Virlets

Virlets are a proposed class of ultra-small, virus-like replicators described in theoretical biology and some science fiction as potential intermediate forms between self-replicating molecules and conventional viruses. The concept is used to explore how minimal genetic systems might evolve into fully fledged viral entities or be replaced by more complex life.

Characteristics commonly attributed to virlets include a minimal genome and a strictly non-cellular lifestyle. Models suggest genomes on the order of hundreds to thousands of bases, encoding only core replication and packaging functions. Virlets are presumed to depend on host cellular machinery or helper factors for replication and assembly, with little to no independent metabolism.

Genetic material in virlet models may be RNA or DNA. Structures are hypothetical and often imagined as enclosed by a host-derived membrane or by a minimal viral capsid borrowed from a helper virus. Some hypotheses allow for non-structural virions that retain replication capability without a conventional protective shell.

Life cycle in these models typically involves attachment to a suitable host, entry and genome replication using host enzymes, and release. Given their presumed reliance on host resources, virlets are described as lacking autonomous energy metabolism and being highly dependent on host cellular processes throughout replication.

Status and debate surround virlets. The idea remains speculative and controversial, with little or no direct empirical evidence of their existence in nature. Proponents use virlets to frame questions about the origins of viruses, genome minimization, and the evolutionary transitions between replicators and infectious agents. They also appear in certain speculative and educational contexts as thought experiments.