Table 1. hth mutants show dose-dependent interactions with ubiquitous En in embryos*.
| Genotype of parent stock | Defective cuticles (%) | hth-like defects (%) | Severely defective (%) |
|---|---|---|---|
| Wild type (300) | 1 | 0 | 0 |
| hth/TM3 (440) | 22 | 22 (3) | 0 |
| hs-En; +/TM3 (254) | 36 | 0 | 23 |
| hs-En; hth/TM3 (392) | 37 | 34 (16) | 7 |
Cuticle defects caused by ectopically expressed En are abrogated by loss of hth function. Cuticles were prepared after a mild heat shock (7 minutes at 37°C) of embryos from crosses of stocks of the indicated genotype. Those showing defects were categorized as follows: ‘defective’ means having clear-cut fusions or deletions in ventral denticle bands (shortened dorsal cuticles resulting in an abnormally curved overall shape, characteristic of most of the hs-En embryos, were not included); ‘hth-like’ refers to characteristic denticle-band defects caused by loss of hth function, which include posteriorward transformations and loss of denticle diversity, particularly loss of row-1 denticles; ‘severely defective’ means multiple fusions or deletions of ventral denticle bands. Numbers in parentheses under ‘genotype’ indicate the number of embryos examined for each. Numbers in parentheses under ‘hth-like defects’ refer to the percentage showing denticle-band fusions in addition to the ‘hth-like’ defects described above.
The 22% defective cuticles with hth alone is in line with the 1/4 expected to be hth/hth, lacking hth function. Comparing hth alone with hth in combination with hs-En, the defective population increased by only 15%, from 22% to 37% (compared with an increase of 35% for hs-En relative to wild type), and half of this 37% (34%-16%=18%) showed only hth-like defects. Strikingly, the percentage of severely defective cuticles was strongly reduced in the hs-En lines by the removal of hth function. Only 7% of hs-En; hth/TM3 progeny showed severe defects, compared with 23% when hth function was wild type. Overall, it appears that hs-En defects are reduced in severity not only in hth homozygotes but in heterozygotes as well, because a reduction in homozygotes would be expected to reduce the percentage of defects by only 25% (in this case 1/4 of 23%, or ∼6%), whereas the reduction of severe defects is from 23% to 7%, a reduction of about 3/4, consistent with a significant reduction in both homozygotes and heterozygotes. The increase in ‘hth-like’ defects is probably due to the partial overlap in the consequences of low-level, ubiquitous En induction and reduced hth or exd function. For example, as seen in Fig. 2, both loss of exd and ectopic En induction result in decreased wg expression. In spite of this, the combination of the two produced fewer severely defective cuticles than hs-En alone, consistent with the conclusion that hth function is required for hs-En to be fully effective. (This experiment was done using a mild heat shock, to test the effect of hth under conditions expected to be sensitive to modification. Stronger heat shocks produce a much higher percentage of ventral cuticle defects in hs-En embryos, without producing defects in wild-type embryos, data not shown.)