Effects of inbreeding in three populations of the dioecious annual Amaranthus cannabinus (Amaranthaceae)

BRAM, M. R. (Patrick Center for Environmental Research, The Academy of Natural Sciences, Philadelphia, PA 19103-1195). Effects of inbreeding in three populations of the dioecious annual Amaranthus cannabinus (Amaranthaceae). J. Torrey Bet. Soc. 129: 298-310. 2002.-The avoidance of inbreeding is thought by many to be a strong selective force leading to the evolution of dioecy. The purpose of this study was to examine the effects of inbreeding on Amaranthus cannabinus (L.) Sauer (water hemp), a dioecious annual. Inbred and outcrossed lines were established in the greenhouse for three New Jersey populations of water hemp, one salt marsh population and two freshwater marsh populations. After two generations of crosses, inbreeding depression was observed in the percentage of germination, leaf size, and plant height. However, the response to inbreeding varied considerably among and within the three populations. Overall, inbreeding depression was low, suggesting that the avoidance of inbreeding was not a strong selective drive for the evolution of dioecy in A. cannabinus. Another selective factor that more likely contributed to the evolution of dioecy in water hemp is increased reproductive efficiency through sex-specific selection.