Two Drosophilids exhibit distinct EGF pathway patterns in oogenesis
Deciphering the evolution of morphological structures is a remaining challenge in the field of developmental biology. The respiratory structures of insect eggshells, called the dorsal appendages, provide an outstanding system for exploring these processes since considerable information is known about their patterning and morphogenesis in Drosophila melanogaster and dorsal appendage number and morphology vary widely across Drosophilid species. We investigated the patterning differences that might facilitate morphogenetic differences between D. melanogaster, which produces two oar-like structures first by wrapping and then elongating the tubes via cell intercalation and cell crawling, and Scaptodrosophila lebanonensis, which produces a variable number of appendages simply by cell intercalation and crawling. Analyses of BMP pathway components thickveins and P-Mad demonstrate that anterior patterning is conserved between these species. In contrast, EGF signaling exhibits significant differences. Transcripts for the ligand encoded by gurken localize similarly in the two species, but this morphogen creates a single dorsolateral primordium in S. lebanonensis as defined by activated MAP kinase and the downstream marker broad. Expression patterns of pointed, argos, and Capicua, early steps in the EGF pathway, exhibit a heterochronic shift in S. lebanonensis relative to those seen in D. melanogaster. We demonstrate that the S. lebanonensis Gurken homolog is active in D. melanogaster but is insufficient to alter downstream patterning responses, indicating that Gurken-EGF receptor interactions do not distinguish the two species’ patterning. Altogether, these results differentiate EGF signaling patterns between species and shed light on how changes to the regulation of patterning genes may contribute to different tube-forming mechanisms.
KeywordsDorsal appendages Scaptodrosophila Drosophila Follicle cells Eggshell Epithelial morphogenesis
We thank Sydney Bowker, Vincent So, and Jill Kumasaka for their help with in situ hybridization experiments; Dr. Scott Roy at San Francisco State University for his advice on Scaptodrosophila gene annotation; Dr. Robert Waterston and Dr. Evan Eichler for the use of their compound microscopes; and Dr. Miriam Osterfield and members of the Berg lab for their helpful discussions. For technical support and advice on imaging, we thank Dr. Nathaniel Peters at the University of Washington W. M. Keck Imaging Center, which is supported by the National Institutes of Health (NIH) grant 1S10 OD016240. We are grateful to FlyBase for exceptional genetic, genomic, and bibliographic resources, the Bloomington Drosophila Stock Center for the MTD-GAL4 driver, the Drosophila Genomics Resource Center for the UASp-attB plasmid, the Drosophila Species Stock Center (formerly at Bowling Green and Tucson but now in San Diego) for providing Scaptodrosophila lebanonensis flies, and Drs. Norbert Perrimon and Christian Ghiglione for the UASp-grk.mb strain. We thank Dr. Tom Jessell and colleagues E. Laufer, S. Morton, and D. Vasiliauskas for the antibody against phosphorylated Smad1/5/8 (P-Mad). We obtained anti-Gurken, anti-Broad, and anti-DE-Cadherin monoclonal antibodies from the Developmental Studies Hybridoma Bank, created by the National Institute of Child Health and Human Development of the National Institutes of Health and maintained by the University of Iowa, Department of Biology
Funding from the National Institutes of Health grant R01-GM079433 (C.A.B.) supported this work.
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Conflict of interest
The authors declare that they have no competing interests.
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