XVI International Botanical Congess

Weeds are important plants with impact on a global scale. Tests showed that two large samples of weeds had much smaller DNA amounts and genome sizes (both means and ranges) than other taxa. How does selection for this operate? The probability of a species being listed as a weed fell significantly with both decreasing C-value and genome size, reaching zero at about 100 pg and 20 pg, respectively. Polyploidy was significantly more frequent in weeds (51%) than in other species (27%), but increased with DNA amount in both. Selection pressure for polyploidy in weeds may partly reflect their increased genetic variability independent of DNA amount. However, it grows strongly with rising DNA amount in both diploids and polyploids, and may act mainly on correlated factors such as faster development. Knowledge of DNA amount can help predict the probability of taxa becoming serious weeds in new environments.