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 autotrophic nonparasites (Arabidopsis, Asarum, Glycine, Malpighia, and Zea), a chlorophyllous hemiparasite (Arceuthobium--Viscaceae), and achlorophyllous holoparasites (Balanophora--Balanophoraceae, Prosopanche--Hydnoraceae, and Rafflesia and Rhizanthes--Rafflesiaceae). Compared with Glycine, the mean number of substitutions per site (K) for five autotrophic angiosperms 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 sequences in the holoparasites are likely attributable to other mechanisms. These might include genetic bottlenecks, effective population size, and/or molecular drive. High substitution rates appear to be associated 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 angiosperms.
Nickrent
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DL
Department of Plant Biology, Southern Illinois University, Carbondale 62901.
Starr
E M
EM
D001483:Base Sequence; D017931:DNA Primers; D004275:DNA, Ribosomal / Q000235:genetics*; D014644:Genetic Variation; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D018521:Plant Physiological Phenomena; D010944:Plants / Q000235:genetics*; D016133:Polymerase Chain Reaction / Q000379:methods; D012337:RNA, Ribosomal, 18S / Q000737:chemistry / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid
D017931:DNA Primers; D004275:DNA, Ribosomal; D012337:RNA, Ribosomal, 18S