The evolution of gastrulation morphologies

During gastrulation, early embryos specify and reorganise the topology of their germ layers. Surprisingly, this fundamental and early process does not appear to be rigidly constrained by evolutionary pressures; instead, the morphology of gastrulation is highly variable throughout the animal kingdom. Recent experimental results demonstrate that it is possible to generate different alternative gastrulation modes in single organisms, such as in early cnidarian, arthropod and vertebrate embryos. Here, we review the mechanisms that underlie the plasticity of vertebrate gastrulation both when experimentally manipulated and during evolution. Using the insights obtained from these experiments we discuss the effects of the increase in yolk volume on the morphology of gastrulation and provide new insights into two crucial innovations during amniote gastrulation: the transition from a ring-shaped mesoderm domain in anamniotes to a crescent-shaped domain in amniotes, and the evolution of the reptilian blastoporal plate/canal into the avian primitive streak.

Automated summary

During gastrulation, the topology of germ layers in early embryos is specified and reorganized, and recent experiments have shown that different alternative modes of gastrulation can be generated in single organisms, such as early cnidarian, arthropod and vertebrate embryos. This process is not rigidly constrained by evolutionary pressures, allowing for high variability in gastrulation morphology throughout the animal kingdom. This review discusses the mechanisms underlying the plasticity of vertebrate gastrulation, its effects on yolk volume, and two crucial innovations during amniote gastrulation: the transition from a ring-shaped mesoderm domain to a crescent-shaped domain, and the evolution of the reptilian blastoporal plate/canal into the avian primitive streak.