It is not only the sporadic and unpredictable appearance of myxomycete fruiting bodies that is fascinating, but also the fact that at some stages of their life cycle they share characteristics with animals (they move about and feed), while their reproductive structures, like fungi, produce spores.
Fruiting bodies¹ produce spores² from which emerge one to four amoebae. Amoebae are single-celled organisms with a distinct nuclei. Their shape is ever-changing, a result of the expansion and retraction of temporary protrusions on their body. These protrusions, called pseudopodia (Gk pseudes false; podos foot), allow amoebae to creep through the porous spaces in soil and organic material.
The amoebae take one of two forms. They are either myxamoebae³ or swarm cells⁴. (Swarm cells have two thread-like structures—one short, one long—called flagella.) Each form is capable of converting to the other depending on conditions: they are flagellated swarm cells when their surroundings are wet and myxamoebae when they are dry. They feed by engulfing other micro-organisms, principally bacteria.
Myxamoebae and swarm cells do not increase in size but instead divide by binary fission (i.e. the division of one cell into two identical cells), a common method of asexual reproduction in single-celled organisms. Their populations can reach extraordinary numbers of between 10 and 1000 and sometimes more than 10 000 per gram of soil.
Myxamoebae can change to dormant structures called microcysts⁵ if growing and feeding is not possible, either because of lack of food or harsh physical conditions e.g. dehydration. Microcysts are moderately durable cells that can quickly resume feeding when favourable conditions return.
Eventually two compatible myxamoebae from different populations fuse to form a diploid zygote⁶. This involves both the fusion of the protoplasm of the two cells, as well as fusion of their nuclei. At first the resulting zygote is either amoeboid or flagellated, depending on the cells involved in its formation. Flagellated zygotes quickly become amoeboid. The zygote feeds and grows in mass until it ultimately produces a plasmodium⁷. This is accompanied by synchronous nuclear division. If the plasmodium is small it may have several hundred nuclei, if large, the number of nuclei can be in the millions. A plasmodium can revert to a dormant structure called a sclerotium⁸ when conditions are unfavourable.
Variations from the basic life cycle can be common. For example, meiosis may not occur during spore formation resulting in the entire lifecycle being carried out in the diploid state (apomixis). The myxamoebae do not function as gametes but grow, undergo synchronous nuclear division and ultimately produce a plasmodium. The formation of myxamoebae and swarm cells directly from plasmodia has also been observed.