hostsymbiont

The term host-symbiont, also written hostsymbiont, refers to the relationship between a host organism and one or more symbionts that live in or on the host. Symbionts may be mutualists, providing benefits; commensals, neutral; or parasites, harming the host. The term emphasizes the dependence of the symbiont on the host and the potential effect on host fitness. Interactions can be intracellular (endosymbionts) or extracellular (ectosymbionts) and can range from obligatory to facultative.

Endosymbionts such as mitochondria and plastids originated from ancestral bacteria; many modern endosymbionts include Buchnera in aphids, Wolbachia in insects. Gut microbiota are examples of facultative symbionts that influence digestion and immunity. Modes of transmission: vertical (parent to offspring) and horizontal or environmental acquisition. Types: mutualism (benefits both), commensalism (host unaffected), parasitism (harmful).

The host-symbiont relationship can shape host biology by supplying nutrients, aiding defense, or affecting development; symbiont genomes often undergo reduction due to intracellular lifestyle. Coevolution can lead to highly specialized partnerships and dependency, influencing ecology, evolution, and even speciation. In agriculture and medicine, manipulating symbionts can affect pest control, disease, and host health.

Research relies on metagenomics, sequencing, microscopy, and experimental manipulation. Ethical and ecological considerations include unintended disruption of beneficial communities and risks from releasing modified organisms.