Modularity in Development

Modularity is found in all facets of life and is very important in the biological development of an organism. Simon (1962)(in Gilbert, 7th ed.) argued that modularity is important in nature for the design and description of complex systems at all levels. He gave the example of two matchmakers to explain how modularity is important in the stable development of complicated system.

Living beings are not composed of hierarchies. “Each entity is an organized array of constituent modular parts, and at the same time, the constituent of a larger module.” (Gilbert S., 7th ed.) These modules interact among themselves to form a larger coherent module. An evolutionary module is characterized as a unit that functions as an individual in respect of three processes of replication, interaction and evolution. (Lewontin (1970), in Bolker, 2000)

One level in which modularity is clearly seen is in the later stages of embryonic development. According to Bonner (1988, in Gilbert 1998, pp.172), “Modularity is associated with ‘gene nets’ that can participate in many different aspects of development.” This happens through discrete and interacting modules. Klingenberg has defined modules as “units that are internally coherent by manifold interactions of their parts, but are relatively autonomous from other such units with which they are connected by fewer or weaker interactions.” (Klingenberg, 2002)

Modules are, therefore, said to be “individualized” units separate from their surroundings. Raff listed the characteristics of developmental modules in his definition. He said that modules must possess certain “discrete specification, hierarchical organization, interactions with other modules, a particular physical location within a developing organism, and the ability to undergo transformations on both developmental and evolutionary time scales.” (Raff 1996, qtd. in Bolker, 2000) He explained that modules are “dynamic entities” which represent localized processes as seen in morphogenetic fields rather than “simply incipient structures” like organ rudiments.

Modules are found to have external connectivity along with internal integration. So, modules allow for the three processes of dissociation, duplication and divergence, and cooption. (Raff 1997, qtd in Gilbert, 1998) Dissociation lets one module change without affecting other modules and that, in turn, permits heterochrony. Dissociation also allows allometry, letting different parts grow at different rates.

The principles of duplication and divergence are seen in the variations of themes produced by morphogenetic fields. These are seen in different sizes and shapes of teeth, and the difference in hind limb and forelimb. Modularity also permits cooption, such that the same module can create both, jaws in fishes and mammalian middle ear cartilage. (Gould 1990, in Gilbert 1998)

Modules can also undergo developmental and evolutionary change separately from other modules. The signaling interactions within modules over a distance are carried out by morphogens (Neumann, Kersberg, Ferguson, Gudon and Bouiillot, qtd.  Klingenberg, 2002) These are proteins of families like FGF, hedgehog, Wnt or TGF-?, or molecules like retinoic acid. These are termed as “panacrine” factors. These factors stimulate the transcription factors in cells through the signal transduction cascades between them.

This brings about a stimulation of a specific feature or factor in the cells in a module. Each module in an embryo makes a different level of a living entity. E.g. a cell is a part of tissue and organelles are parts of cells. Organelles must function to make a coherent cell and cells must function to make a coherent tissue. Modular units lets different parts of the embryo develop without interfering with other units. So development occurs through discrete and interacting modules.

According to Keller (1986) (in Bolker, 2000), the best-characterized module is the dorsal marginal zone (DMZ) of the “Xenopus” gastrula. It fulfills all the necessary criteria for a module. It can be physically isolated from the rest of the embryo and still undergo shape change, so it can be said that convergent extension is intrinsic to DMZ; the force-generating function is uniquely localized to DMZ; all cells within the DMZ interact to produce overall shape change; and lastly, this region can be identified in other amphibian embryos and related fish, like sturgeon.

Other modules, apart from morphogenetic fields, are imaginal discs, like the wing imaginal disc of Drosophila; cell lineages like inner cell mass or trophoblast, insect parasgments, and vertebrate organ rudiments, somites, rhombomeres in flies, RTK-Ras or Wnt, or IP3 pathways are also considered to be developmental and evolutionary modules.

Katherine Anderson first discovered a morphogenetic determinant i.e. an mRNA for the snake protein in her laboratory of Cristiane N?sslein Volhard. “She rescued eggs from homozygous snake mothers by injecting them with small amounts of cytoplasm from wild eggs.” (Gilbert, 1998) Instead of an entire dorsal cuticle being developed, the dorsoventral pattern was restored in them. They also carried out experiments on Drosophila. They showed that a morphogen could be stored as mRNA, but it could be localized to a region of cytoplasm.

Modules can associate with other modules in new ways. Examples at molecular level are proteins like ?-catenin, which can be either a part of Wnt pathway or a cell adhesion factor. At cellular level, the Hedgehog module, which is used to make a border in insect blastoderm, is later used in making eyespots of the wing (Keys et al 1999, in Gilbert, seventh ed.). Buffering of modules is also seen, e.g. the “double assurance” of Spemann (1927) (in Gilbert, 7th ed.)

Modules play an important role in evolution. Evolutionary biologists consider modules as sub-units or components of a larger system. Averof and Patel (1994) showed that the pattern of Ubx and abd-A Hox gene expression correlates with the presence or absence of the modification of thoracic limbs and feeding maxillipeds.(Gilbert, 1998) The maxillipeds form only when the genes are inactive. In vertebrates, the distinction between cervical and thoracic vertebrae, and that between cervical and lumbar vertebrae is mediated by Hox genes.  This is apart from the main role in development. Thus, modular units play important parts in an embryo.

Works Cited

• Gilbert, Scott. “Modules: Key Pieces in the Integration of Developmental and Evolutionary Biology.” Chapter 23. Developmental Biology. Seventh Edition. [Online]<www.devbio.com/article.php?id=222>
• Klingenberg, Christian P. 2002. “Integration, modules, and development: molecules to morphology to evolution.” [Online]<www.flywings.org.uk/PDF%20files/New%20Modules%20&%20Integration.pdf>
• Bolker, J.A. 2000. “Modularity in Development and Why It Matters to Evo-Devo.” American Zoologist, 40:770–776, 2000. [Online]<icb.oxfordjournals.org/cgi/content/full/40/5/770>
• Gilbert, Scott F. 1998. “Conceptual Breakthroughs in Developmental Biology.” Journal of Biosciences, 23, No. 3, Sep 1998, pp. 169-176. Indian Academy of Sciences. [Online]<www.ias.ac.in/jbiosci/september1998/JB3b.pdf>