Dictyostelium discoideum

Lore & Background

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. In the wild, D. discoideum can be found in soil and moist leaf litter. Its primary diet consists of bacteria, such as Escherichia coli. The life cycle begins when spores are released from a mature sorocarp. Myxamoebae hatch from the spores under warm and moist conditions. During the vegetative stage, myxamoebae divide by mitosis as they feed on bacteria. When the supply of bacteria is depleted, the myxamoebae enter the aggregation stage. During aggregation, starvation initiates the production of glycoproteins and adenylyl cyclase; cyclic AMP is secreted to attract neighboring cells. The migration stage begins once the amoebae have formed a tight aggregate, forming a motile pseudoplasmodium (slug). The slug becomes differentiated into prestalk and prespore cells that move to the anterior and posterior ends, respectively. Anterior-like cells are dispersed throughout the posterior region of the slug and form the very bottom of the fruiting body. After the slug settles, the culmination stage begins. The prestalk cells and prespore cells switch positions to form the mature fruiting body: the stalk is made up of cells from the anterior end of the slug, and the cells from the posterior end form the spores. The mature fruiting body is 1–2 mm tall. D. discoideum is also capable of sexual reproduction. It has three different mating types. Heterothallic mating occurs when two amoebae of different mating types fuse to form a giant zygote cell, which then forms a macrocyst. Inside the macrocyst, the giant cell divides first through meiosis, then through mitosis to produce many haploid amoebae. Homothallic strains AC4 and ZA3A can also produce macrocysts, but it is unclear how they become homothallic.

Reader's Guide

D. discoideum is commonly used as a model organism because many of its genes are homologous to human genes, yet its life cycle is simple. It can be observed at organismic, cellular, and molecular levels. It is used to study cell differentiation, chemotaxis, apoptosis, cell sorting, pattern formation, phagocytosis, motility, and signal transduction. It is also a model to investigate the evolution of unicellular organisms to multicellular life-style.

Did You Know?

• The slug is about 2–4 mm long and composed of up to 100,000 cells.
• The fruiting body is 1–2 mm tall and the entire culmination stage lasts 8–10 hours.
• Anterior-like cells form the very bottom of the fruiting body, not the caps of spores.
• Sexual reproduction is rare under laboratory conditions, but recombination is widespread in natural populations.