Testing the vulnerability of the phylotypic stage: On modularity and evolutionary conservation

Abstract

The phylotypic stage is the developmental stage at which vertebrates most resemble each other. In this study we test the plausibility of the hypotheses of Sander [1983, Development and Evolution, Cambridge University Press] and Raff [1994, Early Life on Earth, Columbia University Press; 1996, The Shape of Life, University of Chicago Press] that the phylotypic stage is conserved due to the intense and global interactivity occurring during that stage. First, we test the prediction that the phylotypic stage is much more vulnerable than any other stage. A search of the teratological literature shows that disturbances at this stage lead to a much higher mortality than in other stages, in accordance with the prediction. Second, we test whether that vulnerability is indeed caused by the interactiveness and lack of modularity of the inductions or, alternatively, is caused by some particularly vulnerable process going on at that time. From the pattern of multiple induced anomalies we conclude that it is indeed the interactiveness that is the root cause of the vulnerability. Together these results support the hypotheses of Sander and Raff. We end by presenting an argument on why the absence of modularity in the inductive interactions may also be the root cause of the conservation of the much discussed temporal and spatial colinearity of the Hox genes. J. Exp. Zool. (Mol. Dev. Evol.) 291:195–204, 2001. © 2001 Wiley‐Liss, Inc.

Salamander limb development: Integrating genes, morphology, and fossils

“…They proposed that although much of the development of the limb is similar in all tetrapods, the limbs of amphibians develop as a semiautonomous unit, while in amniotes, limb development primarily takes place in the phylotypic stage in which many parts of the embryo are patterned and a strong interaction between secondary fields and transient structures like the limb bud, somites, notochord, and neural tube takes place. The limb buds occur at comparable stages in all investigated developmental series of amniotes, in which the organization of the embryo is still strongly dependent on interactions between different embryonic units (Galis and Metz, 2001;Galis et al, 2003b). Under this hypothesis, a number of transient structures, which are only present in the early embryo, are necessary for limb development and these interactions cannot be repeated at a later stage, because the structures necessary for induction and interaction of limb structures have disappeared or differentiated (Galis et al, 2003b).…”

Fast running restricts evolutionary change of the vertebral column in mammals

“…Any weakening of this selection in slow and domesticated species, due to the mitigated fitness effects of lumbosacral abnormalities, probably leads to a sharp decrease in robustness. This sharp decrease can in part be explained by the high interactivity and low modularity of the vulnerable early organogenesis stage, when lumbosacral vertebral identities are determined (33,34). Moreover, the early irreversibility of the determination of vertebral identity further limits the buffering potential (5).…”

The ontology of organisms: Mechanistic modules or patterned processes?

“…These modular sub-systems might be considered central (in the aforementioned sense) to an evo-devo ontology on account of their being ontogenically explanatory with respect to the most contemporarily important "units" of evolutionhomologues, discrete morphological features whose broad phenotypic similarity among their various instances is underwritten by shared structural-cum-causal developmental mechanisms which exhibit a traceable phylogenetic lineage. 2 For if the evolutionary process can be conceived as the successive propagation and progression of homologue variation and canalisation, then these highly integrated genetic regulatory networks (GRNs) embedded in the "bottleneck" of ontogenesis (Galis & Metz 2001;Kalinka et al 2010), in virtue of their exerting downstream spatial and temporal regulatory control via the production of transcription factors whose patterns of expression causally specify the particularised developmental pathways of those morphological structures, are surely prime candidates for being the "real players" in an adequate evo-devo ontology (Brandon 1999;Brigandt 2007;McCune & Schimenti 2012;Wagner 2014).…”