Reassessment of genome size in turtle and crocodile based on chromosome measurement by flow karyotyping: close similarity to chicken

Abstract

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.

Figure 1.

Bivariate flow karyotypes of (a) GGA, (b) TSC and (c) CNI. Smaller chromosomes in GGA and TSC were detected in higher resolution. CNI-1 is absent in this setting and appears in a higher position at lower resolution. Peak positions for HSA-4, -17 and -19 are marked by small circles.

Figure 2.

(a) GGA, CNI and TSC chromosomes classified into two groups based on GC content show strong relation to size. The CNI small chromosomes are larger than GGA and TSC microchromosomes, but have a higher GC content than the CNI large chromosomes. (b) The correlation between TG size and GC content is shown for the three species and compared with HSA.