Physical mapping of the elephant X chromosome: conservation of gene order over 105 million years.
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.
Comparative Genomics Group, Research School of Biology, The Australian National University, GPO Box 475, ACT 2601, Canberra, Australia.
Claudia Leticia Rodríguez
CL
Delgado
Paul D
PD
Waters
Clément
C
Gilbert
Terence J
TJ
Robinson
Jennifer A Marshall
JA
Graves
D000818:Animals; D004606:Elephants / Q000235:genetics*; D019143:Evolution, Molecular; D005260:Female; D006801:Humans; D008297:Male; D010802:Phylogeny; D020161:Physical Chromosome Mapping; D013997:Time Factors; D014960:X Chromosome; D049951:X Chromosome Inactivation