Warm-blooded isochore structure in Nile crocodile and turtle

Area of Science:

• Genomics
• Molecular Evolution
• Comparative Genomics

Background:

• Vertebrate genomes exhibit compositional heterogeneity with GC-rich and GC-poor isochores.
• GC content in coding sequences correlates with their host isochores.
• GC-rich isochores are rare in amphibians and fish, leading to the homeothermy hypothesis for their prevalence in mammals and birds.

Purpose of the Study:

• To investigate the evolutionary origins of GC-rich isochores.
• To test the hypothesis that GC increase is an adaptation to homeothermy.
• To analyze isochore structure and molecular evolution in cold-blooded reptiles.

Main Methods:

• Sequencing of marker protein genes in the Nile crocodile (Crocodylus niloticus) and the red-eared slider turtle (Trachemys scripta elegans).
• Analysis of base composition at third codon positions.
• Comparative analysis of isochore structure across different vertebrate groups.

Main Results:

• GC-rich isochores were identified in both the Nile crocodile and the red-eared slider turtle.
• The presence of GC-rich isochores in these cold-blooded species refutes the homeothermy hypothesis.
• Isochore structure similarity between the turtle and crocodile is greater than between the chicken and crocodile, questioning turtle phylogeny.
• A regular molecular clock was observed in Archosauria.

Conclusions:

• The increase in GC content is likely due to a mutational bias predating homeothermy adaptation and turtle/crocodile divergence.
• The findings challenge the direct link between homeothermy and GC-rich isochores.
• The study provides insights into reptile phylogeny and evolutionary timescales within Archosauria.