Physarum polycephalum
Slime Mold
Slime mold (Physarum polycephalum) Credit: Photo courtesy of genome.gov (National Human Genome Research Institute)
Habitat
Slime molds are important heterotrophs (cannot synthesize their own food) in the decomposition of organic matter in temperate and tropical forests. Physarum and other acellular slime molds are composed of a syncytial mass of protoplasm (called a plasmodium) with no cell walls in their main vegetative state, although they can take on a variety of different microscopic and macroscopic forms (see Biology section below). Depending on the species, the plasmodium may range from only a few millimeters in diameter to well over 12 inches (30 cm) across. The plasmodium moves like a giant amoeba, flowing over the surface as it ingests dead leaves and wood. The bright yellow Physarum polycephalum commonly studied in general botany courses and research laboratories is typically found in damp, shady areas of temperate forests, although they may move to bright areas to "fruit." Some tropical slime molds are bioluminescent and glow in the dark.
Biology
Physarum polycephalum is a member of the class/superclass Myxogastridae (or myxomycetes) commonly referred to as plasmodial or true slime molds. Although historically classified as fungi, molecular data now clearly show that they are most closely related to the cellular slime molds (Dictyostelidae). Together they form the supergroup Amoebozoa, which also includes amoebae with lobose (broad) pseudopodia (e.g. Acanthamoeba), archamoebae (e.g. Entamoeba), and pelobionts (flagellated amoebae without mitochondria).
The vegetative stage (macroplasmodium) is a large, single cell containing multiple dipoid nuclei that divide precisely at the same time. Macroplasmodia migrate by protoplasmic streaming which reverses every 30-60 s. Plasmodia engulf bacteria, myxomycete amoebae and other microbes. They also secrete enzymes for digesting the engulfed material. Under adverse conditions, plasmodia can reversibly transformed into a dormant hardened mass (sclerotium) that can survive for long periods. Alternatively, starvation of the macroplasmodium in the presence of light induces differentiation into specialized sacs (sporangia), a highly regulated process, which includes the complete conversion of the macroplasmodium into "fruiting bodies", or fructifications. Haploid spores are produced inside the fructification by meiosis (chromosome reduction by 1/2 of that of the diploid cell). The germinated spore can transform into either an amoeba-like myxamoeba cell or a flagellated swarm cell (mxyoflagellate). Myxamoebae can reversibly transform into mxyoflagellates in liquid environments, or form cysts upon drying; cysts can transform into flagellates if the cell wall has not fully developed prior to re-exposure to liquids. Two myxamoeba cells of different mating types may join together in a cellular fusion (mating, plasmogamy) followed by nuclear fusion (karyogamy). This mixing of cellular contents (protoplasts) represents a very primitive form of sexual reproduction, providing a source of genetic variability. The fusion of two haploid cells results in a diploid zygote, which transforms into a developing plasmodium by nuclear division in the absence of cellular division (cytokinesis). In the laboratory, apogamic strains of Physarum have been derived in which haploid amoebae are able to grow into haploid plasmodia without forming a zygote. In these mutant strains, apogamic development is suppressed at temperatures above 30C, but formation of a diploid plasmodium through mating is still possible. The switch between haploid and diploid stages is an important tool for genetic analysis.
Sequencing Summary
The NHGRI has requested a draft assembly of Physarum polycephalum. The Genome Institute has assessed the genomic landscape, and a sequencing plan has been formulated. The National Human Genome Research Institute (NHGRI), National Institutes of Health (NIH) have provided funding for the sequence characterization of the Physarum polycephalum genome.
Contacts
| Name | Affiliation |
|---|---|
| Patrick Minx | The Genome Institute, Washington University School of Medicine |
| Jonatha Gott | Department of Molecular Biology & Microbiology, Case Western Rerserve University |
Sequences & Maps
Assemblies
| Name | Date | Description | Blast DBs |
| Physarum_polycephalum-3.1 | Apr 21, 2008 | 14.3X | contigs supercontigs |
Related Links
Sequence Data
Others
