Originally published as Genetics Published Articles Ahead of Print on July 13, 2008.

Genetics, Vol. 179, 1721-1723, July 2008, Copyright © 2008
doi:10.1534/genetics.108.090746

A Key Transcription Cofactor on the Nascent Sex Chromosomes of European Tree Frogs (Hyla arborea)

Department of Ecology and Evolution, University of Lausanne, 1015 Lausanne, Switzerland

1 Corresponding author: Department of Ecology and Evolution, Biology Bldg., University of Lausanne, 1015 Lausanne, Switzerland.
E-mail: nicolas.perrin{at}unil.ch

We show that MED15, a key component of the transcription complex Mediator, lies within the nonrecombining segment of nascent sex chromosomes in the male-heterogametic Hyla arborea. Both X and Y alleles are expressed during embryonic development and differ by three frame-preserving indels (eight amino acids in total) within their glutamine-rich central part. These changes have the potential to affect the conformation of the Mediator complex and to activate genes in a sex-specific way and might thus represent the first steps toward the acquisition of a male-specific function. Alternatively, they might result from an ancestral neutral polymorphism, with different alleles picked by chance on the X and Y chromosomes when MED15 was trapped in the nonrecombining segment.