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Fig. 5. Pair-rule gene activity is reduced in egl mutant embryos. (A) Distribution of transcripts (as indicated; red) in Drosophila blastoderm embryos laid by wild-type mothers (egl⁺/egl⁺) or partial loss-of-function egl mothers (egl^3e/egl^WU50). mRNA alone is shown in monochrome images. Pair-rule and wg transcripts accumulate apically in wild-type embryos but are detected readily in the basal cytoplasm of mutant embryos (white arrowheads), although some apical enrichment of pair-rule transcripts is observed in most stripes (red arrowheads). The bottom panels show Run protein, which is normally apical (red arrow, left) and nuclear, but can also be detected in the basal cytoplasm in egl mutant embryos (red arrow, right). (B) The frequency of segmentation phenotypes in first instar larvae caused by inactivation of one copy of a pair-rule gene is significantly enhanced when embryos are laid by an egl mutant mother. For wild-type and mutant maternal genotypes, eve and h mutations are associated with partial deletions of even-numbered segments, whereas ftz mutations had similar effects on odd-numbered segments. We observed similar interactions with an additional h mutant allele, h³¹ (data not shown). The low frequency of larvae with defects in zygotically wild-type embryos typically had small notches in either even- or odd-numbered segments or partial fusion of segments. The frequency of cuticular defects caused by heterozygosity for the gap genes kni and Kr and the segment polarity gene wg are not altered significantly by the egl mutant background although the identity of segments affected by gap gene mutations differ slightly. For example, Kr¹/+ normally gives defects in T3, A1 and A2 but in egl mutants defects in A3 at the expense of defects in A1 and A2 were also observed. These alterations may reflect changes in the distribution of maternal RNA determinants or reduced pair-rule activity. Numbers above the bars are total number of larvae scored. ^*P<0.05; ^***P<0.001 (Fisher's exact test). egl is provided only maternally to the blastoderm. Experiments were conducted at 25°C, except for those involving ftz, which were carried out at 29°C, because only a few defects were seen in either genotype at 25°C. (C) Representative examples of first instar larvae with one inactive copy of h (hⁱ²²) laid by wild-type or egl mutant mothers. Red asterisks show missing regions of ventral denticle belts in abdominal segments 4 and 6 (A4 and A6). Scale bar in C: 50 µm for A (except Run protein, 30 µm); 385 µm for C.

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