Fig. 5.

The developmental hourglass model and embryos representative of phylotypic periods in vertebrates. In the developmental hourglass model (middle, originally proposed by Duboule [64]), embryogenesis proceeds from the bottom to the top, and evolutionary divergence becomes minimal at the mid-embryonic, organogenesis phase. The conserved mid-embryonic phase has been predicted to define the body plan for each animal phylum [64] and has therefore been named the phylotypic satage [64, 65, 66, 181]. However, further studies are required to fully verify this hypothesis, and a recent study indicated that the hypothesis may be better applied to vertebrates than to chordates [63]. Embryos representative of most conserved vertebrate stages are shown at the left. Curiously, these stages can also be identified as the most conserved stages when comparing groups of species smaller than at the vertebrate level. No consensus has been reached on the mechanism of this mid-embryonic conservation, but two independent studies have implied possible contributions by developmental constraints [63, 74]. In other words, extensive reuse of the same genetic machinery could have imposed limitations on the evolutionary diversification process through pleiotropic constraint (right, modified from Hu et al. [63]). In each developmental stage, grey and black dots represent genetic components that are pleiotropically expressed in other stages and are shared (blue vertical lines) by multiple developmental processes