Figure 1. The two major hypotheses about how developmental processes are conserved against evolutionary changes.In both models, embryogenesis proceeds from the bottom to the top, and the width represents the phylogenetic diversity of developmental processes, which are deduced from morphological similarities. (a) The funnel-like model predicts conservation at the earliest embryonic stage. During embryogenesis, diversity increases additively and progressively. This model is based upon the extreme case of developmental burden or generative entrenchment, in which the viability of any developmental feature depends on an earlier one (arrows). (b) The hourglass model predicts conservation of the organogenesis stage. Circles beside the model indicate inductive general features of signals observed during each stage. During this stage, a highly intricate signalling network is established consisting of inductive signals, including the Hox genes11, which leads to conservation of the animal body plan12. Figure 1b was adapted with permission from 11 and 12. (c) Hypothetical data supporting the funnel-like (left) and hourglass (right) models. For both examples, the transcriptome data of M. musculus embryos (early, middle and late stages) were compared with X. laevis embryos (early, middle and late stages) in an all-to-all manner. The data, which are consistent with the funnel-like model, show that the transcriptome similarity is highest in the early versus early comparison (shaded point on the blue line; left). Data that are consistent with the hourglass model show that the transcriptome similarity is highest in the middle versus middle comparison (right).
Image published in: Irie N and Kuratani S (2011)
Copyright © 2011, Nature Publishing Group, a division of Macmillan Publishers Limited. This image is reproduced with permission of the journal and the copyright holder. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike license
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