1) What is Evo-Devo? (by Rey J. Rosa Morales)

           The great morphological complexity and diversity that we see in multicellular organisms is produced by developmental processes that have evolved in response to natural selection. But how do these developmental processes evolve? Direct development  occurs when embryos develop directly into adult-like forms instead of progressing through a larval stage (Indirect development). This striking divergence in developmental mode has evolved independently in many animal lineages, including sea urchins, ascidians, frogs, and salamanders. The evolutionary forces and genetic mechanisms promoting such radical, and sometimes rapid, changes in development and life history have mystified biologists for over a century. Comparisons of embryogenesis and larval morphogenesis, especially among marine invertebrates, are central topics in both classical developmental biology and modern evolutionary developmental biology.

            These examples suggest several questions: What are the selective pressures that favor such a novel evolutionary trajectory? How could such a profound alteration of early development evolve so many times? And, perhaps most challenging, what genetic and developmental processes are involved in these evolutionary alterations? It is likely that selection for rapid development promotes the evolution of direct development. But even though some of the genes that underlie these alternative developmental trajectories are beginning to be uncovered, the developmental mechanisms involved and more importantly, the reasons why these mechanisms are apparently more flexible in some groups of organisms than others are still mysteries.

            The field of evolutionary developmental biology, or EDB (often called "evo-devo"), seeks to understand the mechanisms by which development has evolved, both in terms of developmental processes (for example, what novel cell or tissue interactions are responsible for novel morphologies in certain taxa) and in terms of evolutionary processes (for example, what selection pressures promoted the evolution of these novel morphologies). Two of the main questions or themes that concern evolutionary developmental biologists are, first, what role has developmental evolution played in the history of life On Earth? and second, do the developmental trajectories that produce phenotypes bias the production of variation or constraint trajectories of evolutionary change? Natural selection acts on phenotypes produced by development, but ultimately we want to understand how the modes by which development produces those phenotypes affect evolutionary potentials and trajectories. 

Direct versus indirect development. (A) A pluteus larva of the indirect-developing sea urchin Heliocidaris tuberculata. (B) A nonfeeding larva of the direct developing congeneric species H. erythrogramma. In H. erythrogramma, the ancestral larval mode has been lost and embryos initiate the program for adult morphogenesis without an intervening pluteus stage. None of the complex morphological features of the pluteus are present in H. erythrogramma larvae, yet these two species are so closely related that they can be interbred in the laboratory. (Photos courtesy of R. Raff.)

Introductio to Evo-Devo (YouTube video: 8min 52seg)

References:

1. Futuyma, D.J. (2005). Evolution. Sinauer Associates, Inc. Publishers Sunderland, Massachusetts U.S.A.
2. Hall, Brian K. (2000). "Evo-devo or devo-evo-does it matter". Evolution & Development 2 (4): 177–178.
3. Palmer, RA (2004). "Symmetry breaking and the evolution of development". Science 306 (5697): 828–833.
4. Prum, R.O., Brush, A.H. (March 2003). "Which Came First, the Feather or the Bird?". Scientific American 288 (3): 84–93.

2) Hox Genes and Evo-Devo (by Rey J. Rosa Morales)

Biologists dating back to Geoffroy Saint-Hilaire (1772-1844), Karl Ernst von Baer (1792- 1876), and Darwin himself were fascinated by the patterns of similarity and divergence in development among species. However until quite recently, the fields of evolutionary biology and developmental biology preceded along mostly separate parts, with seemingly distinct research programs and methodologies. In the past three decades, however, burgeoning information about the genetic mechanisms of morphogenesis in model organisms, as well as the molecular genetic techniques developed to obtain that information, have been integrated with many strands of evolutionary research to form the highly interdisciplinary field now known as Evolutionary Developmental Biology EDB.

Figure 1. A) Wild-type D. melanoganster 

with a single pair of wings and a pair of 

winglike structure, halteres. B) A mutant 

fly was experimentally produce by combining 

several mutations in the regulatory region

of the Ultrabithorax (Ubx) gene. The third

thoracic segment has been transfonned, 

bearing wings instead of  haltares. 

(Photo by E.B. Lewis).  

The discovery and characterization of the Hox cluster of homeobox genes in animals in the 1970 and 1980 marks the dawn of modern EDB. The Hox genes are the best known class of homeotic selector genes, which control the patterning of specific body structures. Hox genes control the identity of segments along the anterior-posterior body axis of all metazoans. Mutations in the Hox genes often cause transformations of one type of segment into another. In Drosophila melanoganster for example, a mutation of the Ultrabithorax (Ubx) gene transforms the third thoracic segment (T3), which normally bears the tiny halteres (the Drosophila homologue of the hindwing of four-winged insects), into a second thoracic segment (T2), which bears  wings (Figure 1 a, b). A  mutation in another Hox gene, Antennapedia (Antp), causes the misexpression of Antp protein in the cells that normally give rise to the antennae, resulting in the replacement of antennae with legs  (Figure 2). Antp is normally expressed only in the second thoracic segment (TI), where it controls the development of T2-specific body structures, including legs.

Figure 2. A) Frontal view of the head of a wild-type Drosophila melanogaster, showing normal
antennae and mouthparts. B) Head  of a fly carrying Antennapedia mutation, which converts antennae into legs. (Photo by F.R. Turner).

     In  Drosophila, the Hox genes occur in two complexes (clusters) of genes on chromosome 3, termed the Antennapedia complex and the bithorax complex. The pioneering genetic work on the bithorax complex was done between the 1940s and the 1970s by E. B. Lewis, and that on the Antennapedia complex in the 1970s and 1980s by Thomas Kaufman and his colleagues. These investigators found that the genes in both complexes control the anterior-posterior identity of segments corresponding to their order on the chromosome  (Figure 3).  They also discovered that the eight Drosophila Hox genes are members of a single gene family, and that the proteins they encode share a particular amino acid sequence that binds DNA, subsequently named the homeobox (in the gene) or the homeodamain (in the protein). This finding supported Lewis's idea, proposed in the 1960s, that the Hox genes regulate the transcription of other genes. Other researchers were stunned to discover that all other animal phyla also possess a set of Hox genes. These genes have homeodomain sequences similar to those of their homologues in Drosophila and have the same gene order and orientation as in Drosophila (except that they form a single gene complex in most animals). Mammals have four Hox gene complexes (denoted Hoxn, Hoxb, Hoxc, and Hoxd) in different parts of the genome, and a total of 13 different Hox genes (as opposed to only 8 in Drosophila), although not all of the complexes have all 13 members (Figure 4).

             Figure 3                                        Figure 4

Figure 3. Hox gene expression in Drosophila. In the center is a map of the genes of the Antennapedia and bithorax complexes, with their functional domains shown in color (© 2008 Sinauer Associates Sadava, D. et al. Life: The Science of Biology, 8th ed.)
 

Figure 4. (right panel) Probable evolution of the metazoan Hox gene complex. Vertical white lines delineate currently  accepted groups of orthologous Hox genes. Important gene duplication events are indicated by the labeled tick marks (Image from the book, Evolution 2009).



References:

1. Carroll, S. B., S. D. Weatherbee, and J. A. Langeland. 1995. Homeotic genes and the  regulation and evolution of insect wing number. Nature 375, 58-61.
2. Carroll, S. B., J. K. Grenier, and S. D. Weatherbee. 2001. From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Blackwell Science, Malden, MA.
3. Depew, D. J. and B. H. Weber. 1994. Darwinism Evolving: Systems Dynamics and the Genealogy of Natural Selection MIT Press, Cambridge, MA.
4. Gould, S. J. 1977. Onthogeny and Phylogeny. Harvard University Press, Cambridge, MA.
5. Warren, R. W., L. Nagy, Selegue, J. Gates, and S. B. Carrol. 1994. Evolution of homeotic gene function in flies and butterflies. Nature 372: -158-461.
6. Weatherbee, S. D., G. Halder; Kim, A. Hudson, and S. B. Carroll. 1998. Ultrabithorax regulates genes at several levels of the wing patterning hierarchy to shape the development of the Drosophila haltere. Genes Devel. 12: 1474-1482.
7. Wilkins, A. S. 2002. Tile Evolution of Developmetal Pathways. Sinhauer Associates, Sunderland, MA.