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Bikont Slime Molds
The bikont slime molds belong to a broad group of eukaryotes (organisms with complex cells) of uncertain origin. They are only distantly related to the other slime molds and are sometimes called cellular slime molds.
These organisms spend most of their life as independent ameoba-like organisms. In times of stress, individual cells release a signal molecule called cyclic adenosine monophosphate (cAMP), which causes the cells to aggregate into a diffuse mass called a pseudoplasmodium. This mass further aggregates into a slug (an aggregate of individual cells that resembles a garden slug). After moving to a more promising area, a fruiting body forms and develops a stalk with a spherical structure (called a sorocarp) at the end. This releases numerous spores, which spread about the vicinity and germinate, thus returning to the original single-celled state.
Unikont Slime Molds
The unikont slime molds are distant relatives of fungi and animals. Unlike the bikont slime molds, which exist as discrete cells throughout their entire life cycle, the unikont slime molds form an amorphous plasma containing thousands of nuclei as part of their life cycle. Hence, these are often called plasmodial slime molds.
Plasmodial molds begin as single-celled organisms that resembles a flagellate (flagellum-containing) or amoeboid organism. Upon receiving a cellular signal, these cells fuse together as pairs. These fused pairs then swarm together to form a single giant cell containing many nuclei called a plasmodium. Under times of stress such as food shortage, the plasmodium can form stalks capped by a sporing body called a capillitum. Alternately, it can produce numerous hardy microcysts, which can survive times of drout. The capillita release spores, which disperse into the environment and activate as single-celled organisms.
Typical Slime Mold Life Cycle
Slime molds survive by consuming micro-organisms, usually bacteria, in their environment. Plasmodial slime molds can actively feed as single cells or plasmodia (which can take advantage of nutrient-rich areas), whereas the cellular slime molds only feed in their independent single-celled stage. Once the environment is depleted of nutrients, the slime mold aggregates and forms sporulating "fruiting bodies" to disperse progeny to a more promising habitat.
Cellular slime molds germinate (mate) before forming amoeba-like single cells, whereas plasmodial molds can exist as haploid (ungerminated) or diploid (germinated) cells indefinitely before merging into a plasmodium. In both cases, the molds have evolved to rapidly exploit an environment and then disperse once the nutrients have been consumed.
Signaling in Slime Molds
All types of slime molds engage in complex cellular signaling in order to control different aspects of their life cycle. cAMP is the signaling molecule that promotes aggregation; cAMP is used as a signaling molecule in many organisms, including humans.
Once an aggregate is formed, additional signals control the movement of the mold. For instance, the "slug" form of cellular slime molds detects ammonia. Since ammonia signals decomposition (and therefore bacteria), this is a good way for the mold to get close to a food source before dispersing. Complex signaling systems within a plasmodium enable it to make cellular calculations and effectively process environmental information, such as the location of nutrients.