Dictyostelium are Dictyostelid single- and multi-celled eukaryotic, phagotrophic bacterivores usually present and often abundant in terrestrial ecosystems and are a normal component of the microflora that help in soil balance between bacteria and soils. The amoeba are often grouped as slime molds. The order is the Dictyosteliida (Dictyostelid cellular slime molds or social amoebae. ) Dictyosteliida contains organisms that hover on the borderline between uni- and multicellularity. The protists are often found on organic matter or in soils and caves. Typically, cells grow separately and independently but interact to form multi-cellular structures when challenged by adverse conditions such as starvation. Up to 1,000,000 cells signal each other by releasing chemoattractants such as cyclic AMP (cAMP) or glorin, and coalesce by chemotaxis to form an aggregate that becomes surrounded by an extracellular matrix and may move collectively before differentiating into a fruiting body. Basic processes of development such as differential cell sorting, pattern formation, stimulus-induced gene expression, and cell-type regulation are common to Dictyostelium and metazoans. For further detail see family Dictyostelid.
The cellular slime molds were formerly considered to be fungi following their discovery in 1869 by Brefeld. Although they resemble fungi in some respects, they have been included in the Protista kingdom. Individual cells resemble small amoebae in their movement and feeding, and so are referred to as myxamoebae. D. discoideum is the most studied of the genus.
Most of its life, this haploid social amoeba undergoes a vegetative cycle, preying upon bacteria in the soil, and periodically dividing mitotically. When food is scarce, either the sexual cycle or the social cycle begins. Under the social cycle, amoebae aggregate in response to cAMP by the thousands, and form a motile slug, which moves towards light. Ultimately the slug forms a fruiting body in which about 20% of the cells die to lift the remaining cells up to a better place for sporulation and dispersal.
When starved for their bacterial food supply and exposed to dark, moist conditions, heterothallic or homothallic strains can undergo sexual development that results in the formation of a diploid zygote. Heterothallic mating has been best studied in Dictyostelium discoideum and homothallic mating has been best studied in Dictyostelium mucoroides (strain DM7). In the heterothallic sexual cycle, amoebae aggregate in response to cAMP and sex pheromones, and two cells of opposite mating types fuse, and then begin consuming the other attracted cells. Before they are consumed, some of the prey cells form a cellulose wall around the entire group. When cannibalism is complete, the giant diploid cell is a hardy macrocyst which eventually undergoes recombination and meiosis, and hatches hundreds of recombinants. In D. mucoroides (DM7) homothallic mating, cells are directed towards sexual development by ethylene.
Professor John Tyler Bonner has spent a lifetime researching the slime molds and created a number of fascinating videos in the 1940s to show the life cycle; he has mostly studied D. discoideum. In the videos, intelligence appears to be observed as the single cells, after separation, regroup into a cellular mass. The time-lapse film captivated audiences; indeed, Bonner when giving conferences has stated that the film “always stole the show”. The video is available on YouTube.
Taxonomy of D.sp is complicated. It has also been confused by the different forms in the life cycle stages and by the similar Polysphondylium spp. Below are some reported examples.
- Dictyostelium caveatum (Wadell 1982)
- Dictyostelium discoideum
- Dictyostelium iregularis (Olive, Nelson and Stoianovitch 1967)
- Dictyostelium lacteum
- Dictyostelium minutum
- Dictyostelium mucoroides
- Dictyostelium polycephalum
- Dictyostelium purpureum
- Dictyostelium rosarium
- Landolt. C. (2006) Dictyostelid Cellular Slime Molds from Caves. Journal of Cave and Karst studies v. 68 no. 1 pp. 22-26.
- Kessin, R (1944). ISBN 0-521-58364-0.
- O'Day DH, Keszei A (May 2012). "Signalling and sex in the social amoebozoans". Biol Rev Camb Philos Soc 87 (2): 313–29. doi:10.1111/j.1469-185X.2011.00200.x. PMID 21929567.