Ocalewicz K
Department of Ichthyology, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland.
Cytogenet Genome Res. 2013;141(2-3):114-25. doi: 10.1159/000354278. Epub 2013 Aug 24.
In fishes, as in other vertebrate species, the DNA component of the telomeres consists of the tandemly repeated TTAGGG motif. The length of the telomeric arrays in fishes ranges from 2 to 25 kb and shortens with age in some of the species. To date, chromosomal distribution of the telomeric DNA sequences has been examined in approximately 80 fish species of which about 42% show additional telomeric hybridization signals far from the chromosomal termini. Based on the chromosomal location, such internally located telomeric repeats may be classified into 4 categories: (1) telomeric DNA sequences located at the pericentromeric regions, (2) interstitial telomeric sites observed between centromeres and the bona fide telomeres, (3) telomeric DNA sequences that scatter along the nucleolus organizer regions, and (4) telomeric DNA repeats interspersed with the entire chromosomes. Most of the pericentromeric and interstitial telomeric sequences in fish are possible relicts of chromosome fusion events. The origin of the telomeric sequences co- localizing with the major rDNA sequences or scattered along the whole chromosomes is not clear. Internally located telomeric repeats are considered as 'hot spots' for recombination and thus may potentially increase the rates of chromosome breaks and rearrangements leading to the various chromosomal polymorphisms in fishes. FISH with telomeric probe applied to metaphase spreads of androgenetic specimens that hatched from eggs exposed to ionizing radiation before insemination enabled the detection of small radiation-induced fragments of maternal chromosomes. Remnants of the irradiated chromosomes were found to be ring chromosomes with the interstitial telomeric signals, telomerless rings, fragments with fused sister chromatids, and linear fragments with telomeres detected at both of their ends. The increasing availability of techniques enabling the study of fish telomeres and telomerase and the easy access to numerous fish species strongly confirm that these animals are promising models in research concerning the role of telomeres and telomerase in vertebrate aging, repair of ionizing radiation-induced DNA double strand breaks, and chromosomal rearrangements.
与其他脊椎动物物种一样，鱼类端粒的DNA成分由串联重复的TTAGGG基序组成。鱼类端粒阵列的长度在2至25 kb之间，并且在某些物种中会随着年龄的增长而缩短。迄今为止，已经在大约80种鱼类中检测了端粒DNA序列的染色体分布，其中约42%的鱼类在远离染色体末端的位置显示出额外的端粒杂交信号。根据染色体位置，这种位于内部的端粒重复序列可分为4类：(1)位于着丝粒周围区域的端粒DNA序列；(2)在着丝粒和真正端粒之间观察到的间质端粒位点；(3)沿着核仁组织区散布的端粒DNA序列；(4)散布在整个染色体中的端粒DNA重复序列。鱼类中大多数着丝粒周围和间质端粒序列可能是染色体融合事件的遗迹。与主要rDNA序列共定位或散布在整个染色体上的端粒序列的起源尚不清楚。位于内部的端粒重复序列被认为是重组的“热点”，因此可能会增加染色体断裂和重排的速率，从而导致鱼类出现各种染色体多态性。将端粒探针应用于受精前暴露于电离辐射的卵子孵化出的雄核发育标本的中期染色体铺片，通过荧光原位杂交能够检测到小的辐射诱导的母本染色体片段。发现受辐射染色体的残余物为具有间质端粒信号的环状染色体、无端粒环、具有融合姐妹染色单体的片段以及两端均检测到端粒的线性片段。随着能够研究鱼类端粒和端粒酶的技术越来越容易获得，以及能够方便地获取众多鱼类物种，有力地证实了这些动物是研究端粒和端粒酶在脊椎动物衰老、电离辐射诱导的DNA双链断裂修复以及染色体重排中的作用的有前途的模型。
