A tradeoff between sexual and asexual reproduction in the dioecious clonal macrophyte Vallisneria americana: Environmental and genetic influences

Life history theory has long predicted that tradeoffs should exist between sexual and asexual reproduction in clonal organisms. Preliminary research on the submersed aquatic macrophyte Vallisneria americana suggested that in females from a single population grown in the greenhouse, the production of fruit and tubers (the asexual propagule and sole perennating organ) was inversely related. To investigate the nature of the relationship between sexual and asexual reproductive allocation further, I (1) examined the relationship between sexual reproductive effort (SRE) and asexual reproductive effort (ARE) in males and females; (2) manipulated male and female SRE by removing immature inflorescences (males) or flowers (females) in the early stages of development to examine the effect on ARE; (3) examined the effect of dissolved inorganic carbon or sediment availability on male and female reproductive allocation patterns; (4) tested for a genetic basis to the tradeoff observed between sexual and asexual reproduction in females; and (5) conducted a field survey of sites with different physical characteristics to examine what effect site characteristics play in limiting pollination success among natural populations.; A tradeoff in the form of a negative correlation between SRE and ARE was consistently found among females grown under different greenhouse conditions, but this tradeoff was not observed in males. When SRE was manipulated in females by removing floral buds, biomass that might have gone into developing fruits was allocated to tuber production, so that there was no difference in total reproductive effort (TRE: sexual plus asexual) between manipulated and unmanipulated plants. No differences were observed in ARE between males with buds removed and buds intact. This tradeoff appears to be related to the timing of reproductive investment, where fruit development overlaps with tuber development in females, but there is little overlap between inflorescence production and tuber production among males.; When males and females were grown under different resource conditions, I found that in response to increased carbon or mineral nutrient availability, males tended to decrease the mass specific number of tubers produced and increase the mean size of tubers. In contrast, females increased the mean size of fruits, but did not change the mean size or mass specific number of tubers. Females did increase TRE in response to increased carbon or mineral nutrient availability, but the balance between sexual and asexual reproduction was not affected.; The genetic basis of several traits related to the tradeoff between sexual and asexual reproduction among females was investigated within 13 clonal and 12 sexually produced families. The intraclass correlations for SRE, ARE, and the balance between them showed that there was significant genetic variation in these traits. In addition, a significant negative genetic correlation between SRE and ARE was estimated from the family means of these traits. These results suggested that the tradeoff observed in females between sexual and asexual reproduction had a significant genetic component. (Abstract shortened by UMI.)...