Wnt6 is required for maxillary palp formation in Drosophila
© Doumpas et al.; licensee BioMed Central Ltd. 2013
Received: 24 July 2013
Accepted: 23 September 2013
Published: 3 October 2013
Wnt6 is an evolutionarily ancient member of the Wnt family. In Drosophila, Wnt6 loss-of-function animals have not yet been reported, hence information about fly Wnt6 function is lacking. In wing discs, Wnt6 is expressed at the dorsal/ventral boundary in a pattern similar to that of wingless, an important regulator of wing size. To test whether Wnt6 also contributes towards wing size regulation, we generated Wnt6 knockout flies.
Wnt6 knockout flies are viable and have no obvious defect in wing size or planar cell polarity. Surprisingly, Wnt6 knockouts lack maxillary palps. Interestingly, Wnt6 is absent from the genome of hemipterans, correlating with the absence of maxillary palps in these insects.
Wnt6 is important for maxillary palp development in Drosophila, and phylogenetic analysis indicates that loss of Wnt6 may also have led to loss of maxillary palps on an evolutionary time scale.
KeywordsDrosophila Wnt6 Maxillary palps
During animal development, tissue growth is tightly controlled, leading to adults of defined sizes and proportions. Tissue growth is regulated by a combination of patterning cues, which give each tissue a specific identity and hence size, and environmental cues, which are sensed through nutrient responsive pathways and act to proportionately scale the whole animal [1–4]. Despite intense interest in understanding how tissue growth is controlled, the underlying molecular mechanisms are only partly understood. The Drosophila wing has become one model system that is frequently used to study this problem. Growth of the fly wing is promoted via signals emanating from the anterior/posterior (A/P) and dorsal/ventral (D/V) compartment boundaries, such as Dpp and wingless respectively . Although wingless appears to be the main growth-promoting signal emanating from the D/V boundary, a second Notch-induced signal at the D/V boundary also non-autonomously induces wing growth . The wingless paralog Wnt6 is also expressed at the D/V boundary . Therefore, we decided to test whether Wnt6 might constitute this second signal.
Drosophila is one of the model systems in which Wnt signaling and function have been most intensively studied. Drosophila has seven Wnt genes. Of these, the best understood is wingless, the founding member of the class. Wingless has a myriad of functions during development. One function is to pattern epidermal cells to form repetitive patterns of naked cuticle and denticle belts, small tooth-like structures used for traction during larval crawling [7, 8]. Wingless is also necessary for development of all imaginal discs – the tissues resident in the larva which will give rise to adult tissues during metamorphosis. For instance, in the wing disc, early expression is responsible for specifying the wing primordium whereas later expression sets up the D/V axis of the wing [8, 9]. The remaining Wnts are comparatively less well studied; nonetheless, some functions are known for four of the remaining Wnts: Wnt3 is involved in axon guidance as well as salivary gland migration, Wnt2 regulates salivary gland migration, tracheal development and testis morphogenesis, Wnt8 is part of the Toll/Dorsal signaling network which both specifies the D/V axis of the embryo and participates in the immune response, whereas Wnt4 is critical for the regulation of cell motility during ovarian morphogenesis (reviewed in ). The functions of Wnt6 and Wnt10, however, are not known.
Wnt6 function has been studied in Xenopus, where it was found to be expressed in tissues close to and inside the developing heart, where it regulates heart organogenesis . Since, to our knowledge, Wnt6 mutant flies have not been reported, we generated Wnt6 knockout flies. We find that Wnt6 knockout flies, however, do not have growth defects in the wing. Instead, they completely lack maxillary palps. Together with antenna, maxillary palps are one of two olfactory epithelia in Drosophila. Recent studies suggest maxillary palps might also be involved in taste enhancement . The function of maxillary palps as an olfactory organ is well conserved throughout insects. For instance, mosquitos use maxillary palps to smell CO2, which is used for host seeking behavior . Hence, since Wnt6 knockout flies lack maxillary palps, they might constitute a useful tool for studying olfaction and behavior [13, 15].
Wnt6 knockout flies lack maxillary palps
Wnt6 can activate canonical Wnt signaling and is required for proper positioning of wing margin chemosensory bristles
Wnt6 is expressed in the maxillary palp anlage
Evolutionary loss of Wnt6 correlates with loss of maxillary palps
Wnt6 appears to have a specific role during Drosophila development, promoting maxillary palp formation. This is surprising, given that Wnt6 is quite ancient and present in most bilaterians [25, 26]. One possible interpretation is that the Wnt6 function might be redundant in most parts of the animal, perhaps due to overlapping expression with wingless, whereas Wnt6 expression in the maxillary palp might have been acquired in insects in a non-redundant fashion. This specific function in promoting maxillary palp formation might serve as a useful tool for studying the contribution of maxillary palps to olfaction and behavior. The maxillary palp contribution is currently assayed by surgical removal of the palps, whereas this could now also be accomplished genetically .
As previously noted , the Wnt6 gene is located directly adjacent to the wingless gene, raising the possibility that it arose as a genomic duplication of wingless. Accordingly, Wnt6 expression overlaps with that of wingless in numerous places . One possible reason for the overlapping expression patterns could be that Wnt6 expression is induced by wingless signaling; however, our data suggest this is not the case (Figure 3B). Instead, it is likely that they either share enhancer elements or that regulatory elements were also duplicated alongside the open reading frame. Since wingless and Wnt6 have similar expression patterns and presumably transcriptional regulation, and since the anti-wingless monoclonal antibody 4D4, the most widely-used in the field to detect wingless, appears to cross-react with Wnt6, some caution might be warranted in interpreting results with this antibody.
Given that Wnt6 is able to induce canonical wingless signaling in S2 cells (Figure 2A), we were surprised that Wnt6 is quite poor at inducing wingless signaling in the wing disc (Figure 2E-F’). Consistent with this observation, expression of UAS-Wnt6 with various GAL4 drivers such as patchedts-GAL4 (with GAL80ts) and nubbin-GAL4 cause pupal lethality; however, this does not yield obvious morphological defects in the resulting wings (not shown), suggesting that the lethality is likely due to expression in other parts of the body. In contrast, Wnt6 expression in the central nervous system, including the maxillary palp, induces obvious significant morphological effects. One possible explanation could be that a component required for Wnt6 signaling might be expressed at higher levels in the nervous system compared to wing discs.
The specific absence of Wnt6 from the aphid A. pisum and the plant lice D. citri, both belonging to the order Hemiptera, a group that has lost maxillary palps, suggests that Wnt6-loss could have been the underlying genetic alteration leading to this morphological change. In hemipterans, the mouthparts are modified to form a tube-like structure for piercing. The tube, formed by the labrum and labium, comprises piercing-sucking structures formed by the modified mandible and the maxilla . The evolutionary loss of the maxillary palps was one of many structural modifications leading to the specialized hemipteran mouthparts. The loss of the maxillary palps could have compromised the sense of smell in hemipteran ancestors, but this may have been compensated by the elaboration of sensory structures on the labium . The specific phenotype of the Wnt6 knock-out in Drosophila contrasts with the pleiotropic effects of other secreted signaling molecules including wingless. This means that the deletion of the gene, rather than tinkering with its regulatory regions, could have resulted in a subtle morphological change, the loss of the maxillary palp, contributing to the morphological evolution of the beak-like hemipteran mouthparts.
Although Wnt6 expression overlaps substantially with that of wingless, it appears to play a critical role in maxillary palp growth, but not wing growth. Phylogenetic analysis suggests that loss of Wnt6 also correlates with loss of maxillary palps on an evolutionary timescale.
All oligo sequences are listed in Additional file 3.
Generation of Wnt6KOand UAS-Wnt6 flies
Wnt6KO flies were generated by homologous recombination-based targeting using the 'ends-out’ strategy as previously described . Based on this strategy, 4 kb upstream and downstream flanks were amplified by PCR using the oligos described in Additional file 3, sequenced, and cloned into the pRK1 vector . Knockout flies were then back-crossed to the w1118 reference strain for five generations before studying. To generate UAS-Wnt6 flies, the Wnt6 ORF was amplified by PCR using the oligos listed in Additional file 3, and cloned into the EcoRI\NotI sites of pUAST .
Luciferase reporter assays
Luciferase reporter assays were based on the Promega pGL3 reporter system with a Wnt-responsive LEF7 firefly luciferase reporter and a renilla normalization control as previously described . Wingless and Wnt6 expression were achieved by co-transfecting pac-wg or pUAST-Wnt6 + pMT-GAL4/VP16, respectively .
S2 cells were grown in Express Five Serum Free Medium (Life Technologies, Carlsbad, California, USA) and transfected with Effectene (Qiagen, Venlo, Netherlands).
Antibodies used were mouse anti-wingless (4D4 Developmental Studies Hybridoma Bank); rat anti-Dll (Sean Carroll lab, R.M Bock Laboratories, 1525 Linden Drive, Madison, WI 53706) and guinea pig anti-sens .
The D. citri genome (Diaci1.1, 12× coverage) and transcriptome assemblies (Diaci_transcriptome_0.9) were downloaded from the International Asian Citrus Psyllid Genome Consortium website  and searched with TBLASTN with arthropod Wnt6 query sequences. The five top hits were re-blasted to the Uniprot and GenBank databases. D. citri Wnt1 is on the genomic scaffold scaffold5281.1|size8182|ref0095796|ref0108241.
planar cell polarity
Anti-wg antibody was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa. ND is part of the Hartmut Hoffmann-Berling International Graduate School of Molecular and Cellular Biology (HBIGS), University of Heidelberg.
- Shingleton AW: The regulation of organ size in Drosophila: physiology, plasticity, patterning and physical force. Organogenesis. 2010, 6: 76-87. 10.4161/org.6.2.10375.PubMedPubMed CentralView ArticleGoogle Scholar
- Schwank G, Basler K: Regulation of organ growth by morphogen gradients. Cold Spring Harb Perspect Biol. 2010, 2: a001669-10.1101/cshperspect.a001669.PubMedPubMed CentralView ArticleGoogle Scholar
- Lecuit T, Le Goff L: Orchestrating size and shape during morphogenesis. Nature. 2007, 450: 189-192. 10.1038/nature06304.PubMedView ArticleGoogle Scholar
- Mirth CK, Riddiford LM: Size assessment and growth control: how adult size is determined in insects. BioEssays. 2007, 29: 344-355. 10.1002/bies.20552.PubMedView ArticleGoogle Scholar
- Giraldez AJ, Cohen SM: Wingless and Notch signaling provide cell survival cues and control cell proliferation during wing development. Development. 2003, 130: 6533-6543. 10.1242/dev.00904.PubMedView ArticleGoogle Scholar
- Janson K, Cohen ED, Wilder EL: Expression of DWnt6, DWnt10, and DFz4 during Drosophila development. Mech Dev. 2001, 103: 117-120. 10.1016/S0925-4773(01)00323-9.PubMedView ArticleGoogle Scholar
- Bejsovec A: Wingless/Wnt signaling in Drosophila: the pattern and the pathway. Mol Reprod Dev. 2013, doi: 10.1002/mrd.22228Google Scholar
- Gonsalves FC, DasGupta R: Function of the wingless signaling pathway in Drosophila. Methods Mol Biol. 2008, 469: 115-125. 10.1007/978-1-60327-469-2_10.PubMedView ArticleGoogle Scholar
- Neumann C, Cohen S: Morphogens and pattern formation. BioEssays. 1997, 19: 721-729. 10.1002/bies.950190813.PubMedView ArticleGoogle Scholar
- Murat S, Hopfen C, McGregor AP: The function and evolution of Wnt genes in arthropods. Arthropod Struct Dev. 2010, 39: 446-452. 10.1016/j.asd.2010.05.007.PubMedView ArticleGoogle Scholar
- Lavery DL, Martin J, Turnbull YD, Hoppler S: Wnt6 signaling regulates heart muscle development during organogenesis. Dev Biol. 2008, 323: 177-188. 10.1016/j.ydbio.2008.08.032.PubMedPubMed CentralView ArticleGoogle Scholar
- Song E, de Bivort B, Dan C, Kunes S: Determinants of the Drosophila odorant receptor pattern. Dev Cell. 2012, 22: 363-376. 10.1016/j.devcel.2011.12.015.PubMedView ArticleGoogle Scholar
- Shiraiwa T: Multimodal chemosensory integration through the maxillary palp in Drosophila. PloS One. 2008, 3: e2191-10.1371/journal.pone.0002191.PubMedPubMed CentralView ArticleGoogle Scholar
- Lu T, Qiu YT, Wang G, Kwon JY, Rutzler M, Kwon HW, Pitts RJ, van Loon JJ, Takken W, Carlson JR, Zwiebel LJ: Odor coding in the maxillary palp of the malaria vector mosquito Anopheles gambiae. Curr Biol. 2007, 17: 1533-1544. 10.1016/j.cub.2007.07.062.PubMedPubMed CentralView ArticleGoogle Scholar
- Ayer RK, Carlson J: Olfactory physiology in the Drosophila antenna and maxillary palp: acj6 distinguishes two classes of odorant pathways. J Neurobiol. 1992, 23: 965-982. 10.1002/neu.480230804.PubMedView ArticleGoogle Scholar
- Bejsovec A: Flying at the head of the pack: Wnt biology in Drosophila. Oncogene. 2006, 25: 7442-7449. 10.1038/sj.onc.1210051.PubMedView ArticleGoogle Scholar
- Bartscherer K, Pelte N, Ingelfinger D, Boutros M: Secretion of Wnt ligands requires Evi, a conserved transmembrane protein. Cell. 2006, 125: 523-533. 10.1016/j.cell.2006.04.009.PubMedView ArticleGoogle Scholar
- Zhang J, Carthew RW: Interactions between Wingless and DFz2 during Drosophila wing development. Development. 1998, 125: 3075-3085.PubMedGoogle Scholar
- Lebreton G, Faucher C, Cribbs DL, Benassayag C: Timing of Wingless signalling distinguishes maxillary and antennal identities in Drosophila melanogaster. Development. 2008, 135: 2301-2309. 10.1242/dev.017053.PubMedView ArticleGoogle Scholar
- Potapov M, Gao Y, Deharveng L: Taxonomy of the Cryptopygus complex. I. Pauropygus - a new worldwide littoral genus (Collembola, Isotomidae). ZooKeys. 2013, 304: 1-16.PubMedView ArticleGoogle Scholar
- Trautwein MD, Wiegmann BM, Beutel R, Kjer KM, Yeates DK: Advances in insect phylogeny at the dawn of the postgenomic era. Annu Rev Entomol. 2012, 57: 449-468. 10.1146/annurev-ento-120710-100538.PubMedView ArticleGoogle Scholar
- International Aphid Genomics Consortium: Genome sequence of the pea aphid Acyrthosiphon pisum. PLoS Biol. 2010, 8: e1000313-10.1371/journal.pbio.1000313.View ArticleGoogle Scholar
- Janssen R, Le Gouar M, Pechmann M, Poulin F, Bolognesi R, Schwager EE, Hopfen C, Colbourne JK, Budd GE, Brown SJ, Prpic NM, Kosiol C, Vervoort M, Damen WG, Balavoine G, McGregor AP: Conservation, loss, and redeployment of Wnt ligands in protostomes: implications for understanding the evolution of segment formation. BMC Evol Biol. 2010, 10: 374-10.1186/1471-2148-10-374.PubMedPubMed CentralView ArticleGoogle Scholar
- International Asian Citrus Psyllid Genome Consortium website. [http://www.psyllid.org/]
- Sidow A: Diversification of the Wnt gene family on the ancestral lineage of vertebrates. Proc Natl Acad Sci USA. 1992, 89: 5098-5102. 10.1073/pnas.89.11.5098.PubMedPubMed CentralView ArticleGoogle Scholar
- Holland PW, Garcia-Fernandez J, Williams NA, Sidow A: Gene duplications and the origins of vertebrate development. Dev Suppl. 1994, 125-133.Google Scholar
- Charro MJ, Alcorta E: Quantifying relative importance of maxillary palp information on the olfactory behavior of Drosophila melanogaster. J Comp Physiol A. 1994, 175: 761-766.PubMedView ArticleGoogle Scholar
- Nusse R: An ancient cluster of Wnt paralogues. Trends Genet. 2001, 17: 443-PubMedView ArticleGoogle Scholar
- Zhao L, Dai W, Zhang C, Zhang Y: Morphological characterization of the mouthparts of the vector leafhopper Psammotettix striatus (L.) (Hemiptera: Cicadellidae). Micron. 2010, 41: 754-759. 10.1016/j.micron.2010.06.001.PubMedView ArticleGoogle Scholar
- Huang J, Zhou W, Watson AM, Jan YN, Hong Y: Efficient ends-out gene targeting in Drosophila. Genetics. 2008, 180: 703-707. 10.1534/genetics.108.090563.PubMedPubMed CentralView ArticleGoogle Scholar
- Brand AH, Perrimon N: Targeted gene expression as a means of altering cell fates and generating dominant phenotypes. Development. 1993, 118: 401-415.PubMedGoogle Scholar
- Gross JC, Chaudhary V, Bartscherer K, Boutros M: Active Wnt proteins are secreted on exosomes. Nature Cell Biol. 2012, 14: 1036-1045. 10.1038/ncb2574.PubMedView ArticleGoogle Scholar
- Kozopas KM, Samos CH, Nusse R: DWnt-2, a Drosophila Wnt gene required for the development of the male reproductive tract, specifies a sexually dimorphic cell fate. Genes Dev. 1998, 12: 1155-1165. 10.1101/gad.12.8.1155.PubMedPubMed CentralView ArticleGoogle Scholar
- Tautz D, Pfeifle C: A non-radioactive in situ hybridization method for the localization of specific RNAs in Drosophila embryos reveals translational control of the segmentation gene hunchback. Chromosoma. 1989, 98: 81-85. 10.1007/BF00291041.PubMedView ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.