metagenesis

Metagenesis is a biological phenomenon in which an organism's life cycle alternates between two or more generations that are morphologically or functionally distinct. The generations typically alternate between phases that reproduce asexually and sexually, or between life stages that inhabit different hosts. The term is used most often to describe life cycles with two distinct generations, though definitions vary by field.

Classic examples occur in certain cnidarians of the hydrozoan lineage (for instance, Obelia and related species) where a sessile polyp colony reproduces asexually to produce free-swimming medusae, which in turn reproduce sexually to produce new polyps. In other groups, the generations may be more or less morphologically distinct, with different reproductive modes.

In parasitology, metagenesis describes life cycles in which sexual reproduction occurs in one host while an asexual stage occurs in another; many trematodes and cestodes display this pattern, with snails or other invertebrates serving as intermediate hosts that sustain asexual multiplication. Such life cycles can involve alternation between aquatic or terrestrial habitats and different host species, aiding transmission and adaptation.

The concept is related to, but distinct from, the broader idea of alternation of generations found in plants and some algae, where sporophyte and gametophyte generations alternate. In botany, alternation of generations is the standard term, while metagenesis is sometimes used to emphasize life cycles that include distinct, alternate generations and, in some contexts, separate hosts.

Metagenesis thus describes the broad pattern of alternating generations or life stages, with implications for ecology, transmission, and evolution.