Fig. 4. Localisation of dipteran transcripts upon injection into Drosophila blastoderm embryos mediated by Egl-dependent mRNA localisation signals. (A) Drosophila embryos injected with h transcripts from dipteran species and fixed 8-11 minutes later, showing representative examples of different efficiencies of mRNA localisation. Ten to 30 embryos were imaged for each transcript and used to categorise the extent of apical RNA enrichment. (A, bottom panels) Anopheles-h transcripts localise very efficiently upon injection (left), but localisation of this transcript is prevented by prior injection with antibodies that specifically inhibit the function of the Drosophila Egl protein (right). (B) Summary of the efficiency of localisation signals within dipteran eve, h and wg transcripts upon injection into Drosophila plotted onto the phylogenetic tree: +++, very efficient localisation; ++, efficient localisation; +, weak localisation; -, no apical enrichment. The efficiency of apical transport in this assay mirrors endogenous efficiencies of transcript localisation (compare with Figs 2 and 3). Both Clogmia-eve1 and Clogmia-eve2 fail to localise in this assay. All localising transcripts are dependent on Egl function. The occurrence of pair-rule mRNA localisation signals does not appear to be related to the absolute size of the embryo. For example, h transcripts contain localisation signals in Megaselia and Drosophila (length of the embryo is 400-500 µm), but not in Haematopota (1500 µm) or Coboldia (260 µm). Scale bar: 50 µm.