Cassandra Extavour

Cassandra G. Extavour

Professor of Organismic and Evolutionary Biology
Professor of Molecular and Cellular Biology

Phone: 617-496-1935
E-mail:
Office: Biological Labs, Rm 4103
16 Divinity Ave

Lab Website: http://www.extavourlab.com


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.


Recent 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)

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 (in press) [PDF of accepted MS]

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 doi:10.1098/rspb.2013.2673 published online March 22 (2014)

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 doi 10.1007/s00427-013-0463-7 published online January 10 (2014)

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)

Gupta, T. and Extavour, C.G. Identification of a putative germ plasm in the crustacean Parhyale hawaiensis. EvoDevo 4(1): 34 (2013

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 (in press) doi:10.3791/51073 (2014) [PDF of accepted MS]

Extavour, C.G. Live long and prosper: Germline stem cell maintenance revisited. BioEssays 35 (9): 763 (2013)

Sharma, P.P., Schwager, E.E., Giribet, G.G., Jockusch, E. and Extavour, C. 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 *

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)

Zeng, V. Ewen-Campen, B., Horch, H.W., Roth, S., Mito, T., and Extavour, C.G. Gene discovery in a hemimetabolous insect: a de novo transcriptome for the cricket Gryllus bimaculatus. PLoS ONE 8(5): e61479 (2013)

Ewen-Campen, B., Jones, T., and Extavour, C.G. Evidence against a germ plasm in the milkweed bug Oncopeltus fasciatus, a hemimetabolous insect. Biology Open (Company of Biologists) doi: 10.1242/bio.20134390, Epub 18 April (2013) (* equal author contribution)

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)

Ewen-Campen, B., Srouji, J.R., Schwager, E.E. and Extavour, C.Goskar 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*

Green II, D.A., and Extavour, C.G. Convergent Evolution of a Reproductive Trait Through Distinct Developmental Mechanisms in Drosophila. Developmental Biology doi: 372(1): 120-130 (2012)

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)

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 opisthosomaEvolution and Development (2012) 14(5): 450-463

Sarikaya, D.P., Belay, A.A., Ahuja,A. Green, 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).

Zeng, V., Villanueva, K.E., Ewen-Campen, B., Alwes, F., Browne, W.E. and Extavour, C.GDe novo assembly and characterization of a maternal and developmental transcriptome for the emerging model crustacean Parhyale hawaiensis. BMC Genomics 12 (1): 581 (2011).

Kainz, F., Ewen-Campen, B., Akam, M. and Extavour, C. G. Delta/Notch signaling is not required for segment generation in the basally branching insect Gryllus bimaculatus. Development 138 (22): 5015-5026 (2011).

Alwes, D., 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 139 (1): 110-123 (2011).

Extavour, C.G. Long-Lost Relative Claims Orphan Gene: oskar in a Wasp. PLoS Genetics Epub April 28. doi/10.1371/journal.pgen.1002045 (2011).

Green, D.A., Sarikaya, D.P. and Extavour, C.G. Counting in oogenesis. Cell and Tissue Research 344 (2): 207-212 (2011).

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*

Ewen-Campen, B., Schwager, E.E., and Extavour, C.G. The molecular machinery of germ line specification. Molecular Reproduction and Development. 77 (1): 3-18 (2010).

Extavour, C.G. Oogenesis: Making the Mos of Meiosis. Current Biology 19 (2): R489-R491 (2009).

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).

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).

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).

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).

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*

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 *

Extavour, C.G. Hold the germ cells, I’m on duty (on germ cell specification and caste determination in a polyembryonic wasp). BioEssays 26 (12): 1263-1267 (2004). (invited review)

Extavour, C.G. Not just a tasty snack (review of "Evolutionary Developmental Biology of Crustacea", ed. G. Scholtz.) BioEssays 26 (11): 1256-1258 (2004).

Extavour, C.G. and Akam, M. Mechanisms of germ cell specification across the metazoans: epigenesis and preformation. Development 130 (24): 5869-5884 (2003). (invited review)

Extavour, C.G. and García-Bellido, A. Germ cell selection in genetic mosaics in Drosophila melanogaster. P.N.A.S. 98 (20): 11341-11346 (2001).


Books and Book Chapters


Srouji, J. R. and Extavour, C. Redefining stem cells and assembling germ plasm: key transitions in the evoution of the germ line. Chapter 16, pp. 360-397 in Key Transitions in Animal Evolution, eds. R. DeSalle and B. Schierwater. CRC Press, Taylor & Francis Group; Science Publishers (2010)

Extavour, C. Urbilaterian Reproduction: the evolution of metazoan germ cell specification mechanisms. Chapter 17, pp. 321-342 in Evolving Pathways: Key Themes in Evolutionary Developmental Biology. Alessandro Minelli and Giuseppe Fusco, eds. Cambridge University Press (2008).


News Features


Byrne, P. Debate on Evolution of Multicellular Organisms Starts to Gain Focus. Scientific American, September 30 (2013). Original article published as " Debating the Evolution of Multicellularity" in Quanta Magazine, September 25 (2013)

Biological Time. Exhibit by the Ernst Mayr Library, July 2013 - present.

Smith, D. Cooperating to study Cooperation. The Scientist, February 20 (2013)

Maxmen, A. Transcriptomics for the Animal Kingdom: Using RNA-Seq to study nonmodel organisms. The Scientist, July 1 (2013)

Abouheif, E. Evolution: oskar Reveals Missing Link in Co-optive Evolution  Current Biology 23(1): R24-R25 (2013).

Bondar, C . and Extavour, C. Nerd Corner Podcast Interview for Carin Bondar's Biology Blog. (2010).

Tintori, S. and Extavour, C. CreatureCast Episode 2 "Why Cells Cooperate" about the evolution and development of multicellular organisms and germ cells. NPR's Science Friday. (2009).

Schaffer, L. Shooting Stars: Check out these faculty ballers! The Harvard Crimson Fifteen Minutes Magazine April 30 (2009).

Dolgin, E. Management for Beginners: So you’re a princial investigator – now what? The Scientist 22 (7): 75-77 (2008).

Bjorn, G. Ready, Set, Hire. Nature 451: 740-741 (2008).

Travis, J. A Close Look at Urbisexuality. Science 316: 391-391 (2007).

Hines, P. J. and Kadereit, S. Topic of the Month: Asymmetric Cell Division. International Society for Stem Cell Research. January 2004: 1-3 (2004).


Courses Taught


MCB 291. Genetics, Genomics and Evolutionary Biology

OEB 51. The Biology and Evolution of Invertebrate Animals

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