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Professor of Organismic & Evolutionary Biology

Cassandra Extavour

Professor of Organismic & Evolutionary Biology

Professor of Molecular and Cellular Biology

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Germ cells play a unique role in gamete production, heredity and evolution. Germ cells are likely also the closest wild type in vivo equivalent to laboratory-maintained stem cells. To understand the mechanisms that specify germ cells is therefore a central challenge in developmental and evolutionary biology.

Data from model organisms show that germ cells can be specified either by maternally inherited determinants or by inductive signals. Although the inheritance mode is seen in most model organisms, it is actually likely to be the less prevalent mode of germ cell specification, and inductive germ cell specification may be ancestral to the Metazoa.

Among the invertebrates, the only arthropod in which the germ line has been studied in detail is the dipteran Drosophila melanogaster. In this fruit fly, germ line precursors form as pole cells at the posterior end of the embryo very early in development. However, it is not clear whether this mechanism of germ cell specification is widespread among, or representative of, all arthropods. Moreover, there is great variation in the time and place of germ cell specification across all multicellular animals. My interests are the evolution and development of reproductive systems, and specifically, the mechanisms of initial specification of primordial germ cells. By using molecular markers, functional genetic analysis, and cellular analysis to study the embryonic development and reproductive systems of multiple emerging arthropod laboratory systems (spiders, crickets, milkweed bugs, amphipods and fruit flies), we hope to add to our understanding of which mechanisms may have been basal to arthropods, and ultimately to metazoans, in the specification of the germline.


Selected Publications

  • Modrell, M.S., Price, A L., Havemann, J., Extavour, C.G., Gerberding, M. and Patel, N.H. Germline replacement following ablation of the primordial germ cells in Parhyale hawaiensis. (in revision)
  • Kao, D., Lai, A., Stamataki, E., Rosic, S., Konstantinides, N., Jarvis, E., Di Donfrancesco, A., Pouchkina-Stancheva, N., Semon, M., Grillo, M., Bruce, H., Kumar, S., Siwanowicz, I., Le, A., Lemire, A., Extavour, C.G., Browne, W., Wolff, C., Averof, M., Patel, N.H., Sarkies, P., Pavlopoulos, A., Aboobaker, A.* (2016). The genome of Parhyale hawaiensis: a model for animal development regeneration, immunity and ligno-cellulose digestion. bioRxiv 065789 http://dx.doi.org/10.1101/065789 PDF
  • Whittle, C.A. and Extavour, C.G. Refuting the hypothesis that the acquisition of germ plasm accelerates animal
    evolution. Nature Communications 7: 12637 (2016) PDF PubMed
  • Iyer, H., Issigonis, M., Sharma, P.P., Extavour, C.G. and Newmark, P.A. A premeiotic function for boule in the planarian Schmidtea mediterranea. PNAS 113(25): E3509-18 (2016) PDF PubMed
  • Extavour, C.G. and Pick, L. Editorial Overview: Development, regulation and evolution of organ systems. Current Opinion in Insect Science do13: vii-ix (2016) PDF PubMed
  • Whittle, C.A. and Extavour, C.G. Expression-linked patterns of codon usage, amino acid frequency and protein length in the basally branching arthropod Parasteatoda tepidariorum. Genome Biology and Evolution (in press) (2016) PDF of accepted MS PubMed
  • Nakamura, T. and Extavour, C.G.The transcriptional repressor Blimp-1 acts downstream of BMP signaling to generate primordial germ cells in the cricket Gryllus bimaculatus. Development 143(2): 255-263 (2016) * Read more in “In This Issue” of Development * PDF PubMed
  • Whittle, C.A. and Extavour, C.G. Codon and amino acid usage are shaped by selection across divergent model organisms of the Pancrustacea. G3: Genes, Genomes, Genetics 5(11): 2307-2321 (2015) PDF PubMed
  • Sharma, P. P., Tarazona, O.A., Lopez, D.H., Schwager, E.E., Cohn, M.J., Wheeler, W.C. and Extavour, C.G. A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids. Proceedings of the Royal Society B: Biological Sciences 282: 20150698 (2015) PDF PubMed
  • Donoughe, S. and Extavour, C.G. Embryonic development of the cricket Gryllus bimaculatus. Developmental Biology 411(1): 140-156 (2015) PDF PubMed
  • Moczek, A., Sears, K., Stollewerk, A., Wittkopp, P., Diggle, P., Dworkin, I., Ledon-Rettig, C., Matus, D., Roth, S., Abouheif, E., Brown, F. Chiu, C.-H., Cohen, C.S., De Tomaso, T., Gilbert, S., Hall, B., Love, A., Lyons, D., Sanger, T., Smith, J., Specht, C., Vallejo-Marin, M. and Extavour, C.G. The significance and scope of evolutionary developmental biology: a vision for the 21st century. Evolution and Development 17(3): 198-219 (2015) PDF PubMed
  • Sarikaya, D.P. and Extavour, C.G. The Hippo pathway regulates homeostatic growth of stem cell niche precursors in the Drosophila ovary. PLoS Genetics 11(2): e1004962 (2015) * Cover Article * Read more in Biology of Reproduction * PDF PubMed
  • Chipman, A.D., Ferrier, D.E.K and the Strigamia maritima sequencing and annotation consortium (105 authors, including C. G. Extavour (#26) and T. Jones (#49)). The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology 12(11): e1002005 (2014) *Read more in PLoS Biology, NBC News, Tech Times, International Business Times, Astrobiology Magazine, Sci-News, Genome Web, UK Daily Mail, Nature World News, and Reuters PDF PubMed
  • Schwager, E.E., Meng, Y. and Extavour, C.G. vasa and piwi are required for mitotic integrity in early embryogenesis in the spider Parasteatoda tepidariorum. Developmental Biology 402(2): 276-290 (2015) PDF PubMed
  • Sharma, P.P., Schwager, E.E., Extavour, C.G. and Wheeler, W.C. Hox gene duplications correlate with posterior heteronomy in scorpions. Proceedings of the Royal Society B: Biological Sciences 281: 20140661 (2014) PDFPubMed
  • Posnien, N., Zeng V., Schwager, E.E., Pechmann, M., Hilbrant, M., Keefe, J., Damen, W.G.M., Prpic-Schaper, N., McGregor, A.P. and Extavour, C.G. A comprehensive reference transcriptome resource for the common house spider Parasteatoda tepidariorum. PLoS ONE 9(8): e104885. doi:10.1371/journal.pone.0104885 (2014) PDF PubMed
  • Donoughe, S., Nakamura, T., Ewen-Campen, B., Green II, D.A., Henderson, L. and Extavour, C.G. BMP signaling is required for generation of primordial germ cells in an insect. Proceedings of the National Academy of Sciences of the USA 111(11): 4133-4138 (2014) PDF PubMed
  • Green II, D.A. and Extavour, C.G. Insulin Signaling Underlies Both Plasticity and Divergence of a Reproductive Trait in Drosophila. Proceedings of the Royal Society B: Biological Sciences 281(1779): 20132673 (2014) *Read more in the Harvard Gazette PDF PubMed
  • Ahuja, A. and Extavour, C.G. Patterns of molecular evolution of the germ line specification gene oskar suggest that a
    novel domain may contribute to functional divergence in Drosophila. Development, Genes and Evolution 224(2): 65-77 (2014) PDF PubMed
  • Sharma, P.P., Gupta, T., Schwager, E.E., Wheeler, W., and Extavour C.G. Subdivision of arthropod cap-n-collar expression domains is restricted to Mandibulata. EvoDevo 5(1): 3 (2014) PDF PubMed
  • Gupta, T. and Extavour, C.G. Identification of a putative germ plasm in the crustacean Parhyale hawaiensis. EvoDevo 4(1): 34 (2013) PDF PubMed
  • Nast, A. R. and Extavour, C.G. Ablation of a single cell from eight-cell embryos of the amphipod crustacean Parhyale hawaiensis. Journal of Visualized Experiments 16 March, 2014, Issue 85, doi:10.3791/51073 (2014) PDF PubMed
  • Extavour, C.G. Live long and prosper: Germline stem cell maintenance revisited. BioEssays 35(9): 763 (2013) PDF PubMed
  • Ewen-Campen, B., Donoughe, S., Clarke, D.N., and Extavour, C.G. Germ Cell Specification Requires Zygotic Mechanisms Rather Than Germ Plasm in a Basally Branching Insect. Current Biology 23(10): 835-842 (2013) PDFPubMed
  • Zeng, V. Ewen-Campen, B., Horch, H.W., Roth, S., Mito, T., and Extavour, C.G. Developmental Gene Discovery in a Hemimetabolous Insect: De Novo Annotation and Assembly of a Transcriptome for the Cricket Gryllus bimaculatus.PLoS ONE 8(5): e61479 (2013) PDF PubMed
  • Ewen-Campen, B.*, Jones, T.E.*., and Extavour, C.G. Evidence against a germ plasm in the milkweed bug Oncopeltus fasciatus, a hemimetabolous insect. Biology Open (Company of Biologists) 2(6):556-568 (2013) (* equal author contribution) PDF PubMed
  • Sharma, P.P., Schwager, E.E., Giribet, G.G., Jockusch, E. and Extavour, C.G. Distal-less and dachshund pattern both plesiomorphic and derived structures in chelicerates: RNA interference in the harvestman Phalangium opilio (Opiliones) Evolution and Development 15(4): 228-242 (2013)* Cover Article * PDF PubMed
  • Zeng, V. and Extavour, C.G. ASGARD: an open-access database of annotated transcriptomes for emerging model arthropod species. Database doi: 10.1093/database/bas048 (2012) PDF PubMed
  • Ewen-Campen, B., Srouji, J.R., Schwager, E.E. and Extavour, C.G. oskar Predates the Evolution of Germ Plasm in Insects. Current Biology 22(23): 2278-2283 (2012) *Read more in the Harvard Gazette and a Dispatch in Current Biology * Recommended by the Faculty of 1000 * PDF PubMed
  • Green II, D.A., and Extavour, C.G. Convergent evolution of a reproductive trait through distinct developmental mechanisms in Drosophila. Developmental Biology 372(1): 120-130 (2012) PDF PubMed
  • Sharma, P.P., Schwager, E.E., Extavour, C. G. and Giribet, G. Evolution of the chelicera: a dachshund domain is retained in the deutocerebral appendage of Opiliones (Arthropoda, Chelicerata). Evolution and Development 14(6): 522-533 (2012)* Recommended by the Faculty of 1000 * PDF PubMed
  • Sharma, P.P., Schwager, E.E., Extavour, C. G. and Giribet, G. Hox gene expression in the harvestman Phalangium opilio reveals divergent patterning of the chelicerate opisthosoma. Evolution and Development 14(5): 450-463 (2012) PDF PubMed
  • Sarikaya, D.P., Belay, A.A., Ahuja, A., Green II, D.A., Dorta, A. and Extavour, C.G. The roles of cell size and cell number in determining ovariole number in Drosophila. Developmental Biology 363(1): 279-289 (2012) PDF PubMed
  • Zeng, V., Villanueva, K.E., Ewen-Campen, B., Alwes, F., Browne, W.E. and Extavour, C.G. De novo assembly and characterization of a maternal and developmental transcriptome for the emerging model crustacean Parhyale hawaiensis. BMC Genomics 12(1): 581 (2011) PDF PubMed
  • Kainz, F., Ewen-Campen, B., Akam, M. and Extavour, C.G. Notch/Delta signalling is not required for segment generation in the basally branching insect Gryllus bimaculatus. Development 138(22): 5015-5026 (2011) PDF PubMed
  • Alwes, F., Hinchen, B. and Extavour, C G. Patterns of cell lineage, movement, and migration from germ layer specification to gastrulation in the amphipod crustacean Parhyale hawaiensis. Developmental Biology 359(1): 110-123 (2011) PDF PubMed
  • Extavour, C G. Long-Lost Relative Claims Orphan Gene: oskar in a Wasp. PLoS Genetics 7(4): e1002045 (2011) PDFPubMed
  • Green, D.A.*, Sarikaya, D.P.* and Extavour, C.G. Counting in oogenesis. Cell and Tissue Research 344(2): 207-212 (2011) PDF PubMed (* equal author contribution)
  • Ewen-Campen, B., Shaner, N., Panfilio, K., Suzuki, Y., Roth, S. and Extavour, C.G. The maternal and early embryonic transcriptome of the milkweed bug Oncopeltus fasciatus. BMC Genomics 12(1): 61 (2011) * Highly Accessed * PDFPubMed
  • Ewen-Campen, B., Schwager, E.E., Extavour, C.G. The molecular machinery of germ line specification. Molecular Reproduction and Development 77(1): 3-18 (2010) PDF PubMed
  • Extavour, C.G. Oogenesis: Making the Mos of Meiosis. Current Biology 19(12): R489-R491 (2009) PDF PubMed
  • Abzhanov, A., Extavour, C.G., Groover, A., Hodges, S., Hoekstra, H., Kramer, E M., Monteiro, A. Are We There Yet? Tracking the Development of New Model Systems. Trends in Genetics 24(7): 353-360 (2008) PDF PubMed
  • Voronina, E., Lopez, M., Juliano, C. E., Gustafson, E., Song, J. L., Extavour, C.G., George, S., Oliveri, P., McClay, D., and Wessel, G. Vasa protein expression is restricted to the small micromeres of the sea urchin, but is inducible in other lineages early in development. Developmental Biology 314(2): 276-286 (2008) PDF PubMed
  • Extavour, C.G. Evolution of the bilaterian germ line: lineage origin and modulation of specification mechanisms.  Integrative and Comparative Biology 47(5): 770-785 (2007) PDF PubMed
  • Extavour, C.G. Gray Anatomy: phylogenetic patterns of somatic gonad structures and reproductive strategies across the Bilateria.  Integrative and Comparative Biology 47(3): 420-426 (2007) PDF PubMed
  • Extavour, C.G., Pang, K., Matus, D. Q. & Martindale, M. Q.  vasa and nanos expression patterns in a sea anemone and the evolution of bilaterian germ cell specification mechanisms Evolution and Development 7(3): 201-215 (2005) * Nominated as a “must read” by the Faculty of 1000 * PDF PubMed
  • Extavour, C.G. The fate of isolated blastomeres and formation of germ cells in the amphipod crustacean Parhyale hawaiensis. Developmental Biology 277(2): 387-402 (2005) * Cover Article * PDF PubMed
  • Extavour, C.G.  Hold the germ cells, I’m on duty (on germ cell specification and caste determination in a polyembryonic waspBioEssays 26(12): 1263-1267 (2004). PDF PubMed
  • Extavour, C.G. and Akam, M.  Mechanisms of germ cell specification across the metazoans: epigenesis and preformation.  Development 130(24): 5869-5884 (2003) PDF PubMed
  • Extavour, C.G. and García-Bellido, A. Germ cell selection in genetic mosaics in Drosophila melanogasterProceedings of the National Acedemy of Sciences of the USA 98(20): 11341-11346 (2001) PDF PubMed