Chapter Summary

The study of developmental biology seeks to explain how the genome dictates a certain adult morphology. On the one hand, the developmental program must be robust and produce the same morphology for adult members of a species every time. On the other hand, there is a huge amount of morphological diversity within kingdoms, and even phenotypic diversity within some species. Ultimately, these changes in adult morphology occurred by changing the underlying developmental program of different species. How can a robust developmental program tolerate such changes, and how might they contribute to the evolution of diverse morphologies? Recently, there has been a resurgence of interest in this problem, triggered by new technologies of molecular biology and genetics. Modern studies of evolutionary developmental biology investigate how individual genes, gene networks, signaling pathways, life histories, and embryology may generate novelty.

Many developmental genes are conserved from species to species. Some genes have been shown to be functionally equivalent, which means they can biochemically substitute for homologous genes in another species, as in the case of pax6 and eyeless. It is important to understand when doing these types of experiments that this does not mean that the structures of the eyes themselves are homologous, but rather that the underlying genes or gene networks are functional orthologs. By investigating conserved genes and gene networks, scientists try to decipher the minimal "tool kit" necessary for animal development. At the level of individual genes, useful motifs have been conserved, and proteins can be assigned to different gene families the basis of the presence of conserved motifs, such as TGFB and the FGF protein families. Furthermore, members of different protein families sometimes form conserved pathways, like the FGF signaling pathway in the respiratory development of Drosophila and mouse [fig. 17.4]. The HOM/HOX genes have been extensively studied in various organisms, and have been implicated in alterations in patterning, including butterfly wings [figs. 17.5 and 17.6], vertebrate rhombomeres, vertebrate limbs, and the lack of limbs in snakes [fig. 17.7], to name a few.

In addition to studying conserved genes and gene networks, scientists also describe conserved stages and processes of development. Within some groups of animals, there is a conserved phylotypic stage, a stage of development during which different embryos of different species look morphologically similar to each other. In arthropods, the phylotypic stage is the segmented germ band stage that appears after gastrulation [fig. 17.8]. In chordates, the phlotypic stage is the pharyngula, which appears after neurulation [fig. 17.9]. It is important to understand that embryos at the phylotypic stage are not identical; rather, they are more similar to each other during the phylotypic stage than during earlier or later times of development [fig. 17.10].

In order to understand diversification of form and function, it is also important to study features of development that are not conserved. A classic example is the vertebrate limb, which has been extensively studied and used as a basis for taxonomic classification of vertebrates [fig. 17.11]. HOX genes have been shown to be important for the patterning of digits in different vertebrate limbs, and both chicks and mice suffer digit duplication as a result of ectopic ZPA activity [fig. 17.12]. Whole signaling systems may be co-opted and used in novel ways during development, like notch in vertebrates and engrailed in the brittle star [fig. 17.13]. Sometimes entire signaling systems may be modified, as in the case of toll function in both dorsal ventral specification in Drosophila and innate immunity. Other times a similar signaling system can produce different outcomes depending on the embryological context in which it is employed, as in the case of dorsal ventral specification between arthropods and chordates [fig. 17.14]. Novelty can arise due to changes in life history, as in the case of larval versus direct development in sea urchins [figs. 17.15 and 17.16], and through heterochrony, a change in the relative timing of a developmental process. Finally, in some groups novelties arise by invention of new developmental trajectories, as in the case of neural crest cells, which are uniquely found in vertebrates.

Chapter Summary

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