Genetic and evolutionary consequences of harvest in American ginseng, Panax quinquefolius L. (Araliaceae).

摘要

American ginseng (Panax quinquefolius L.) is a wild-harvested perennial plant of the eastern deciduous forest. Harvest supplies world markets with roots used in Asian medicine, but this practice is fatal to plants. The objective of this research was to investigate the genetic and evolutionary consequences of harvest. As seen in animal species, harvest may alter size selection by preferentially removing large individuals. Chapter 2 describes my study of harvest's effects on size selection. From the simulated harvests, I observed that large-sized plants lose their fitness advantages in harvested populations. Harvest pressure could ultimately lead to population divergence if selected traits are genetically-based. As described in Chapter 3, I collected size, reproductive and age data from plants in 12 wild populations. I then used the proportion of seedlings and juvenile plants as a 'harvest index', which was based on the recovery of an experimentally-harvested population. In most study years, the age-size relationship varied with harvest index. In a separate common garden study, I also found that size differences were maintained among populations 3 to 4 years after transplantation, suggesting genetically-based variation. Harvest also reduces genetic diversity, which may lead to increased levels of inbreeding in affected populations. At the same time, unusual levels of outcrossing are possible because of 'restocking' with cultivated seeds. Chapter 4 describes the controlled crosses that I conducted to evaluate the effects of inbreeding and outcrossing with cultivated plants. The smaller size of seedlings produced from self-pollination relative to those from cross-pollination suggested inbreeding depression, but cultivated genotypes may confer accelerated growth not observed in the wild. As described in Chapter 5, I also examined the importance of genetic diversity to population growth rate. Eighteen populations were censused to obtain demographic data and their genetic diversity was assessed using neutral DNA markers (RAPD). Because of the descriptive nature of the data, I used path analysis to model and test for relationships among genetic diversity, population size and harvest pressure, and how these in turn affect population growth rate. From the results of the path analysis, harvest pressure had a negative influence on population growth, whereas genetic diversity contributed positively to population growth rates. Altogether, harvest may have far-reaching, unintended effects for populations of P. quinquefolius in the wild.