Savard L, Li P, Strauss S H, Chase M W, Michaud M, Bousquet J
Centre de Recherche en Biologie Forestière, Faculté de Foresterie et de Géomatique, Université Laval, Ste-Foy, Québec, Canada.
Proc Natl Acad Sci U S A. 1994 May 24;91(11):5163-7. doi: 10.1073/pnas.91.11.5163.
We have estimated the time for the last common ancestor of extant seed plants by using molecular clocks constructed from the sequences of the chloroplastic gene coding for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) and the nuclear gene coding for the small subunit of rRNA (Rrn18). Phylogenetic analyses of nucleotide sequences indicated that the earliest divergence of extant seed plants is likely represented by a split between conifer-cycad and angiosperm lineages. Relative-rate tests were used to assess homogeneity of substitution rates among lineages, and annual angiosperms were found to evolve at a faster rate than other taxa for rbcL and, thus, these sequences were excluded from construction of molecular clocks. Five distinct molecular clocks were calibrated using substitution rates for the two genes and four divergence times based on fossil and published molecular clock estimates. The five estimated times for the last common ancestor of extant seed plants were in agreement with one another, with an average of 285 million years and a range of 275-290 million years. This implies a substantially more recent ancestor of all extant seed plants than suggested by some theories of plant evolution.
我们通过使用由编码核酮糖-1,5-二磷酸羧化酶/加氧酶（rbcL）大亚基的叶绿体基因序列和编码rRNA小亚基（Rrn18）的核基因序列构建的分子钟，估算了现存种子植物最近共同祖先的时间。核苷酸序列的系统发育分析表明，现存种子植物最早的分化可能表现为针叶树-苏铁类植物和被子植物谱系之间的分歧。相对速率检验用于评估谱系间替代率的同质性，结果发现对于rbcL而言，一年生被子植物的进化速率比其他类群更快，因此，这些序列被排除在分子钟的构建之外。利用这两个基因的替代率以及基于化石和已发表分子钟估计的四个分歧时间，校准了五个不同的分子钟。现存种子植物最近共同祖先的五个估计时间相互一致，平均为2.85亿年，范围在2.75 - 2.90亿年之间。这意味着所有现存种子植物的祖先比一些植物进化理论所暗示的要近得多。
