HIGH-RATES OF NUCLEOTIDE SUBSTITUTION IN NUCLEAR SMALL-SUBUNIT (18S) RDNA FROM HOLOPARASITIC FLOWERING PLANTS

Relative rate tests, using Gnetum as a reference taxon, were conducted on nuclear 18S rRNA sequences from 10 angiosperms including autotroph ic nonparasites (Arabidopsis, Asarum, Glycine, Malpighia, and Zea), a chlorophyllous hemiparasite (Arceuthobium--Viscaceae), and achlorophyl lous holoparasites (Balanophora-Balanophoraceae, Prosopanche-Hydnorace ae, and Rafflesia and Rhizanthes-Rafflesiaceae). Compared with Glycine , the mean number of substitutions per site (K) for five autotrophic a ngiosperms is 0.036 whereas for the holoparasites K = 0.126, i.e., 3.5 times higher. Comparisons of autotrophic species with short and long generation times showed no differences in K; hence, divergent rRNA seq uences in the holoparasites are likely attributable to other mechanism s. These might include genetic bottlenecks, effective population size, and/or molecular drive. High substitution rates appear to be associat ed only with those parasitic angiosperms that have developed a highly modified haustorial system and extreme nutritional dependence upon the host. At present, high substitution rates in these parasites confound attempts to determine their phylogenetic position relative to other a ngiosperms.