Other papers by Alexei Sharov

Metasystem Transitions in Biology

Alexei Sharov

1. Duplication and control

"The metasystem transition creates a higher level of organization, the metalevel in relation to the level of organization of the subsystems being integrated"(Turchin 1976).

According to Turchin (1976), metasystem transition requires the following 2 steps:

  1. Duplication of the original system, and
  2. Establishment of control over multiple copies.

In this figure, the initial element duplicates, then differentiation follows. Differentiation is a typical (but not necessary) result of control of elements by the entire system. However, control always changes system components in order to increase the performance of the entire system.

2. Biological examples

2.1. Hypercycle. The idea of a hypercycle was developed by Eigen and Schuster (1979). They noticed that biological organization results from a cooperation of several autocatalytic systems that augment each other.

Each blue loop represents an individual autocatalytic system. Green arrows indicate augmenting effect of one autocatalytic component on another (i.e., the presence of one component increases the rate of selfreproduction of another component). These augmenting effects close in a bigger loop which is a hypercycle. For example, genes that encode proof-reading enzymes increase the precision of self-reproduction of other genes.

Each autocatalytic component benefits from forming a hypercycle because cooperation increases its reproduction rate. The evolution of hypercycles was never observed experimentally. Thus, the only evidence comes from mathematical simulations. Eigen and Schuster (1979) found that homogeneous systems with uniform components may be evolutionary unstable. They differentiate and a new form (e.g., dissipative structures) emerges at the level of the entire hypercycle. Thus, hypercycles may develop by duplicating of the original gene and subsequent differentiation of genes. This process corresponds well to the Turchin's scheme of metasystem transition.

2.2. Volvox. Volvox (Phytomastigophora) is a colonial algae (diam. up to 1 mm) shown on this picture:

Apparently, multicellular organisms originated from colonies of cells like Volvox. A colony is formed of a progeny of a single cell. The colony controls several functions of cells. Cells divide mostly in the direction of the colony surface. There is also coordination of colony movement. The direction of movement is determined by a kind of 'voting' among cells. The colony moves in the direction where the majority of cells want to move.

Volvox produces daughter colonies inside which are clearly visible in movies presented at the Microscopy-UK web page. Probably, protection of daughter colonies from predation is one of the functions of the colony.

2.3. Embryogenesis. Embryonic development of most multicellular organisms goes through the stage of blastula which is a hollow ball of cells that resembles a colony of Volvox. For example, below are photographs of early development of sea urchin taken from the web page of Sea Urchin Fertilization Lab

Cells are not differentiated in early embryos. Any cell if separated, can develop into a full organism. This is the stage of multiplication. Differentiation (control) occurs later. At the moment of fertilization, the cell membrane becomes depolarized. This process creates a gradient which later controls the differentiation of cells. The top side of the blastula eventually develops into the ectoderm and the bottom side develops into the endoderm. Further development involves more complicated processes such as gastrulation. More information on embryogenesis can be found at the Frog Embryology web page.

A similar process of multiplication and differentiation occurred in the evolution of segmentation in invertebrates. Worms (Annelida) have almost even segmentation; additional segments appear as worms grow. In most primitive worms, each segment has all organs of the whole organism: paired appendages (parapodia), gut, nervous system, excretory system, reproductive system. Apparently segmentation originated from unfinished vegetative reproduction (parabiosis). Further evolution of segmented worms led to specialization of segments. Earthworms have special segment that produce slime, and only few segments have reproduction functions (see worm anatomy).

Arthropods originated from worms by further specialization of segments. In insects, segments are highly specialized (see Insect Anatomy). For example, 3 thorax segments have legs, 2 of them also have wings. The head developed from several segments that are hard to distinguish. Check the web page on mechanisms of differentiation of segments and Dynamic development.

Adult of the onion fly

Development of insects recapitulates the metasystem transition that occurred at the evolutionary time scale. Many insect larvae resemble worms with a uniform segmentation.

Larvae of the onion fly

2.4. Neural nets. Unicellular organisms can control their functions (movement, feeding, metabolism) mostly via the network of chemical reactions. Because their body is small, diffusion occurs sufficiently fast to transfer signals from one part to another. Other routs of signal transmission (e.g., electrical) are also possible.

First multicellular organisms apparently had no nervous system (like Volvox). Thus it was a problem how to coordinate movements of the entire organism. Hydra and many other Cnidaria have no central nervous system, instead they have an almost uniform web of nervous cells. These cells have specialized in signal transmission, however all nervous cells are uniform. Maximum they can do is to cause the contraction of the entire body. Here there is no specialization among nervous cells, no higher-level control.

Later, neurons differentiated into sensory neurons and motor neurons, which control muscle contraction. Also there may be interneurons that establish a network of connections between sensory neurons and motor neurons. The entire nervous system become differentiated into the peripheral part and central part (e.g., brain). In vertebrates, cell bodies of sensory neurons are located part-way along their axons outside the spinal cord, and cell bodies of motor neurons are located within the spinal cord. See an overview of the structure of the nervous system.

Diffuse nervous system of a hydra (scheme)

2.6. Slime molds. Slime molds are plasmodial fungi that live in decaying wood or litter. Part of their life cycle they live as single-cell organisms. But then they are able to aggregate together and form a fruit body.

This example is interesting because metasystem transition starts from aggregation of cells rather than from their multiplication. Then cells become differentiated forming a sporangium on a long column.

Fruit body of slime mold

2.7. Insect colonies Insect colonies are families because all members are close relatives. Most colony members are progeny of the same parents. At initial stages of social evolution, individuals in a colony are uniform. For example, colonies of the tent caterpillar have no differentiation among individuals. Caterpillars use pheromones to mark directions from the nest to food (foliage) which is beneficial for the entire colony.

Sociality is more advanced in bees, ants, and termites. As a rule, only one female (a queen) lays eggs in a colony, and numerous workers are sterile. Workers are involved in other functions: foraging, construction, defense, taking care of juveniles. They often have a division of labor and corresponding morphological differences.

This picture shows some forms (casts) of termites. Social insects have very complicated behavior. Their success largely depends on coordinated actions of many individuals. For example, leaf-cutting ants have underground fungus gardens. Ants bring foliage to this garden and collect fungus for food. This is insect agriculture!

3. Symbiosis

Although "multiplication + control" is the most common mechanism of metasystem transition, there is another mechanism, which is symbiosis. Symbiosis, which means "to live together", is a cooperation of several non-similar organisms. For example, lichen is a symbiotic organism formed from algae and fungi.

I call symbiosis a heterogeneous metasystem transition because cooperation become established between different species of organisms. It is opposed to the homogeneous metasystem transition described by Turchin (see above):

Relations in heterogeneous cooperation are asymmetric from the very beginning. Also, there is no differentiation because components are already different. However, components may change considerably in the process of their coevolution.

Other examples of symbiosis

1. eucaryotic cell originated from a symbiosis of several bacterial species. Mitochondria are bacteria that became specialized as energy producers.

This is a transmission electron micrograph of rat liver cells. Part of the nucleus is in the right lower corner. Circular objects are mitochondria.

2. Symbiosis of leaf-cutter ants and fungi.

3. More examples

4. Evolutionary stability of cooperation

Metasystem transition requires cooperation of components. The question is what are necessary conditions for cooperation? The major obstacle on the way of cooperation is possible evolutionary instability. Let us consider cooperating species A and B that produce resources for each other. Species A may "mutate" into a selfish species A1 which will use resources produced by species B without providing help to the species B. As a result, the cooperation between species becomes broken.

Cooperation is evolutionary stable only if specific restrictions are applied on resource exchange. For example, several representatives of species A and B may form small groups, so that communication occurs only among members of a group. If a selfish mutation destroys communication within a group, then this group will become less competitive and eventually will be eliminated in the process of group selection.

Thus, encapsulation makes a metasystem transition possible. Hierarchical systems have several levels of encapsulation and this makes them more stable than systems that have no restrictions for their interaction.

The same is true for homogeneous metasystem transitions. If organisms have no restrictions on their interaction, then cooperation is evolutionary unstable. The simplest restriction mechanism is clustered oviposition and limited dispersal. In this case, organisms interact mostly with their relatives. If an organism has an altruistic behavior, then it is most likely that his neighbors also have an altruistic behavior because they are relatives. Thus, cooperation develops easily among groups of related organisms, a phenomenon known as 'kin selection'.

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Alexei Sharov 11/06/1998