Schmid Michael, Steinlein Claus, Feichtinger Wolfgang, Nanda Indrajit
Department of Human Genetics, University of Würzburg, Würzburg, Germany.
Cytogenet Genome Res. 2017;152(4):180-193. doi: 10.1159/000481554. Epub 2017 Oct 24.
A 7-year cytogenetic study on the leaf litter frog Craugastor fitzingeri from Costa Rica and Panama revealed the existence of highly mobile nucleolus organizing regions (NORs) in their genomes. Silver (Ag)-staining of the active NORs demonstrated an exceptional interindividual pattern of NOR distribution at the telomeres of the chromosomes. All individuals examined showed a different and specific NOR location in their karyotypes. Furthermore, intraindividual variation in the NOR sites was found. This observation suggested the existence of mobile NORs in C. fitzingeri. Confirmation of this phenomenon was possible by systematic FISH analysis using an 18S + 28S rDNA probe. The extremely variable number and position of the NORs in C. fitzingeri is best explained by highly mobile NORs that move freely between the telomeres of the chromosomes. These transpositions must occur preferentially in premeiotic, meiotic, or postmeiotic stages, but also at a lower incidence in the somatic tissues of the animals. It is hypothesized that transposable (mobile) elements are closely linked to the NORs or are inserted into the major 18S + 28S rDNA spacers of C. fitzingeri. When such transposable elements spread by transpositions, they can carry with them complete or partial NORs. The present study provides detailed information on various differential chromosome banding techniques, in situ hybridization experiments, chromosomal hypermethylation patterns, determination of the genome size, and analyses of restriction fragment length polymorphisms of the DNA.
一项针对来自哥斯达黎加和巴拿马的枯叶蛙（Craugastor fitzingeri）进行的为期7年的细胞遗传学研究揭示，其基因组中存在高度可移动的核仁组织区（NORs）。对活跃NORs进行银（Ag）染色显示，NORs在染色体端粒处呈现出异常的个体间分布模式。所有检测的个体在其核型中都显示出不同且特定的NOR位置。此外，还发现了个体内部NOR位点的变异。这一观察结果表明，枯叶蛙中存在可移动的NORs。通过使用18S + 28S rDNA探针进行系统的荧光原位杂交（FISH）分析，证实了这一现象。枯叶蛙中NORs数量和位置的极端变异性，最好的解释是高度可移动的NORs在染色体端粒之间自由移动。这些转座必定优先发生在减数分裂前、减数分裂或减数分裂后阶段，但在动物的体细胞组织中发生率较低。据推测，转座（可移动）元件与枯叶蛙的NORs紧密相连，或者插入到其主要的18S + 28S rDNA间隔区中。当这些转座元件通过转座传播时，它们可以携带完整或部分的NORs。本研究提供了关于各种染色体差异带型技术、原位杂交实验、染色体超甲基化模式、基因组大小测定以及DNA限制性片段长度多态性分析的详细信息。
