The P{mwh.+t38} construct described above expresses DsRed from the 3xP3 promoter, which results in fluorescent photoreceptor cells in eyes, ocelli and Bolwig's organs. Its earliest expression is seen in late embryos in the nascent Bolwig's organs. The 3xP3 promoter can be ectopically activated in other tissues when transgenic constructs are inserted at some genomic sites, but, to our knowledge, the full DsRed expression pattern of the P{mwh.+t38}attPY insertion has not been explored.
Many people ask us for a Y chromosome carrying a transgene expressing a fluorescent protein to use in identifying male embryos. The Y chromosome carrying P{mwh.+t38}attPY is the only one we know about, but, unless it shows ectopic DsRed expression, it will not work for sorting younger embryos.
Even if a Y chromosome exists that provides early embryonic fluorescent protein expression, we would recommend against its use for identifying male embryos because it would also mark XXY females and it would fail to identify X0 males. A more accurate method for sexing embryos is to use a stock expressing GFP from the Sxl establishment promoter to identify females.