Developmental Biology

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 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...