Cassandra Extavour

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Insect egg evolution defies predicted allometric relationships across eight orders of magnitude in size



The evolution of organism size is thought to be predicted by development, ecology, and morphological constraints. However, tests of these predictions have been challenged by restricted taxon sampling in a phylogenetic framework. To overcome these limitations, we generated a database of more than ten thousand observations of insect egg size and shape from the entomological literature and combined them with published genetic datasets, enabling us to perform statistically robust, phylogenetically corrected tests of long-standing predictions in size evolution. We show that across eight orders of magnitude in volume variation, the shape of insect eggs is predicted by their size, but that the allometric relationship between shape and size evolves dynamically. Moreover, in multiple independent insect clades, shape is conserved over orders of magnitude in size variation. We test the predicted relationship between size and development, and show that egg size is not correlated with developmental rate across insects, and that for many insect groups, including flies, egg size is not correlated with adult body size. Finally, we show that the evolution of parasitism and aquatic oviposition both help to explain the diversification of egg size and shape across the insect evolutionary tree.

Samuel H. Church*, Seth Donoughe*, Bruno de Medeiros and Cassandra G. Extavour (* equal contribution)


Cassandra Extavour is a native of Toronto, where she attended the University of Toronto Schools and went on to obtain an Honors BSc at the University of Toronto with a specialist in Molecular Genetics and Molecular Biology, a Major in Mathematics and a Minor in Spanish. She obtained her PhD with Antonio Garcia Bellido at the Severo Ochoa Center for Molecular Biologyat the Autonomous University of Madrid. She performed postdoctoral work first with Michalis Averof at the Institute for Molecular Biology and Biotechnology in Crete, Greece, and subsequently with Michael Akam at the University of Cambridge. At Cambridge she received a BBSRC Research Grant and became a Research Associate in the Department of Zoology. In 2007 she established her independent laboratory as an Assistant Professor in the Department of Organismic and Evolutionary Biology at Harvard University, where she was promoted to Associate Professor in 2011 and to Full Professor in 2014.

The Extavour laboratory studies primarily early animal embryonic development, the genes that control this development, the evolutionary origins of these genes and how their functions have changed over evolutionary time. The lab is particularly interested in the development and evolution of reproductive systems, including both germ cells, which are cells that make eggs and sperm in sexually reproducing animals, and somatic gonad cells, which create the structures to house and protect the germ cells, and regulate egg and sperm production.