Kooistra W H, Medlin L K
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
Mol Phylogenet Evol. 1996 Dec;6(3):391-407. doi: 10.1006/mpev.1996.0088.
Small subunit ribosomal RNA (ssu rRNA) coding regions from 30 diatoms, 3 oomycetes, and 6 pelagophytes were used to construct linearized trees, maximum-likelihood trees, and neighbor-joining trees inferred from both unweighted and weighted distances. Stochastic accumulation of sequence substitutions among the diatoms was assessed with relative rate tests. Pennate diatoms evolved relatively slowly but within the limits set by a stochastic model; centric diatoms exceeded those limits. A rate distribution test was devised to identify those taxa showing an aberrant distribution of base substitutions within the ssu rRNA coding region. First appearance dates of diatom taxa from the fossil record were regressed against their corresponding branch lengths to infer the average and earliest possible age for the origin of the diatoms, the pennate diatoms, and the centric diatom order Thalassiosirales. Our most lenient age estimate (based on the median-evolving diatom taxon in the maximum-likelihood tree or on the average branch length in a linearized tree) suggests that their average age is approximately 164-166 Ma, which is close to their earliest fossil record. Both calculations suggest that it is unlikely that diatoms existed prior to 238-266 Ma. Rate variation among the diatoms' ssu rRNA coding regions and uncertainties associated with the origin of extant taxa in the fossil record contribute significantly to the variation in age estimates obtained. Different evolutionary models and the exclusion of fast or slow evolving taxa did not significantly affect age estimates; however, the inclusion of aberrantly fast evolving taxa did. Our molecular clock calibrations indicate that the rRNA coding regions in the diatoms are evolving at approximately 1% per 18 to 26 Ma, which is the fastest substitution rate reported in any pro- or eukaryotic group of organisms to date.
来自30种硅藻、3种卵菌和6种褐胞藻的小亚基核糖体RNA（ssu rRNA）编码区域被用于构建从非加权和加权距离推断出的线性化树、最大似然树和邻接树。通过相对速率测试评估硅藻中序列替换的随机积累。羽纹硅藻进化相对缓慢，但在随机模型设定的范围内；中心硅藻则超出了这些范围。设计了一种速率分布测试，以识别那些在ssu rRNA编码区域内碱基替换分布异常的分类群。将化石记录中硅藻分类群的首次出现日期与其相应的分支长度进行回归，以推断硅藻、羽纹硅藻和中心硅藻目海链藻目的起源的平均年龄和最早可能年龄。我们最宽松的年龄估计（基于最大似然树中进化速度居中的硅藻分类群或线性化树中的平均分支长度）表明它们的平均年龄约为164 - 166百万年，这与它们最早的化石记录接近。两种计算都表明硅藻不太可能在238 - 266百万年之前就已存在。硅藻的ssu rRNA编码区域之间的速率变化以及与化石记录中现存分类群起源相关的不确定性，对所获得的年龄估计的变化有显著贡献。不同的进化模型以及排除快速或缓慢进化的分类群对年龄估计没有显著影响；然而，纳入异常快速进化的分类群则有影响。我们的分子钟校准表明，硅藻中的rRNA编码区域每18至26百万年以约1%的速率进化，这是迄今为止在任何原核或真核生物群体中报道的最快替换速率。
