Now classified as protists within the supergroup Amoebozoa, these shape-shifting microbes are not true molds or fungi, despite the name and their spore-producing structures. Their life cycle involves both single-celled and complex, temporary multicellular stages. The method of reproduction depends on the type of slime mold, primarily dividing into two distinct groups: the acellular (plasmodial) and the cellular slime molds. Both types rely on the dispersal of microscopic spores to ensure the continuation of their species.
Reproduction in Acellular Slime Molds
The life cycle of acellular slime molds, also known as plasmodial slime molds, is dominated by a diploid, multinucleated feeding stage called the plasmodium. This massive cell is essentially a single, wall-less sack of cytoplasm containing millions of nuclei, and it can grow to cover an area of several square meters while consuming bacteria and organic matter. The plasmodium moves by cytoplasmic streaming, slowly flowing across surfaces like decaying wood or leaf litter as it feeds.
When environmental conditions become unfavorable, typically due to starvation or a decrease in moisture, the plasmodium halts its feeding stage and prepares to reproduce. The entire mass of protoplasm migrates to a drier, often more exposed location, such as the top of a log, which aids in later spore dispersal. The plasmodium then transforms into a collection of stalked, spore-producing structures known as sporangia, or fruiting bodies.
The nuclei within the developing sporangia undergo meiosis, a cell division process that reduces the chromosome number by half, creating haploid nuclei. These haploid nuclei are packaged into thick-walled, uninucleate spores, which are highly resistant to harsh conditions. This transformation is often the end of the parent plasmodium, marking a biological sacrifice for the next generation. The spores are then ready for release into the environment to begin the cycle anew.
Reproduction in Cellular Slime Molds
Cellular slime molds, such as Dictyostelium discoideum, spend their vegetative stage as individual, haploid amoeboid cells that feed on bacteria and divide by mitosis. This unicellular life continues as long as food is abundant in the soil or decaying matter. When the food supply is exhausted, these solitary amoebae enter a remarkable social phase to reproduce.
Starvation is the cue for individual cells to begin releasing a chemical signal, most famously cyclic adenosine monophosphate (cAMP), which acts as an attractant. This signal triggers a collective behavior called chemotaxis, where thousands of surrounding amoebae stream toward the source of the cAMP in a process of aggregation. The aggregated mass forms a temporary, multicellular structure called a pseudoplasmodium, often referred to as a “slug,” because the cells retain their individual cell membranes and identity.
The slug can migrate for hours or days in response to light and temperature, seeking an ideal location for spore dispersal. During this migration, the cells begin a process of differentiation within the slug, specializing into distinct cell types. The cells in the front third of the slug become pre-stalk cells, while the cells in the back two-thirds differentiate into pre-spore cells. The pre-stalk cells form a rigid stalk, lifting the pre-spore cells off the substrate, where they mature into haploid spores within a structure called the sorocarp.
Spore Survival and Germination
The spores produced by both acellular and cellular slime molds are the primary means of survival and dispersal for the species. These microscopic reproductive units are encased in a thick, protective cell wall composed of cellulose, providing exceptional resistance to desiccation and other adverse conditions. The resilience of these spores allows them to remain dormant for extended periods, with some species’ spores capable of surviving for decades before reactivation.
Spores are typically dispersed passively, carried away from the parent organism by wind, water currents, or sometimes by animals. Dispersal to new locations is enhanced by the stalked fruiting bodies, which raise the spore mass above the substrate. Germination occurs when the spore lands in a favorable environment, specifically requiring the return of moisture and the presence of a food source, such as bacteria.
Upon receiving the correct environmental cues, the spore wall ruptures, and the haploid cell emerges, either as a motile, flagellated swarm cell in wet conditions or as a simple amoeboid cell in drier environments. These emergent cells immediately enter the feeding stage, multiplying by mitosis and effectively restarting the vegetative phase of the slime mold life cycle.