A key transcription cofactor on the nascent sex chromosomes of European tree frogs (Hyla arborea)

Abstract

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.

Figure 1.—

Schematic of the X and Y HaMED15 alleles with their conserved N-terminal (KIX-domain) and C-terminal regions (solid boxes). Arrows and shaded boxes indicate the three sex-specific frame-preserving indels within the central polyQ region. Detailed methods and sequences appear in supplemental material.