Body plan

A body plan is the blueprint for the way the body of an organism is laid out. Each species of multicellular organism—plant, fungus, red algae, slime mold, among others—has a body plan. This article is about animal body plans.

An animal's symmetry, its number of body segments and limbs are all aspects of its body plan. One of the key issues of developmental biology is the evolution of body plans as different as those of a starfish, or a mammal, which come from a close common biological heritage-both are deuterostomes. One issue in particular is how radical changes in body plans have occurred over geological time. The body plan is a key feature of an organism's morphology and, since the discovery of DNA, developmental biologists have been able to learn a lot about how genes control the development of structural features through a cascade of processes in which key genes produce morphogens, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups of homeobox genes, which are responsible for laying down the basic body plan in animals. The homeobox genes are remarkably conserved between species as diverse as the fruit fly and man, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in man. The field of animal evolutionary developmental biology, which studies the genetics of morphology in detail, is now a rapidly expanding one [1], with many of the developmental genetic cascades, particularly in the fruit fly drosophila, now catalogued in considerable detail.

Body plan is the basis for distinguishing animal phyla, and there are 35 different basic animal body plans, each corresponding to distinct animal phyla.

Origin

The evolution of animal body plans became inevitable with the emergence of differentiated multicellular animal life in the Ediacaran Era, over 600 million years ago.

All land animals are descended from a common bilaterian ancestor. This animal had a "pipe" or alimentary canal body plan. It is essentially a passage having a mouth at one end, and a cloaca or anus at the other. This is common to organisms as diverse as humans and earthworms, and is derived from their shared bilaterian ancestor. The process of nutrient capture, digestion, and waste disposal is fundamental to the body plan of advanced, free-moving animals. Vertebra, limbs, even brains are supplementary to the pipe. Natural selection has spun off an enormous range of variations on this basic theme, but the pipe model itself remains. The basic symmetry and organization of this body plan apparently gave an ancient organism an enormous advantage at survival and reproduction, and it has been preserved in most animals ever since, with the notable exception of the echinoderms.

The Cambrian explosion refers to the massive increase in different body plans that took place around 530 million years ago. Fossils from this era show all of the body plans in existence today.

Bauplan

Bauplan (German for building plan, blueprint; plural: Baupläne or Bauplaene) is a closely related term in biology referring to the common new and original (homologous) properties of the members of a systematic group (taxon). It is not necessary that a bauplan precisely describes any one particular species of that group.

The concept of bauplan is employed in the studies of morphology, taxonomy, comparative physiology, evolutionary physiology, and, most notably, phylogenetics and evolution. Before the advent of genetic sequencing, the analysis of the bauplan of fossils was the primary method to determine hypothetical relationships and lineages of species, both living and extinct. The idea is that species that are closely related share more common properties, hence a more detailed bauplan. Small differences of bauplan are indicative of species belonging to a parent, child or sibling taxon.

Genetic basis

Similarities and differences in adult shape and form, as well as the developmental pattern of embryos, provide the framework for modern taxonomic classification. These comparisons are the basis of phylogenetic systematics. Embryonic development is relatively consistent among animals with similar body plans, although similar larval forms may give rise to very different adults in some groups. The timing, pattern, and scale of developmental events determine the shape of an organism, and closely related groups are more likely to share structural and developmental similarities than those that are more distantly related. Homologous structures and developmental stages—those that are similar among related groups because they are inherited from a common ancestor—are the basis of modern biological classification.

Animal Examples

An early tetrapod: the Ichthyostega

The current range of body plans is far from exhausting the possible patterns for life: the Ediacaran biota appears to contain numerous species and taxa with body plans quite different from any found in currently living organisms.

The most commonly seen body plan amongst land vertebrates is that of the tetrapod, which include all mammals, birds, amphibians and reptiles. Some vertebrate groups, such as the cetaceans, bats and most birds have been modified (e.g. front limbs become wings or flippers) but, nevertheless, they are still tetrapods.

The invertebrates employ a much more diverse array of body plans, such as seen in insects (six legs, three body parts and an exoskeleton), cephalopods (no skeleton, hydrostatically stiffened tentacles, primary propulsion by squeezing water out of a mantle cavity), echinoderms (fivefold radial symmetry, external skeleton, movement by hydrostatically operated tube feet) and various phyla of "worms" (tube-shaped, movement by expanding and contracting parts of the body).

Fictional

One common theme in science fiction is the appearance of extraterrestrial beings, descriptions of which have ranged from being simple variants on human anatomy to beings with body plans wildly different from any found on Earth. The field of exobiology attempts to bring these and similar speculations into the realm of serious scientific investigation.

See also

• Anatomical terms of location
• Arthropod head problem
• Deep homology
• Definition of Phylum based on body plan
• Evolutionary developmental biology
• Ediacaran biota
• Homeobox
• Macroevolution
• Morphology (biology)
• Sean B. Carroll
• Supernumerary body part
• Symmetry (biology)

External links

• Developmental Biology 8e Online: Patterning of the Mesoderm by Activin

Video

• The Science of Evolution: Sean B. Carroll explains the genetics of the fruit fly body plan.