DNA amounts in two samples of angiosperm weeds

Of the world's 250000 angiosperm species, only about 200 are recognized as important weeds. 4C nuclear DNA amounts were estimated for 39 such species. Success for many important weeds is suggested to reflect several traits kn own to correlate with low DNA C-value, so such weeds may have smaller DNA C -values than other species. Our work tests this hypothesis, comparing DNA a mounts in 156 species recognized as important world weeds or British garden weeds, with 2685 other species. DNA amounts did not differ significantly b etween the two weed samples, but weeds showed highly significant difference s from other species. For example, nuclear DNA amount in weeds (mean 11.74 pg) was smaller than in other species (mean 28.13 pg), and restricted to th e lowest 20 % of their range. Similarly, DNA amount per genome in weeds (me an 3.79 pg) was smaller than in other species (mean 12.14 pg), and restrict ed to the lowest 10 % of their range. As significant differences between we eds and other species remain for almost all subsamples tested, this contras t is widely distributed. So it is important to ask how selection against hi gh nuclear DNA amount and genome size in weeds operates. The probability of a species being a weed fell significantly with increasing nuclear 4C DNA a mount, and mean genome size, reaching zero just above 100 pg, and 19 pg, re spectively. Moreover, polyploidy was significantly more frequent in weeds ( 51 %) than in other species (27 %), increasing with nuclear DNA amount in b oth, reaching 100 % in weeds with the highest 4C DNA amounts, but only 41 % in other species. Thus, selection for polyploidy in weeds may partly refle ct their increased genetic variability, independent of DNA amount. However, such selection pressure grows strongly with rising nuclear DNA amount, and this may act mainly on correlated factors including faster development. (C ) 1998 Annals of Botany Company.