Metamerism (biology) - AbsoluteAstronomy.com

Biology

Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, metamerism is a linear series of body segments fundamentally similar in structure, though not all such structures are entirely alike in any single life form because some of them perform special functions.
In animals, metameric segments are referred to as somite

Somite

A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.- In vertebrates :...

s or metameres. In plants, they are referred to as metamers or, more concretely, phytomer

Phytomer

Phytomers are functional units of a plant continually produced by root and shoot meristems throughout a plant's vegetative life-cycle. A typical phytomer consists of a node to which a leaf is attached, a subtending internode, and an axillary bud at the base of the leaf.Initially, a young plant will...

In animals

In animals, metamery is defined as a mesoderm

Mesoderm

In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

al event resulting in serial repetition of unit subdivisions of ectoderm

Ectoderm

The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...

and mesoderm

Mesoderm

products. Endoderm is not involved in metamery. Segmentation

Segmentation (biology)

Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

is not the same concept as metamerism. Segmentation can be confined only to ectodermally derived tissue, e.g., in the Cestoda tapeworms. Metamerism is far more important biologically since it results in metameres, also called somites, that play a critical role in advanced locomotion.

Metamerism can be divided into two main categories:

homonomous metamery is a strict serial succession of metameres, of which, in fact, there are no true examples in the invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s; however, the Annelida worms
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

, e.g., nereis
Nereis
Nereis is a genus of polychaete worms in the family Nereidae. It comprises many species, most of which are marine, including the sandworm and the common clam worm . Nereis possess setae and parapodia for locomotion. They may have two types of setae, which are found on the parapodia. Acicular setae...

, are used as a model to portray homonomous metamery.
heteronomous metamery is the condition where metameres have grouped together to perform similar tasks. The extreme example of this is the insect head
Head
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

(5 metameres), thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...

, and abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

, where the segments are only as regards structures not crucial to specific segment function (e.g. ladder nerve system or somite
Somite
A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.- In vertebrates :...

s).

Humans and most other chordate

Chordate

Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

s have similar groupings of metameres. Here however, the metameres of each group are essentially fused into one and few repetitive features are seen. Some vestiges – e.g. branchial arch

Branchial arch

In the development of vertebrate animals, the pharyngeal arches are anlage for a multitude of structures. In humans, they develop during the fourth week in utero as a series of mesodermal outpouchings on the left and right sides of the developing pharynx...

es – are still discernible. The process that results in the grouping of metameres is called "tagmitization", and each grouping is called a tagma

Tagma (biology)

In invertebrate biology, a tagma is a specialized grouping of arthropod segments, such as the head, the thorax, and the abdomen with a common function. The segments of a tagma may be either fused or moveable.-Tagmata:...

(plural: tagmata).

In plants

A metamer is one of several segments that share in the construction of a shoot

Shoot

Shoots are new plant growth, they can include stems, flowering stems with flower buds, and leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop...

, or into which a shoot may be conceptually (at least) resolved. In the metameristic model, a plant consists of a series of 'phytons' or phytomer

Phytomer

s, each consisting of an internode and its upper node with the attached leaf. As Asa Gray

Asa Gray

-References:*Asa Gray. Dictionary of American Biography. American Council of Learned Societies, 1928–1936.*Asa Gray. Encyclopedia of World Biography, 2nd ed. 17 Vols. Gale Research, 1998.*Asa Gray. Plant Sciences. 4 vols. Macmillan Reference USA, 2001....

(1850) wrote:

The branch, or simple stem itself, is manifestly an assemblage of similar parts, placed one above another in a continuous series, developed one from another in successive generations. Each one of these joints of stem, bearing its leaf at the apex, is a plant element; or as we term it a phyton,—a potential plant, having all the organs of vegetation, namely, stem, leaf, and in its downward development even a root, or its equivalent. This view of the composition of the plant, though by no means a new one, has not been duly appreciated. I deem it essential to a correct philosophical understanding of the plant.

Some plants, particularly grasses, demonstrate a rather clear metameric construction, but many others either lack discrete modules or their presence is more arguable. Phyton theory has been criticized as an over-ingenious, academic conception which bears little relation to reality. Eames (1961) concluded that "concepts of the shoot as consisting of a series of structural units have been obscured by the dominance of the stem- and leaf-theory. Anatomical units like these do not exist: the shoot is the basic unit." Even so, others still consider comparative study along the length of the metameric organism to be a fundamental aspect of plant morphology

Plant morphology

Plant morphology or phytomorphology is the study of the physical form and external structure of plants. This is usually considered distinct from plant anatomy, which is the study of the internal structure of plants, especially at the microscopic level...

.

Metameric conceptions generally segment the vegetative axis into repeating units along its length, but constructs based on other divisions are possible. The pipe model theory conceives of the plant (especially trees) as made up of unit pipes ('metamers'), each supporting a unit amount of photosynthetic tissue. Vertical metamers are also suggested in some desert shrubs in which the stem is modified into isolated strips of xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

, each having continuity from root to shoot. This may enable the plant to abscise a large part of its shoot system in response to drought, without damaging the remaining part.

In vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

s, the shoot system differs fundamentally from the root system in that the former shows a metameric construction (repeated units of organs; stem, leaf, and inflorescence), while the latter does not. The plant embryo represents the first metamer of the shoot in spermatophyte

Spermatophyte

The spermatophytes comprise those plants that produce seeds. They are a subset of the embryophytes or land plants...

s or seed plants.

Plants (especially trees) are considered to have a 'modular construction,' a module

Modularity

Modularity is a general systems concept, typically defined as a continuum describing the degree to which a system’s components may be separated and recombined. It refers to both the tightness of coupling between components, and the degree to which the “rules” of the system architecture enable the...

being an axis in which the entire sequence of aerial differentiation is carried out from the initiation of the meristem to the onset of sexuality (e.g flower or cone development) which completes its development. These modules are considered to be developmental units, not necessarily structural.

In animals

In plants

See also