Alternation of generations

Definition

A life cycle in which a sexually reproducing haploid phase (gametophyte) alternates with an asexually reproducing diploid phase (sporophyte), with both phases typically multicellular and morphologically distinct. In animals, the term describes analogous cycles where generations differ in reproductive mode, ploidy, or morphology, such as in parasitic flatworms, cnidarians, and some aphids.

Etymology

Example

In aphids (Aphididae), some species exhibit a modified alternation of generations: asexual parthenogenetic females dominate during summer (the 'asexual generation'), while sexual males and oviparous females appear in autumn to produce cold-resistant eggs, completing the cycle. Parasitic trematodes that infect insects (e.g., some Digenea in aquatic insect larvae) alternate between asexual generations in molluscan intermediate hosts and sexual generations in vertebrate definitive hosts.

Synonyms

• metagenesis
• heterogenesis
• digenean life cycle (specific to trematodes)

Related Terms

• Parthenogenesis
• gametophyte
• sporophyte
• Diploid
• haploid
• complex life cycle
• Polyphenism
• holometabolism
• heterogony

Usage Notes

Strictly applied, the term originates from botany and describes the archegoniate-plant life cycle with distinct multicellular phases. In zoological and entomological contexts, usage is looser: 'alternation of generations' often denotes any cyclic shift between sexual and asexual reproduction, or between morphologically distinct generations, regardless of ploidy changes. Specialists may prefer 'heterogony' for aphids specifically, or 'metagenesis' for cnidarians. The term should not be confused with 'alternation of hosts' (heteroxeny), though the two phenomena frequently co-occur in parasite life cycles.