During meiotic prophase, homologous chromosomes must pair (synapse) in order to be separated appropriately during the first meiotic division. It is believed that meiotic sex chromosome inactivation (MSCI) evolves to mask the sex chromosome asynapsis for the heterogametic sex with distinctive heteromorphic sex chromosomes. It is has been shown that during pachytene stage, almost all the protein coding genes on the sex chromosomes are silenced. However, a study from the Yan lab (Song et al.) has found that many X-linked miRNAs are transcribed by Pol II at the pachytene stage, suggesting that mechanisms of MSCI may be different from conventional heterochromatin silencing, and the miRNA on sex chromosomes may undergo distinct regulation. It would be interesting to check the expression of X-linked miRNAs in meiotic mutants in which MSCI is disrupted, which would indicate whether the transcription of these miRNAs is dependent on MSCI.
Xin Chenglin Li