Journal of Molecular Evolution

, Volume 58, Issue 6, pp 653-660

First online:

The Decline of Isochores in Mammals: An Assessment of the GC ContentVariation Along the Mammalian Phylogeny

  • Elise M. S. BelleAffiliated withCentre for the Study of Evolution—School of Life Sciences, University of Sussex Email author 
  • , Laurent DuretAffiliated withLaboratoire de Biométrie et Biologie évolutive, UMR CNRS 5558, Université Claude Bernard Lyon 1
  • , Nicolas GaltierAffiliated withLaboratoire Génome, Populations, Interactions, Adaptations, CNRS Université Montpellier 2, IFREMER UMR 5000
  • , Adam Eyre-WalkerAffiliated withCentre for the Study of Evolution—School of Life Sciences, University of Sussex

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Whether isochores, the large-scale variation of the GC content in mammalian genomes, are being maintained has recently been questioned. It has been suggested that GC-rich isochores originated in the ancestral amniote genome but that whatever force gave rise to them is no longer effective and that isochores are now disappearing from mammalian genomes. Here we investigated the evolution of the GC content of 41 coding genes in 6 to 66 species of mammals by estimating the ancestral GC content using a method which allows for different rates of substitution between sites. We found a highly significant decrease in the GC content during early mammalian evolution, as well as a weaker but still significant decrease in the GC content of GC-rich genes later in at least three groups of mammals: primates, rodents, and carnivores. These results are of interest because they confirm the recently suggested disappearance of GC-rich isochores in some mammalian genomes, and more importantly, they suggest that this disappearance started very early in mammalian evolution.


Isochore Genome GC content Noncoding DNA Mammal Primate