Dictyostelium discoideum
Dictyostelium discoideum is a species of soil-dwelling amoeba belonging to the phylum Amoebozoa, infraphylum Mycetozoa. Commonly referred to as slime mold, D. discoideum is a eukaryote that transitions from a collection of unicellular amoebae into a multicellular slug and then into a fruiting body within its lifetime. Its unique asexual life cycle consists of four stages: vegetative, aggregation, migration, and culmination. The life cycle of D. discoideum is relatively short, which allows for timely viewing of all stages. The cells involved in the life cycle undergo movement, chemical signaling, and development, which are applicable to human cancer research. The simplicity of its life cycle makes D. discoideum a valuable model organism to study genetic, cellular, and biochemical processes in other organisms.
Natural habitat and diet
In the wild, D. discoideum can be found in soil and moist leaf litter. Its primary diet consists of bacteria, such as Escherichia coli, found in the soil and decaying organic matter. Uninucleate amoebae of D. discoideum consume bacteria found in their natural habitat, which includes deciduous forest soil and decaying leaves.Life cycle and reproduction
The life cycle of D. discoideum begins when spores are released from a mature sorocarp. Myxamoebae hatch from the spores under warm and moist conditions. During their vegetative stage, the myxamoebae divide by mitosis as they feed on bacteria. The bacteria secrete folic acid, which attracts the myxamoebae. When the supply of bacteria is depleted, the myxamoebae enter the aggregation stage.During aggregation, starvation initiates the production of protein compounds such as glycoproteins and adenylyl cyclase. The glycoproteins allow for cell-cell adhesion, and adenylyl cyclase creates cyclic AMP. Cyclic AMP is secreted by the amoebae to attract neighboring cells to a central location. As they move toward the signal, they bump into each other and stick together by the use of glycoprotein adhesion molecules.
The migration stage begins once the amoebae have formed a tight aggregate and the elongated mound of cells tips over to lie flat on the ground. The amoebae work together as a motile pseudoplasmodium, also known as a slug. The slug is about 2–4 mm long, composed of up to 100,000 cells, and is capable of movement by producing a cellulose sheath in its anterior cells through which the slug moves. Part of this sheath is left behind as a slimy trail as it moves toward attractants such as light, heat, and humidity in a forward-only direction. Cyclic AMP and a substance called differentiation-inducing factor, help to form different cell types. The slug becomes differentiated into prestalk and prespore cells that move to the anterior and posterior ends, respectively. Once the slug has found a suitable environment, the anterior end of the slug forms the stalk of the fruiting body and the posterior end forms the spores of the fruiting body. Anterior-like cells, which have only been recently discovered, are also dispersed throughout the posterior region of the slug. These anterior-like cells form the very bottom of the fruiting body and the caps of the spores. After the slug settles into one spot, the posterior end spreads out with the anterior end raised in the air, forming what is called the "Mexican hat", and the culmination stage begins.
The prestalk cells and prespore cells switch positions in the culmination stage to form the mature fruiting body. The anterior end of the Mexican hat forms a cellulose tube, which allows the more posterior cells to move up the outside of the tube to the top, and the prestalk cells move down. This rearrangement forms the stalk of the fruiting body made up of the cells from the anterior end of the slug, and the cells from the posterior end of the slug are on the top and now form the spores of the fruiting body. At the end of this 8– to 10-hour process, the mature fruiting body is fully formed. This fruiting body is 1–2 mm tall and is now able to start the entire cycle over again by releasing the mature spores that become myxamoebae.
Sexual reproduction
Although D. discoideum generally reproduces asexually, D. discoideum is still capable of sexual reproduction if certain conditions are met. D. discoideum has three different mating types and studies have identified the sex locus that specifies these three mating types. Type I strains are specified by the gene called MatA, type III strains have two different MatS and MatT genes, and type II strains have three different genes: MatB, MatC, and MatD. Each sex can only mate with the two different sexes and not with its own. By switching out these genes, it was shown that not all genes found in the sex locus are required to cause successful mating. Successful mating occurs when a strain with MatA is paired with one strain that has MatC or MatS, or when one with MatB is paired with one that has MatS. MatD/MatT have no effect on mating compatibility, but it being present in the fused zygote makes macrocyst formation more likely to be successful.When incubated with their bacterial food supply, heterothallic or homothallic sexual development can occur, resulting in the formation of a diploid zygote. Heterothallic mating occurs when two amoebae of different mating types are present in a dark and wet environment, where they can fuse during aggregation to form a giant zygote cell. The giant cell then releases cAMP to attract other cells, then engulfs the other cells cannibalistically in the aggregate. The consumed cells serve to encase the whole aggregate in a thick, cellulose wall to protect it. This is known as a macrocyst. Inside the macrocyst, the giant cell divides first through meiosis, then through mitosis to produce many haploid amoebae that will be released to feed as normal amoebae would. While sexual reproduction is possible, it is very rare to see successful germination of a D. discoideum macrocyst under laboratory conditions. Nevertheless, recombination is widespread within D. discoideum natural populations, indicating that sex is likely an important aspect of their life cycle.
Homothallic D. discoideum strains AC4 and ZA3A are also able to produce macrocysts. Each of these strains express a mating type similar to type III, but with significant divergence. AC4's status as D. discoideum has been questioned, but its SSU rRNA sequence still fall into the range of this species. It is still unclear how these two manage to become homothallic. In addition, type II cells produce homothallic macrocysts after being primed either by a small number of type I cells or by nearby type I cells physically prevented from contacting them. This may be due to the effect of ethelene. Some mutants also generate cyst-like structures on their own. Because these require unnatural conditions, the type IIs are not considered truly homothallic.
Use as a model organism
Because many of its genes are homologous to human genes, yet its life cycle is simple, D. discoideum is commonly used as a model organism. It can be observed at organismic, cellular, and molecular levels primarily because of their restricted number of cell types and behaviors, and their rapid growth. It is used to study cell differentiation, chemotaxis, and apoptosis, which are all normal cellular processes. It is also used to study other aspects of development, including cell sorting, pattern formation, phagocytosis, motility, and signal transduction. These processes and aspects of development are either absent or too difficult to view in other model organisms. D. discoideum is closely related to higher metazoans. It carries similar genes and pathways, making it a good candidate for gene knockout. D. discoideum has also been established as a model organism to investigate the evolution of unicellular organisms to multicellular life-style because of its unique developmental fashion.The cell differentiation process occurs when a cell becomes more specialized to develop into a multicellular organism. Changes in size, shape, metabolic activities, and responsiveness can occur as a result of adjustments in gene expression. Cell diversity and differentiation, in this species, involves decisions made from cell-cell interactions in pathways to either stalk cells or spore cells. These cell fates depend on their environment and pattern formation. Therefore, the organism is an excellent model for studying cell differentiation.
Chemotaxis is defined as a passage of an organism toward or away from a chemical stimulus along a chemical concentration gradient. Certain organisms demonstrate chemotaxis when they move toward a supply of nutrients. In D. discoideum, the amoeba secretes the signal, cAMP, out of the cell, attracting other amoebae to migrate toward the source. Every amoeba moves toward a central amoeba, the one dispensing the greatest amount of cAMP secretions. The secretion of the cAMP is then exhibited by all amoebae and is a call for them to begin aggregation. These chemical emissions and amoeba movement occur every six minutes. The amoebae move toward the concentration gradient for 60 seconds and stop until the next secretion is sent out. This behavior of individual cells tends to cause oscillations in a group of cells, and chemical waves of varying cAMP concentration propagate through the group in spirals.
An elegant set of mathematical equations that reproduces the spirals and the streaming patterns of D. discoideum was discovered by mathematical biologists Thomas Höfer and Martin Boerlijst. Mathematical biologist Cornelis J. Weijer has proven that similar equations can model its movement. The equations of these patterns are mainly influenced by the density of the amoeba population, the rate of the production of cyclic AMP and the sensitivity of individual amoebas to cyclic AMP. The spiraling pattern is formed by amoebas at the centre of a colony who rotate as they send out waves of cyclic AMP.
The use of cAMP as a chemotactic agent is not established in any other organism. In developmental biology, this is one of the comprehensible examples of chemotaxis, which is important for an understanding of human inflammation, arthritis, asthma, lymphocyte trafficking, and axon guidance. Phagocytosis is used in immune surveillance and antigen presentation, while cell-type determination, cell sorting, and pattern formation are basic features of embryogenesis that may be studied with these organisms.
Note, however, that cAMP oscillations may not be necessary for the collective cell migration at multicellular stages. A study has found that cAMP-mediated signaling changes from propagating waves to a steady state at a multicellular stage of D. discoideum.
Thermotaxis is movement along a gradient of temperature. The slugs have been shown to migrate along extremely shallow gradients of only 0.05 °C/cm, but the direction chosen is complicated; it seems to be away from a temperature about 2 °C below the temperature to which they had been acclimated. This complicated behavior has been analyzed by computer modeling of the behavior and the periodic pattern of temperature changes in soil caused by daily changes in air temperature. The conclusion is that the behavior moves slugs a few centimeters below the soil surface up to the surface. This is an amazingly sophisticated behavior by a primitive organism with no apparent sense of gravity.
Apoptosis is a normal part of species development. Apoptosis is necessary for the proper spacing and sculpting of complex organs. Around 20% of cells in D. discoideum altruistically sacrifice themselves in the formation of the mature fruiting body. During the pseudoplasmodium stage of its life cycle, the organism has formed three main types of cells: prestalk, prespore, and anterior-like cells. During culmination, the prestalk cells secrete a cellulose coat and extend as a tube through the grex. As they differentiate, they form vacuoles and enlarge, lifting up the prespore cells. The stalk cells undergo apoptosis and die as the prespore cells are lifted high above the substrate. The prespore cells then become spore cells, each one becoming a new myxamoeba upon dispersal. This is an example of how apoptosis is used in the formation of a reproductive organ, the mature fruiting body.
A recent major contribution from Dictyostelium research has come from new techniques allowing the activity of individual genes to be visualised in living cells. This has shown that transcription occurs in "bursts" or "pulses" rather than following simple probabilistic or continuous behaviour. Bursting transcription now appears to be conserved between bacteria and humans. Another remarkable feature of the organism is that it has sets of DNA repair enzymes found in human cells, which are lacking from many other popular metazoan model systems. Defects in DNA repair lead to devastating human cancers, so the ability to study human repair proteins in a simple tractable model will prove invaluable.