptation

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Because pollen-ovule ratios (P/O's) reflect the predictability of pollinators in a habitat and the efficiency of pollination, large intraspecific differences in P/0's suggest differences in pollinator numbers and/or their efficiency. Plants of Heracleum lanatum, which is andromonoecious, from forests have larger percentages of male flowers than those outside of forests, hence a higher P/0. This difference is associated with differences in the kinds of flower visitors. I suggest the pollen removal by small bees that forage on Heracleum in but not outside the woods may be the selective force that accounts for the larger percentage of male flowers of woods plants. In andromonoecious Caesalpinia the percentage of hermaphroditic flowers in a population ranges from 8-83%, and appears to be ecotypically adapted to levels of pollinator, i.e., butterfly, activity. Nectar secretion is continuous and is the key to successful reproduction, especially in populations with low pollinator activity. Pollination is proportional to foraging time and a function of the pollen carried. The amount of nectar in the flowers reflects pollinator activity; thus in low activity populations there will be more nectar and visits will be longer, thus increasing the likelihood of pollination. Because there are large numbers of male flowers in such populations the pollinators presumably carry more pollen, which also increases the likelihood of pollination. In populations with high pollinator activity large numbers of visits balance the shortness of individual visits. A consequence of this balanced system is that the fecundity of hermaphroditic flowers in quite dissimilar populations is equivalent. Deviations from predicted levels of seed set and fruit set are consistent with below normal levels of pollinator activity. Nectar production in two populations of Calliandra anomala are quite different, with a high elevation population producing far less nectar than a lower elevation population. The low rate of nectar production in the high elevation population is undoubtedly an adaptation that forces the pollinators, i.e., hawkmoths, to visit large numbers of flowers to obtain sufficient nutrients, thus increasing fruit set and maximizing fecundity. The breeding system and pollination biology of Leonotis nepetaefolia are used to explain the distribution of this African plant in Mexico, where it is a roadside weed.