Fig. 2.

Mesoderm spreading defects in ths and pyr single mutants. (A-F) Drosophila embryos of the indicated genotypes stained with anti-Twi (top two rows of each panel) or anti-dpERK (bottom row, detects MAPK) shown as whole-mount (top row) or in cross-section (between 40% and 60% egg length). Stages 7 (left column of each panel), 8 (middle) and 9 (right) are shown. (A) Wild type. Note activation of MAPK at mesoderm-ectoderm contacts and at the mesoderm leading edge (arrows). (B) In embryos lacking both pyr and ths function, mesoderm cells fail to establish initial contact, activate MAPK (arrowheads) or spread to a monolayer. (C) In ths⁷⁵⁹ homozygotes initial contact is normal, but unequal spreading can be observed at stage 8 (arrowheads). No major changes in monolayer formation were observed and MAPK activation was normal (arrows). (D) Embryos lacking ths function and one copy of pyr [ths⁷⁵⁹/Df(2R)ED2238] exhibit similar defects to ths⁷⁵⁹ homozygotes. (E) Pyramus is required for equal dorsal spreading and monolayer formation (arrowheads). MAPK activation is strongly reduced in the absence of pyr function (arrows). (F) Similar defects are observed in pyr¹⁸/Df(2R)ED2238 embryos. Note the strong reduction of dpERK staining in leading edge cells (arrows). (G-I) pyr and ths are expressed in distinct domains during mesoderm spreading. In situ hybridisation of stage 8 embryos with antisense probes against ths (G) and pyr (H,I). (G,H) Cross-sections. (I) Fluorescent detection of pyr RNA (red) in an embryo expressing twi::CD2 (green). The right-hand panel shows a reconstructed cross-section. pyr expression is restricted to dorsal ectoderm (arrows in H, arrowheads in I) and ventral ectodermal patches (arrowheads in H).