Multicellular organism

A multicellular organism is an

In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox), as a great part of species have the capacity for somatic embryogenesis (e.g., land plants, most algae, many invertebrates).^[26][10]

Origin hypotheses

One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. 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 cells in one organism (this mechanism is observable in Drosophila). A third hypothesis 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. This is what plant and animal embryos do as well as colonial choanoflagellates.^[27][28]

Because the first multicellular organisms were simple, soft organisms lacking bone, shell, or other hard body parts, they are not well preserved in the fossil record.

This theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other that they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism.^[32] Each respective organism would become a separate lineage of differentiated cells within the newly created species.^{[citation needed]}

This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between

The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently 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 protista 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.^[36] For example, in one species 25–35 cells reproduce, 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; colonial protists have been dubbed "pluricellular" rather than "multicellular".^[5]

The synzoospore theory

Some authors suggest that the origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial

About 800 million years ago,[38] a minor genetic change in a single molecule called guanylate kinase protein-interaction domain (GK-PID) may have allowed organisms to go from a single cell organism to one of many cells.^[39]

The role of viruses

Genes borrowed from

Oxygen availability hypothesis

This theory suggests that the oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life.^[45] This hypothesis is based on the correlation between the emergence of multicellular life and the increase of oxygen levels during this time. This would have taken place after the Great Oxidation Event but before the most recent rise in oxygen. Mills^[46] concludes that the amount of oxygen present during the Ediacaran is not necessary for complex life and therefore is unlikely to have been the driving factor for the origin of multicellularity.^{[citation needed]}

Snowball Earth hypothesis

A snowball Earth is a geological event where the entire surface of the Earth is covered in snow and ice. The term can either refer to individual events (of which there were at least two) or to the larger geologic period during which all the known total glaciations occurred.

The most recent snowball Earth took place during the Cryogenian period and consisted of two global glaciation events known as the Sturtian and Marinoan glaciations. Xiao et al.^[47] suggest that between the period of time known as the "Boring Billion" and the snowball Earth, simple life could have had time to innovate and evolve, which could later lead to the evolution of multicellularity.

The snowball Earth hypothesis in regards to multicellularity proposes that the Cryogenian period in Earth's history could have been the catalyst for the evolution of complex multicellular life. Brocks^[48] suggests that the time between the Sturtian Glacian and the more recent Marinoan Glacian allowed for planktonic algae to dominate the seas making way for rapid diversity of life for both plant and animal lineages. Complex life quickly emerged and diversified in what is known as the Cambrian explosion shortly after the Marinoan.^{[citation needed]}

Predation hypothesis

The predation hypothesis suggests that to avoid being eaten by predators, simple single-celled organisms evolved multicellularity to make it harder to be consumed as prey. Herron et al.^[49] performed laboratory evolution experiments on the single-celled green alga, Chlamydomonas reinhardtii, using paramecium as a predator. They found that in the presence of this predator, C. reinhardtii does indeed evolve simple multicellular features.^{[citation needed]}

Experimental evolution

It is impossible to know what happened when single cells evolved into multicellular organisms hundreds of millions of years ago. However, we can identify mutations that can turn single-celled organisms into multicellular ones. This would demonstrate the possibility of such an event. Unicellular species can relatively easily acquire mutations that make them attach to each other—the first step towards multicellularity. Multiple normally unicellular species have been evolved to exhibit such early steps:

• millimeters. Changes in multiple genes were identified. In addition, the authors reported that only anaerobic cultures of snowflake yeast evolved this trait, while the aerobic ones did not.^[52]
• A range of green algae species have been experimentally evolved to form larger clumps. When Chlorella vulgaris is grown with a predator Ochromonas vallescia, it starts forming small colonies, which are harder to ingest due to the larger size. The same is true for Chlamydomonas reinhardtii under predation by Brachionus calyciflorus and Paramecium tetraurelia.

C. reinhartii normally starts as a motile single-celled propagule; this single cell asexually reproduces by undergoing 2–5 rounds of mitosis as a small clump of non-motile cells, then all cells become single-celled propagules and the clump dissolves. With a few generations under Paramecium predation, the "clump" becomes a persistent structure: only some cells become propagules. Some populations go further and evolved multi-celled propagules: instead of peeling off single cells from the clump, the clump now reproduces by peeling off smaller clumps.^[53]

Advantages

Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism.^{[citation needed]}

Whether all of these can be seen as advantages however is debatable: The vast majority of living organisms are single celled, and even in terms of biomass, single celled organisms are far more successful than animals, although not plants.^[54] Rather than seeing traits such as longer lifespans and greater size as an advantage, many biologists see these only as examples of diversity, with associated tradeoffs.^{[citation needed]}

Gene expression changes in the transition from uni- to multicellularity

During the evolutionary transition from unicellular organisms to multicellular organisms, the expression of genes associated with reproduction and survival likely changed.^[55] In the unicellular state, genes associated with reproduction and survival are expressed in a way that enhances the fitness of individual cells, but after the transition to multicellularity, the pattern of expression of these genes must have substantially changed so that individual cells become more specialized in their function relative to reproduction and survival.^[55] As the multicellular organism emerged, gene expression patterns became compartmentalized between cells that specialized in reproduction (germline cells) and those that specialized in survival (somatic cells). As the transition progressed, cells that specialized tended to lose their own individuality and would no longer be able to both survive and reproduce outside the context of the group.^[55]

See also

• Bacterial colony
• Embryogenesis
• Organogenesis
• Unicellular organism

References