The evolution of oocyte patterning in insects: multiple cell-signaling pathways are active during honeybee oogenesis and are likely to play a role in axis patterning Megan J. Wilson, à Helen Abbott, and Peter K. Dearden Laboratory for Evolution and Development, Genetics Otago, Department of Biochemistry, University of Otago, PO Box 56, Dunedin 9054, Aotearoa, New Zealand à Author for correspondence (email: meganj.wilson@otago.ac.nz) SUMMARY In Drosophila it is well established that signaling between the germline and surrounding follicle cells establishes the axes of the future embryo and is required for patterning of the eggshell. However, little is known about how this is achieved in other insects. Genome sequencing studies imply that maternal axis determination may be rapidly evolving, as a number of Drosophila maternal patterning genes are absent from the genomes of other insects. We have examined the distribution and function of six developmental signaling pathways present, and active, in honeybee ovarioles. We have confirmed an evolutionarily conserved role for transforming growth factor-a--epidermal growth factor receptor signaling in dorsalFventral (DV) patterning. We also found evidence for the involvement of Dpp/Mad and JNK--MAPK pathways in DV patterning, unlike Drosophila. Several of these pathways are also active in the germarium, implicating them in germ and somatic stem cell maintenance and proliferation, similar to their activities in Drosophila ovaries. INTRODUCTION In insects, like other bilaterians, the establishment of the an- terior–posterior (AP) and dorsal–ventral (DV) axes are early events in development. Once established, axes are maintained throughout embryogenesis and underpin the body plan of the adult. In many insects, the information required for pattern- ing the embryo is laid down in the egg, while still in the ovary. While maternal patterning events have been well studied in the insect model Drosophila, little is known about these path- ways in other insects. Examination of the genomes of several insects, including the honeybee, has revealed that several maternal patterning genes that influence axis formation in Drosophila are absent, implying other mechanisms of axis formation act in these insects (Dearden et al. 2006). This fast evolution of early developmental events is counter-intuitive as all of the later processes of development rely on them. How then are novel genes and pathways co-opted into axis forma- tion to produce conserved outputs? To address this question it is important to discover the genes and mechanisms that are used to specify the axes in insects other than Drosophila. The insect ovary consists of trophic or nurse cells (NCs) (a supporting cell lineage that provides nutrients, mRNA and protein to the developing egg), the oocyte, and follicle cells (FCs). Honeybee and Drosophila ovaries are polytrophic; the NCs are grouped together and located near the oocyte. In Drosophila ovaries the NCs and oocyte are contained within the same chamber and connected by ring canals that transport components into the oocyte. In honeybees, however, the NCs and some associated FCs form a separate chamber connected by a cytoplasmic bridge to the oocyte, which is also sur- rounded by a layer of FCs (Dearden 2006). In Drosophila, axes are established through cell signaling and localized RNAs. Early in oogenesis, cell signaling events control main- tenance and proliferation of somatic and germ cells, and act in differentiation of FC populations. Several cell-signaling path- ways have been shown to be involved in oogenesis; including two MAPK pathways (gurken/ERK and p38 MAPK), dec- apentaplegic (Dpp), hedgehog (hh), and Notch-signaling pathways (Ruohola et al. 1991; Forbes et al. 1996; Twombly et al. 1996; Suzanne et al. 1999; Lynch et al. 2010). Later, cell– cell signaling between the overlying FC layer and the oocyte patterns the eggshell and establishes the axes of the oocyte and thus the future embryo. Transcripts synthesized by the neighboring NCs are transported into the oocyte and local- ized by polarized microtubule networks established through oocyte–FC signaling and oocyte nucleus positioning. The localization of maternal mRNAs has a critical role in pat- terning the axes of the future embryo. Gurken is a good example of a rapidly evolving axis for- mation gene. Gurken activates the epidermal growth factor receptor (EGFR) Torpedo in FCs at the posterior and EVOLUTION & DEVELOPMENT 13:2, 127–137 (2011) DOI: 10.1111/j.1525-142X.2011.00463.x & 2011 Wiley Periodicals, Inc. 127