The Developmental Hourglass in the Evolution of Embryogenesis

Abstract

Embryogenesis is the process of transformation of a single fertilized egg into a differentiated, complex organism. This requires coordinated cleavage of the fertilized egg, followed by patterning and cell-fate specification, to establish the adult body plan. Based on morphological studies and, more recently, comparative gene expression analyses, the evolution of embryogenesis in animals, and to some degree in plants, has been proposed to follow a developmental hourglass model. In this model, less conserved early events are followed by a highly conserved phylotypic stage at the narrow waist of the hourglass where species within a phylum have similar morphologies and gene expression patterns. Variation in later stages of embryogenesis then follows, providing the diversity of morphologies found in adult forms. As the phylotypic stage is the most conserved, it implies there may be greater evolutionary constrains during mid-embryogenesis compared with the less conserved early and late stages. These constraints may relate to morphological events, and/or the underlying gene regulatory networks, at the different stages of embryogenesis, as well as the requirement for embryogenesis to produce viable offspring adapted to environmental variation.