A comparison of transplantable bicoid activity and partial bicoid homeobox sequences in several Drosophila and blowfly species (Calliphoridae)

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Abstract
In order to test for bicoid-like activity in insects other than Drosophila melanogaster, anterior egg cytoplasm from the following species was injected into cleavage stage embryos from mutant D. melanogaster lacking a functional bicoid (bcd) product: six other Drosophila species, the housefly, three blowfly species, the primitive cyclorrhaphic dipteran Megaselia, and the honeybee Apis mellifera; preliminary tests were made with four lower dipterans (Nematocera). Rescue effects were only observed with the drosophilids, housefly, and two of the three blowfly species. Rescue was stronger with the drosophilids than with the other flies as donors. Where checked (D. pseudoobscura), a positive correlation was found between the amount of cytoplasm injected and the number of pattern elements formed, suggesting threshold effects upon target genes as with the endogenous bcd product. By polymerase chain reaction, fragments from a bcd-orthologous homeobox were cloned from the three blowfly species. The derived sequence of 43 amino acids was identical in all blowflies and the housefly but differed at 4 positions from the orthologous D. melanogaster sequence. Localization of the mRNA recognized by the respective fragments in the blowflies Lucilia and Phormia resembled that known from D. melanogaster, while Calliphora — the blowfly species lacking rescue activity —showed remarkable differences of localization in both ovarian follicles and the deposited egg cell. This surprising divergence within a morphologically rather uniform family of cyclorrhaphic dipterans should be of interest from both functional and evolutionary points of view.