INTRASPECIFIC VARIATION IN POLLEN-OVULE RATIOS AND NECTAR SECRETION-PRELIMINARY EVIDENCE OF ECOTYPIC ADAPTATION1 ROBERT WILLIAM CRUDEN2 ABSTRACT Because pollen-ovule ratios (P/O's) reflect the predictability of pollinators in a habitatand the efficiency of pollination, large intraspecific differences in P/O's suggest differencesin pollinator numbers and/or their efficiency. Plants of Heracleum lanatum, which is an-dromonoecious, from forests have larger percentages of male flowers than those outside offorests, hence a higher P/O. This difference is associated with differences in the kinds offlower visitors. I suggest the pollen removal by small bees that forage on Heracleum in butnot outside the woods may be the selective force that accounts for the larger percentage ofmale flowers of woods plants. In andromonoecious Caesalpinia the percentage of hennaphroditic flowers in a popula-tion ranges from 8-83%, and appears to be ecotypically adapted to levels of pollinator, i.e.,butterfly, activity. Nectar secretion is continuons and is the key to successful reproduction,especially in populations with low pollinator activity. Pollination is proportional to foragingtime and a function of the pollen carried. The amount of nectar in the flowers reflects pol-linator activity; thus in low activity populations there will be more nectar and visits will belonger, thus increasing the likelihood of pollination. Because there are large numbers of maleflowers in such populations the pollinators presumably carry more pollen, which also increasesthe likelihood of pollination. In populations with high pollinator activity large numbers ofvisits balance the shortness of individual visits. A consequence of this balanced system isthat 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 normallevels of pollinator activity. Nectar production in two populations of Calliandra anomala are quite different, with ahigh elevation population producing far less nectar than a lower elevation population. Thelow rate of nectar production in the high elevation population is undoubtedly an adaptationthat forces the pollinators, i.e., hawkmoths, to visit large numbers of flowers to obtain sufficientnutrients, thus increasing fruit set and maximizing fecundity. The breeding system and pollination biology of Leonotis nepetaefolia are used to explainthe distribution of this African plant in Mexico, where it is a roadside weed. The objective of this paper is to communicate initial results that suggest thattwo reproductive characteristics, "pollen-ovule ratios" and "nectar secretion," mayhave adapted ecotypically in response to differences in the kinds and levels ofactivity of their pollinators. Some of the data is of a preliminary nature, i.e.,one year's observations, and in the future the present interpretation may requiremodification or reinterpretation. FLORAL CHARACTERISTICS REVIEWED For at least 15 years botanists have known that xenogamous flowers reflectreciprocal evolution with a pollinator class (Pijl, 1960, 1961). Floral morphologyreflects the size and foraging behavior of the pollinator class. Flower color andodor reflect the visual and olfactory sensitivity of the pollinator class. For exam-The contribution of Sharon Hermann-Parker in collecting and discussing much of thedata presented in this paper is greatly appreciated. 2 Department of Botany, University of Iowa, Iowa City, Iowa 52240.ANN. MissounI BOT. GARD. 63: 277-289. 1976.
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