5 April 2019
Cassandra Extavour
Departments of Organismic and Evolutionary Biology &
Molecular and Cellular Biology
Harvard University
Animal development takes place within eggs, which are single cells of various shapes and sizes. Given that cell behavior is influenced not just by genetic factors, but also by the physical and mechanical properties of their environments, we hypothesize that the shapes and sizes of eggs are likely to impact early embryogenesis. An important step towards testing this hypothesis is to gather and analyze data on the distribution of extant egg shapes and sizes. The evolution of the size in cells and cellular organisms 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. We discuss how these data may be used, in combination with a model of nuclear behavior in early insect embryos generated from empirical observations of cricket development, to begin to generate frameworks for predicting early embryonic cell behaviors as a function of egg size and shape.