In situ hybridization for mpz transcripts (purple) at 3dpf (A) and 6dpf (B) in transverse spinal cord sections showed that mpz expression increased in the lateral spinal cord of developing embryos during nascent myelination in larvae. In order to assess when mpz promoter activity was specific to oligodendrocyte lineage cells, the whole spinal cord of Tg(mpz:Gal4;UAS:Kaede) larvae was photoconverted at 2.5dpf and larvae allowed to develop until 6dpf. Presence of photoconverted, stabilized Kaede protein in the spinal cord of Tg(mpz:Gal4;UAS:Kaede) larvae at 6dpf showed that the mpz reporter had been active before 2.5dpf in numerous spinal cord neural progenitors including oligodendrocyte lineage cells (C, E, red cells; converted Kaede). However, the mpz reporter was active only in myelinating oligodendrocytes after 2.5dpf (D, E, green cells; unconverted Kaede). (scale bar = 50μM)