Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.
Biol Lett. 2012 Aug 23;8(4):631-5. doi: 10.1098/rsbl.2012.0141. Epub 2012 Apr 4.
The genome size in turtles and crocodiles is thought to be much larger than the 1.2 Gb of the chicken (Gallus gallus domesticus, GGA), according to the animal genome size database. However, GGA macrochromosomes show extensive homology in the karyotypes of the red eared slider (Trachemys scripta elegans, TSC) and the Nile crocodile (Crocodylus niloticus, CNI), and bird and reptile genomes have been highly conserved during evolution. In this study, size and GC content of all chromosomes are measured from the flow karyotypes of GGA, TSC and CNI. Genome sizes estimated from the total chromosome size demonstrate that TSC and CNI are 1.21 Gb and 1.29 Gb, respectively. This refines previous overestimations and reveals similar genome sizes in chicken, turtle and crocodile. Analysis of chromosome GC content in each of these three species shows a higher GC content in smaller chromosomes than in larger chromosomes. This contrasts with mammals and squamates in which GC content does not correlate with chromosome size. These data suggest that a common ancestor of birds, turtles and crocodiles had a small genome size and a chromosomal size-dependent GC bias, distinct from the squamate lineage.
根据动物基因组大小数据库，龟鳖类动物的基因组大小被认为比鸡（Gallus gallus domesticus，GGA）的 1.2Gb 大得多。然而，GGA 巨染色体在红耳滑龟（Trachemys scripta elegans，TSC）和尼罗鳄（Crocodylus niloticus，CNI）的核型中表现出广泛的同源性，并且鸟类和爬行动物的基因组在进化过程中高度保守。在这项研究中，从 GGA、TSC 和 CNI 的流式核型中测量了所有染色体的大小和 GC 含量。从总染色体大小估计的基因组大小表明，TSC 和 CNI 分别为 1.21Gb 和 1.29Gb。这修正了以前的高估，并揭示了鸡、龟和鳄之间相似的基因组大小。对这三个物种中每条染色体的 GC 含量的分析表明，较小的染色体的 GC 含量高于较大的染色体。这与哺乳动物和有鳞目动物形成对比，在这些动物中，GC 含量与染色体大小不相关。这些数据表明，鸟类、龟鳖类和鳄类的共同祖先具有较小的基因组大小和依赖于染色体大小的 GC 偏倚，与有鳞目动物的谱系不同。
