Developmental onset of the U7 mutant phenotype. (A) RNA isolated from w1118 control embryos and subjected to Northern analysis with a 32P-labeled U7 RNA probe. (Lanes 1–3) equal numbers of 0- to 2-, 2- to 4-, and 4- to 8-h-old embryos, respectively. (B) Equal amounts of total RNA extracted from different stages of development and subjected to Northern analysis with a 32P-labeled H3 probe. (Lane 1) Embryos collected overnight from U714/TM3 heterozygous parents and allowed to age for 3 h. (Lanes 2–4) Homozygous first, second, and third instar U714 mutant larvae, respectively. (Lanes 5,6) 1- to 2-d-old homozygous U714 and w1118 control adult females, respectively. Note that the poly A+ H3 mRNA is first detectable in small amounts at the second larval instar stage, and that wild-type H3 mRNA is absent by the third larval instar stage. (C) U7 Northern analysis of RNA isolated from homozygous U714 or w1118 control larvae and adults. (Odd lanes) U714 homozygous mutants collected from U714/TM3 P[act-GFP] heterozygous parents. Note that the detectable U7 snRNA is maternally derived. (Even lanes) w1118 control animals. Hybridization of the blot with a U1 probe was used as a loading control. (D) H3 Northern analysis of RNA isolated from eye imaginal discs (lanes 1–3), salivary glands (lanes 4–6), or whole larvae (lanes 7,8) of the indicated genotypes.