Acellular Parasites

Some General Characteristics

Acellular parasites—also known as genetic parasites, parasitic genetic elements or selfish genetic elements— are genetic elements that generally provide no direct benefit to their host cell. They tend to behave independently of the other genetic material in the cell, and in a manner that maximizes their own abundance and spread, often to the detriment of the host cell. Acellular parasites consist of DNA or RNA (or in some cases DNA and RNA). Some may be packaged in other materials, such as proteins and lipids. They range from simple nucleotide sequences capable of getting themselves replicated, to highly adapted life forms, such as Escherichia virus T4 (Figure 1).

Acellular parasites can be thought of as genetic elements that lack cellular "homes" of their own. To be biologically active and be able to complete their life cycles they need to reside in host cells. All cells have their own resident genetic material: bacterial cells have bacterial genomes, archaean cells have archaean genomes, and eukaryotic cells have eukaryotic genomes. Cells and their genomes have evolved together as inseparable units. Because acellular parasites lack "homes" of their own, they are always "guests" in others' cells or wanderers (if they are capable of existing outside of cells). Like all guests, acellular parasites incur costs on their hosts, because the hosts must expend energy and resources to sustain their "guests." In some cases the "guests" bring special tools to improve the lives of their hosts or by happy accident they cause changes in the host cell which increase the fitness of the host. In other cases they wreck havoc and destruction on their host or at best act as a slow drain on the host's energy and resources. Some hosts are able to "silence" their acellular parasites so that they have little impact on the host cell. On occasion, acellular parasites may be "domesticated" by their host and become essential components of their host's genome.

Although often labeled as "not alive", some acellular parasites can communicate and coordinate their actions with others of their kind (Duddy and Bassler 2021); some are parasitized by other acellular parasites (La Scola et al. 2008); and some are known to possess an immune system (Levasseur et al. 2016, Brandes and Linial 2019, Chelkha et al. 2021).