Liss M, Kirk D L, Beyser K, Fabry S
Genetik, Universität Regensburg, Universitätsstrasse 31, D-93040 Regensburg, Germany.
Curr Genet. 1997 Mar;31(3):214-27. doi: 10.1007/s002940050198.
Three nuclear spliceosomal introns in conserved locations were amplified and sequenced from 28 strains representing 14 species and 4 genera of volvocalean green algae. Data derived from the three different introns yielded congruent results in nearly all cases. In pairwise comparisons, a spectrum of taxon-specific sequence differences ranging from complete identity to no significant similarity was observed, with the most distantly related organisms lacking any conserved elements apart from exon-intron boundaries and a pyrimidine-rich stretch near the 3' splice site. A metric (SI50), providing a measure of the degree of similarity of any pair of intron sequences, was defined and used to calculate phylogenetic distances between organisms whose introns displayed statistically significant similarities. The rate of sequences divergence in the introns was great enough to provide useful information about relationships among different geographical isolates of a single species, but in most cases was too great to provide reliable guides to relationships above the species level. A substitution rate of approximately 3 x 10(-8) per intron position per year was estimated, which is about 150-fold higher than in nuclear genes encoding rRNA and about 10-fold higher than the synonymous substitution rate in protein-coding regions. Thus, these homologous introns not only provide useful information about intraspecific phylogenetic relationships, but also illustrate the concept that different parts of a gene may be subject to extremely different intensities of selection. The intron data generated here (1) reliably resolve for the first time the relationships among the five most extensively studied strains of Volvox, (2) reveal that two other Volvox species may be more closely related than had previously been suspected, (3) confirm prior evidence that particular isolates of Eudorina elegans and Pleodorina illinoisensis appear to be sibling taxa, and (4) contribute to the resolution of several hitherto unsettled issues in Chlamydomonas taxonomy.
从代表团藻目绿藻4个属14个物种的28个菌株中扩增并测序了位于保守位置的3个核剪接体内含子。几乎在所有情况下，来自3个不同内含子的数据都产生了一致的结果。在两两比较中，观察到一系列分类群特异性的序列差异，从完全相同到无显著相似性，关系最远的生物体除了外显子 - 内含子边界和3'剪接位点附近富含嘧啶的区域外，没有任何保守元件。定义了一个度量标准（SI50），用于衡量任何一对内含子序列的相似程度，并用于计算其内含子显示出统计学显著相似性的生物体之间的系统发育距离。内含子中的序列分歧率足够大，足以提供有关单个物种不同地理分离株之间关系的有用信息，但在大多数情况下，又太大而无法为物种水平以上的关系提供可靠的指导。估计每个内含子位置每年的替代率约为3×10^(-8)，这比编码rRNA的核基因高约150倍，比蛋白质编码区域的同义替代率高约10倍。因此，这些同源内含子不仅提供了有关种内系统发育关系的有用信息，还说明了一个基因的不同部分可能受到截然不同的选择强度这一概念。此处生成的内含子数据（1）首次可靠地解析了5个研究最广泛的团藻菌株之间的关系，（2）揭示另外两个团藻物种可能比之前怀疑的关系更密切，（3）证实了先前的证据，即秀丽多球藻和伊利诺多星藻的特定分离株似乎是姐妹分类群，（4）有助于解决衣藻分类学中几个迄今未解决的问题。
