Chromosome Research

, 17:917 | Cite as

Physical mapping of the elephant X chromosome: conservation of gene order over 105 million years

  • Claudia Leticia Rodríguez Delgado
  • Paul D. Waters
  • Clément Gilbert
  • Terence J. Robinson
  • Jennifer A. Marshall Graves


All therian mammals (eutherians and marsupials) have an XX female/XY male sex chromosome system or some variant of it. The X and Y evolved from a homologous pair of autosomes over the 166 million years since therian mammals diverged from monotremes. Comparing the sex chromosomes of eutherians and marsupials defined an ancient X conserved region that is shared between species of these mammalian clades. However, the eutherian X (and the Y) was augmented by a recent addition (XAR) that is autosomal in marsupials. XAR is part of the X in primates, rodents, and artiodactyls (which belong to the eutherian clade Boreoeutheria), but it is uncertain whether XAR is part of the X chromosome in more distantly related eutherian mammals. Here we report on the gene content and order on the X of the elephant (Loxodonta africana)—a representative of Afrotheria, a basal endemic clade of African mammals—and compare these findings to those of other documented eutherian species. A total of 17 genes were mapped to the elephant X chromosome. Our results support the hypothesis that the eutherian X and Y chromosomes were augmented by the addition of autosomal material prior to eutherian radiation. Not only does the elephant X bear the same suite of genes as other eutherian X chromosomes, but gene order appears to have been maintained across 105 million years of evolution, perhaps reflecting strong constraints posed by the eutherian X inactivation system.


sex chromosome evolution afrotheria X chromosome X added region 



X conserved region


X added region


Y conserved region


Y added region


Million years ago


Pseudoautosomal region


PAR boundary


Testis determining factor


X chromosome inactivation


Fluorescence in situ hybridization


Bacterial artificial chromosome


Children's Hospital Oakland Research Institute


Virginia Mason Research Center


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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Claudia Leticia Rodríguez Delgado
    • 1
    • 2
  • Paul D. Waters
    • 1
  • Clément Gilbert
    • 3
    • 4
  • Terence J. Robinson
    • 3
  • Jennifer A. Marshall Graves
    • 1
  1. 1.Comparative Genomics Group, Research School of BiologyThe Australian National UniversityCanberraAustralia
  2. 2.Centro de Ciencias GenómicasUniversidad Nacional Autónoma de MéxicoCuernavaca MorelosMéxico
  3. 3.Evolutionary Genomics Group, Department of ZoologyUniversity of StellenboschMatielandSouth Africa
  4. 4.Department of BiologyUniversity of Texas at ArlingtonArlingtonUSA

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