Nickrent D L, Starr E M
Department of Plant Biology, Southern Illinois University, Carbondale 62901.
J Mol Evol. 1994 Jul;39(1):62-70. doi: 10.1007/BF00178250.
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.
以买麻藤属作为参考分类群，对来自10种被子植物的核18S rRNA序列进行了相对速率测试，这些被子植物包括自养非寄生植物（拟南芥、细辛、大豆、金虎尾、玉米）、一种含叶绿素的半寄生植物（油杉寄生——桑寄生科）以及不含叶绿素的全寄生植物（蛇菰——蛇菰科、肉苁蓉——锁阳科、大王花属和寄生花属——大花草科）。与大豆相比，5种自养被子植物每个位点的平均替换数（K）为0.036，而全寄生植物的K值为0.126，即高出3.5倍。对世代周期短和长的自养物种进行比较，K值没有差异；因此，全寄生植物中rRNA序列的差异可能归因于其他机制。这些机制可能包括遗传瓶颈、有效种群大小和/或分子驱动。高替换率似乎仅与那些已发育出高度特化的吸器系统且对宿主有极端营养依赖的寄生被子植物有关。目前，这些寄生虫中的高替换率使得确定它们相对于其他被子植物的系统发育位置的尝试变得复杂。
