McLain D K, Wesson D M, Collins F H, Oliver J H
Department of Biology, Georgia Southern University, Statesboro 30460-8042, USA.
Heredity (Edinb). 1995 Sep;75 ( Pt 3):303-19. doi: 10.1038/hdy.1995.139.
Evolution of the rDNA spacer, ITS 2, is examined by comparing 17 DNA sequences of the ticks, Ixodes scapularis and I. pacificus. The distribution of fixed interspecific differences and the relative frequency of base changes vs. insertions/deletions (indels) matches the distribution and relative frequency for intraspecifically variable sites. This suggests that most intraspecific variation is not effectively selected against. The base composition of the ITS 2 transcript is G- and U-biased. But, 5-base regions enriched (> 80 per cent) for A or U occur more frequently than expected while G- and C-enriched regions occur less frequently than expected. Enriched sequences may be prone to replication slippage, accounting for the A/T bias in insertions. Slippage-mediated gains and losses of A/T-rich tandem repeats apparently account for most indels. Minimum-energy conformations of the two species' folded transcripts share major structural features. Structural inertia arises from intramolecular base pairing within stems that allows most mutations to be absorbed as new bulges off stems. Yet, there is evidence of selection to maintain the conformation. First, intraspecifically variable sites are concentrated at the ends of stems in loops and intersections, structures that do not contribute to intramolecular base pairing. Moreover, some indels that have become fixed in one species compensate for the presence of conformation-destabilizing indels. However, high rates of sequence evolution within stems and absence of compensatory base evolution contraindicates selective constraint. Degenerate dispersed and tandem copies of two subrepeats, each approximately 20 bases long, may account for much of the ITS 2 sequence. These are approximately inverses of each other and are, consequently, capable of significant intramolecular hydrogen bonding to produce folded transcripts of low energy. Evolution of the ITS 2 sequence may largely entail replication slippage-mediated gains and losses of these repeats or their composite subrepeats.
通过比较肩突硬蜱和太平洋硬蜱的17个DNA序列，研究了核糖体DNA间隔区ITS 2的进化情况。固定种间差异的分布以及碱基变化与插入/缺失（indel）的相对频率与种内可变位点的分布和相对频率相匹配。这表明大多数种内变异没有受到有效选择。ITS 2转录本的碱基组成偏向于鸟嘌呤（G）和尿嘧啶（U）。但是，富含腺嘌呤（A）或尿嘧啶（U）（>80%）的5碱基区域出现的频率高于预期，而富含鸟嘌呤（G）和胞嘧啶（C）的区域出现的频率低于预期。富含这些碱基的序列可能易于发生复制滑移，这解释了插入中A/T的偏向性。由滑移介导的富含A/T的串联重复序列的增加和减少显然是大多数插入/缺失的原因。两个物种折叠转录本的最小能量构象具有主要的结构特征。结构惯性源于茎干内的分子内碱基配对，这使得大多数突变能够作为茎干上新的凸起被吸收。然而，有证据表明存在选择以维持构象。首先，种内可变位点集中在环和交叉点处茎干的末端，这些结构对分子内碱基配对没有贡献。此外，一些在一个物种中固定下来的插入/缺失补偿了破坏构象的插入/缺失的存在。然而，茎干内的高序列进化速率以及缺乏补偿性碱基进化表明不存在选择性限制。两个亚重复序列的简并分散和串联拷贝，每个亚重复序列大约20个碱基长，可能占ITS 2序列的大部分。它们彼此大致相反，因此能够进行显著的分子内氢键结合，以产生低能量的折叠转录本。ITS 2序列的进化可能在很大程度上涉及这些重复序列或其复合亚重复序列的复制滑移介导的增加和减少。
