Multicellular organism

In this image, a wild-type Caenorhabditis elegans is stained to highlight the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell. Each cell is specialized to do a certain job for that organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Plantae and Animalia (except for specialized organisms such as Myxozoa).

Evolutionary history

Early life was most probably single celled. Multicellularity has evolved independently dozens of times in the history of Earth, for example in plants and animals^[1]. Multicellularity exists in both prokaryotes and eukaryotes, and first appeared several billion years ago in cyanobacteria. In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Mesoproterozoic is thought to have precipitated the development and rise of multicellular life.

Multicellular organisms, especially long-living animals, also face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process.

Hypotheses for origin

There are various mechanisms which are disputed as being the first responsible for the emergence of multicellularity, but it is difficult to say which is correct. This is because all the suggested mechanisms are viable, but establishing which was responsible for the first multicellular life requires mostly speculation.^[2]

One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected and specialized cells in one organism (this mechanism is observable in Drosophila). A third theory is that, as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues.

Because the first multicellular organisms would have lacked hard body parts, they are not well preserved in fossil records^[3]. The earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian B Formation in Gabon.^[4]

Until recently phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. This is very inexact, as current multicellular organisms such as animals and plants are 500 million years removed from their single celled ancestors. This allows both divergent and convergent evolutionary processes a huge amount of time to mimic similarities and differences between groups of modern and ancestral species that don't actually exist. While modern phylogenetics uses more sophisticated techniques such as alloenzymes, satellite DNA and other molecular markers, they are still rather imprecise over such huge timescales. Nevertheless, it is hypothesized that the evolution of multicellularity in most, if not all, extant clades could have happened in one of three distinct ways:

The Symbiotic Theory

This theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single celled organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to their genomes being incorporated into one, multicellular, organism^[5]. Each respective organism would become a separate lineage of differentiated cells within the newly created species.

This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones. In these cases it is extremely doubtful if either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is theorized to have occurred (e.g. mitochondria and chloroplasts in animal and plant cells - endosymbiosis) it has only happened extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming the composite lichen, while dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.

The Cellularization (Syncytial) Theory

This theory states that a single unicellular organism could have developed internal membrane partitions around each of its nuclei^[6] Many protists such as the ciliates or slime moulds can have several nuclei, lending support to this hypothesis. However, simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions: the macronucleus serves the organism's needs while the micronucleus is used for sexual-like reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not the other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium.

The Colonial Theory

The third explanation of multicellularisation is the Colonial Theory proposed by Haeckel in 1874. This theory claims that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, presumably land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division^[7]. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation^[8]

The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently numerous times (in 16 different protoctistan phyla). For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protozoa are Volvocaceae, such as Eudorina and Volvox (the latter of which consists of up to 500 — 50,000 cells (depending on the species), only a fraction of which reproduce^[9] (in one species 25 — 35, 8 asexually and around 15 — 25 sexually). However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct. This problem plagues most hypotheses of how multicellularisation could have occurred.

However, most scientists accept that multicellular organisms, from all phyla, evolved by the colonial mechanism.

See also

• Cellular differentiation
• Organogenesis
• Embryogenesis
• Eukaryote
• Bacterial colony
• Nicole King, a biologist studying the evolution of multicellularity

References

External links

• Tree of Life Eukaryotes