In embryology a phylotypic stage or phylotypic period is a particular developmental stage or developmental period during mid-embryogenesis where embryos of related species within a phylum express the highest degree of morphological and molecular resemblance. Recent molecular studies in various plant and animal species quantified the expression of genes covering crucial stages of embryo development and found that during the morphologically defined phylotypic period the evolutionary oldest genes, genes with similar temporal expression patterns, and genes under strongest purifying selection are most active throughout the phylotypic period.
Historical origins of concept
The idea that embryos of different species have similar morphologies at some point during development can be traced back to Aristotle. Aristotle observed a number of developing vertebrate embryos, noting in his text The Generation of Animals that the morphological differences among the different embryos arose late in development. In 1828, Karl Ernst von Baer created his laws of embryology, which summarized the results of his comparative embryogenesis studies. In his first law, he proposed that the more general characters of a group appear earlier in their embryos than the more special characters. In 1866, Ernst Haeckel proposed that each developing organism passes through the evolutionary stages of its ancestors, i.e., ontogeny recapitulates phylogeny. The hypothesis that different organisms pass through the developmental stages of closely related organisms is outdated. However, the idea that early stages of development are conserved among species, with increasing divergence as development progresses, has influenced modern evolutionary and developmental biology. The early conservation or funnel model of development (see below) is closely tied to these historical origins.
Phylotypic period The first formulation of the phylotypic period concept came in 1960 from Friedrich Seidel's Körpergrundgestalt, which translates to "basic body shape." In 1977, Cohen defined the phyletic stage as the first stage that reveals the general characters shared by all members of that phylum. Klaus Sander revised this concept in 1983 and named it the phylotypic stage, which is "the stage of greatest similarity between forms which, during evolution, have differently specialized both in their modes of adult life and with respect to the earliest stages of ontogenesis." Note that this definition demonstrates his support for the hourglass model (see below). Recent papers refer to the phylotypic period, or the phylotypic stage, as a period of maximal similarity between species within each animal phylum. While this concept was originally devised using morphological comparisons of developing embryos from different species, the period of maximal similarity has recently been identified using molecular evidence. The phylotypic period has been identified using conservation of gene expression, estimates of gene age, gene sequence conservation, the expression of regulatory genes and transcription factors, and the interconnectivity of genes and proteins.
Funnel and hourglass models The funnel model is the hypothesis that the most conserved stage of development (the phylotypic period) occurs at the beginning of embryogenesis, with increasing divergence as development progresses. This is also known as the early conservation model of development. Evidence for an alternative model arose from careful comparisons of the temporal divergence in morphology of the embryos of different species. For example, Klaus Sander noticed that the "incredible variation in larvae and adults" of insects occurs after they "develop from nearly identical rudiments in the germ band stage." The most conserved stage of development, the germ band stage, occurs near the middle of development rather than at the beginning, supporting a mid-developmental period of maximal similarity between species. This model, called the hourglass model, is the idea that early embryos of different species display divergent forms but their morphologies converge in the middle of development, followed by a period of increasing divergence.
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