Sajdak S L, Reed K M, Phillips R B
Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA.
J Mol Evol. 1998 Jun;46(6):680-8. doi: 10.1007/pl00006348.
This study was designed to characterize further the nontranscribed intergenic spacers (NTSs) of the 5S rRNA genes of fish and evaluate this marker as a tool for comparative studies. Two members of the closely related North American Great Lakes cisco species complex (Coregonus artedi and C. zenithicus) were chosen for comparison. Fluorescence in situ hybridization found the ciscoes to have a single multicopy 5S locus located in a C band-positive region of the largest submetacentric chromosome. The entire NTS was amplified from the two species by polymerase chain reaction with oligonucleotide primers anchored in the conserved 5S coding region. Complete sequences were determined for 25 clones from four individuals representing two discrete NTS length variants. Sequence analysis found the length variants to result from presence of a 130-bp direct repeat. No two sequences from a single fish were identical. Examination of sequence from the coding region revealed two types of 5S genes in addition to pseudogenes. This suggests the presence of both somatic and germline (oocyte) forms of the 5S gene in the genome of Coregonus. The amount of variation present among NTS sequences indicates that accumulation of variation (mutation) is greater in this multicopy gene than is gene conversion (homogenization). The high level of sequence variation makes the 5S NTS an inappropriate DNA sequence for comparisons of closely related taxa.
本研究旨在进一步表征鱼类5S rRNA基因的非转录基因间隔区（NTSs），并评估该标记作为比较研究工具的价值。选择了北美五大湖白鲑属密切相关的两个物种（阿氏白鲑和顶峰白鲑）进行比较。荧光原位杂交发现，白鲑有一个单一的多拷贝5S基因座，位于最大亚中着丝粒染色体的C带阳性区域。使用锚定在保守5S编码区的寡核苷酸引物，通过聚合酶链反应从这两个物种中扩增出整个NTS。测定了代表两个不同NTS长度变体的四个个体的25个克隆的完整序列。序列分析发现，长度变体是由一个130 bp的直接重复序列的存在导致的。来自同一条鱼的序列没有两个是相同的。对编码区序列的检查发现，除了假基因外，还有两种类型的5S基因。这表明在白鲑基因组中存在5S基因的体细胞形式和种系（卵母细胞）形式。NTS序列之间存在的变异量表明，在这个多拷贝基因中，变异（突变）的积累比基因转换（同质化）更大。高水平序列变异使得5S NTS成为用于密切相关分类群比较的不合适DNA序列。
