A Window of MHM Demethylation Correlates with Key Events in Gonadal Differentiation in the Chicken.

The MHM (male hypermethylated) region on the chicken Z chromosome is of interest in terms of sex determination both because hypermethylation of the promoter in males results in female-specific expression, and in females the non-coding transcripts accumulate in the region of a locus that encodes a key candidate gene for testis differentiation in birds - DMRT1. However, it has yet to be established whether MHM plays a role in either sex determination or gonadal development in chicken. Here, we sought to establish the MHM methylation pattern during chicken gonadal development and determine how this pattern correlates with the expression profiles of genes involved in sex determination and gonadal differentiation. As expected, we found that MHM methylation levels were higher in male than in female gonads at most stages of development. We also showed that there was no significant difference in the methylation patterns between the right and left gonad and that the pattern of methylation in female gonads was not affected by sex reversal. However, we did find an unexpected window where MHM promoter methylation levels were very low in both males and females. This developmental window coincided with a stage of gonad development where there is a dramatic increase in expression of key testis genes in males and of key ovary genes in females. The current study raises the possibility that the non-coding RNA transcripts of MHM may affect testis and ovary differentiation by influencing sex-specific gene expression.