Abstract
The concept of the phylotypic stage has been strongly integrated into developmental biology, thanks mostly to drawings presented by Haeckel (Anthropogenie oder Entwicklungsgeschichte des Menschen, 1874). They are printed in every textbook as proof of the existence of the phylotypic stage and the fact of its conservation, albeit many times criticized as misleading and simplifying (Richardson in Develop Biol 172:412–421, 1995, Richardson et al. in Anat Embryo 196:91–106, 1997; Bininda-Emons et al. in Proc R Soc Lond 270:341–346, 2003). Although generally accepted by modern biology, doubt still exists concerning the very existence or the usefulness of the concept. What kind of evolutionary and developmental horizons does it open indeed? This article begins with the history of the concept, discusses its validity and draws this into connotation with the idea of a memory activated throughout the development. Barbieri (The organic codes. An introduction to semantic biology, 2003) considers the phylotypic stage to be a crucial boundary when the genetic program ceases to suffice for further development of the embryo, and supracellular memory of the body plan is activated. This moment clearly coincides with the commencing of the modular development of the embryo. In this article the nature of such putative memory will be discussed.
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Notes
The authors argue from the perspective of geometric morphometrics with the impossibility to find any quantitative measure how to compare the similarities among organisms passing through the blastula stage and the phylotypic period. The differences among organisms before and after the phylotypic period are supposed to be higher- but the same measures cannot be defined for all compared species: some of the variables are not defined for all species involved; some traits at later stages are too complex to compare between each other, like the human lips or the bird’s beak etc.
The developmental events being taken into account were transformations (i.e., first appearance of a defined morphology or morphogenetic movement), as they take place during the whole of the mid-embryonic period. Most of these developmental events shared features present in all the species studied: the first dataset consisting of 14 vertebrate species and 41 developmental events and the second 14 mammal, plus two amniote outgroups with 116 developmental events (Bininda-Emons et al. 2003).
For a criticism on the conservation of phylotype, based on interspecific variation in amphibians observed during the development of neural crest, see Collazo (2000).
In the same issue of Nature, Kalinka et al. (2010) confirmed the phylotypic status of the germband stage in insects, comparing the expression levels of selected genes and their specific temporal relationships among six Drosophila species, revealing that the temporal gene expression pattern is the most conservative during the mid-embryonic period.
Colinearity means that the order of the gene cluster on the chromosome corresponds to the order of their expression along the antero-posterior axis of the organism. In vertebrates, colinearity is not only spatial, but also temporal, as genes corresponding to the anterior part of the body are expressed earlier than the genes corresponding to the posterior parts.
Alternative splicing of the gene transcript provides yet another source of trans-factor heterogeneity: the differences in products of a single Ubx gene, i.e. different proteins are spliced from the same gene. In D. melanogaster six such different isomorphs were observed (Alonso 2008). Ubx determines the segment specificity for many cell types, in epidermis, central and peripheral nervous system and mesoderm. The transcript isomorphs of Ubx gene differ in the presence of short additional regions (microexons): isomorphs containing microexons are expressed especially in epidermis, mesoderm and peripheral nervous systems. Isomorphs lacking the microexons are expressed only in central nervous systems. Functional specificity of the selector genes is therefore generated also on the level of RNA splicing.
Two related ascidian species undergo different developments: either a conventional tadpole larva, or a tailless larva (Swalla and Jeffery 1996). In addition, changes in sea urchin cytoplasmic determinants can generate sea urchins that develop without larvae, yet accomplishing a normal adult (Gilbert 2003).
In his book Organic codes, Barbieri (2003) distinguished several different types of memories. The first level is genetic – in DNA. The second type of the memory works on the basis of different epigenetic codes, e.g. histone code. Such codes are created and re-written thanks to quasi-digital marks, such as histone modification or DNA methylation (Markoš and Švorcová 2009). Epigenetic memory determines the state of every cell in the body and maintains their differentiation. Barbieri also speaks about the neuronal memory and the memory of the immune system at the supracellular level. In his opinion, such memories represent deposits of epigenetic information acquired in ontogeny.
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This study was supported by the project of the Czech Ministry of education MSM 0021620845, and the GPSS Major Awards Programme, a joint programm of the Interdisciplinary University of Paris and Elon University.
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Švorcová, J. The phylotypic stage as a boundary of modular memory: non mechanistic perspective. Theory Biosci. 131, 31–42 (2012). https://doi.org/10.1007/s12064-012-0149-0
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DOI: https://doi.org/10.1007/s12064-012-0149-0