Abstract
Over the course of evolution, organism size has diversified markedly. Changes in size are thought to have occurred because of developmental, morphological and/or ecological pressures. To perform phylogenetic tests of the potential effects of these pressures, here we generated a dataset of more than ten thousand descriptions of insect eggs, and combined these with genetic and life-history datasets. We show that, across eight orders of magnitude of variation in egg volume, the relationship between size and shape itself evolves, such that previously predicted global patterns of scaling do not adequately explain the diversity in egg shapes. We show that eggsize is not correlated with developmental rate and that, for many insects, egg size is not correlated with adult body size. Instead, we find that the evolution of parasitoidism and aquatic oviposition help to explain the diversification in the size and shape of insect eggs. Our study suggests that where eggs are laid, rather than universal allometric constants, underlies the evolution of insect egg size and shape.
Highlights
Church, Samuel H., Seth Donoughe, Bruno A
We show that size is not correlated with developmental rate, and that for many insects, egg size is not correlated with adult body size either
Changes in egg size have been studied in relation to changes in other aspects of organismal biology[9], including adult body size[10,11,12], features of adult anatomy[13], and offspring fitness via maternal investment[14]
Summary
This hypothesis predicts a reduction in relative cross-sectional area as eggs get larger, which has been proposed as a solution to the need for eggs to pass through a narrow opening during oviposition[13, 19] To test these hypotheses about the physical scaling of size and shape, we began by modeling the individual evolutionary history of each morphological trait. For egg size and aspect ratio, an ‘EarlyBurst’ (EB) model in which evolutionary rate decreases over time, best describes the observed data This change in rate is distributed non-uniformly over the insect phylogeny, with some clades evolving faster than others Evolutionary forces beyond the constraints of physical scaling (e.g. development or ecology) are required to explain egg morphological diversification
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