9629774 Delph Some flowering plant species have separate male and female individuals and are termed dioecious. In addition to differing in their primary function (males produce pollen, whereas females produce seeds), males and females often exhibit sexual dimorphism in other traits (e.g., petal or leaf size, frequency of reproduction, and physiological traits). As a consequence of producing fruit, females of most dioecious species invest more biomass in reproduction than do males. This higher reproductive effort by females should mean that females will invest less in other structures than do males. Consequently, they should have a higher "cost of reproduction" than males; i.e., females often exhibit relatively slower rates of growth and/or higher rates of mortality. The investigators plan to study the dioecious plant, Silene latifolia, which does not appear to fit the pattern of females having a higher cost of reproduction. In this species, males grow less vegetatively and die at a younger age than females, even though females of this species do invest relatively more biomass in reproductive structures. In addition, in this species males produce 10 to 15 times more flowers than females, and male flowers are relatively small. The investigators will test the hypothesis that production of many, small flowers imposes a high cost of reproduction for the males, and that this cost can not be predicted simply by measuring biomass (i.e., carbon) invested in reproductive structures, because physiological differences between the sexes may also exist. The investigators will test this hypothesis in two ways: 1) by growing plants of known parentage in a common garden and greenhouse to observe whether variation in morphological and physiological traits is genetically based and correlated with flower number, and to observe relative longevity, and 2) by conducting an artificial selection experiment to reduce the magnitude of differences in flower size and number that exist between the sexes and obse rve whether certain physiological traits thought to be responsible for the cost of reproduction (such as respiration rate) also change with the change in the morphological traits. This will be the first test in plants of a phenomenon that has been reported for some animals: that sexual selection (selection for obtaining many mates) imposes a survival cost on males. Furthermore, the proposed research will contribute to our understanding of the evolution of sexual dimorphism in plants.
