Choanoflagellate

A single-celled or colonial eukaryote with a collar of microvilli surrounding a beating flagellum. Choanoflagellates are the closest living unicellular relatives of animals.

Choanocyte: A flagellated collar cell that lines sponges and moves water through their bodies. Its resemblance to choanoflagellates once suggested a direct cellular link between sponges and their relatives.
Eukaryote: An organism whose cells contain a nucleus and other membrane-bound compartments. Choanoflagellates are eukaryotes, with the cellular organization shared by animals.
Animal: A multicellular eukaryote whose development typically includes a blastula stage. Animals are the closest major living group to choanoflagellates.
Salpingoeca rosetta: A marine choanoflagellate that can grow as single cells or form multicellular rosettes. Its experimentally tractable life cycle exposes how environmental cues trigger colony formation.
Animal origins: The evolutionary emergence of animals from their unicellular ancestors. Choanoflagellates are living relatives that help constrain reconstructions of this transition.
Flagellum: A whip-like cellular structure that propels cells or moves fluid along their surfaces. Its beating draws water and suspended bacteria through the surrounding microvillar collar.
Opisthokont: A member of the eukaryotic lineage that includes animals, fungi, and their closest relatives. Choanoflagellates belong to the opisthokonts, placing them near animals on the tree of life.
Holozoa: A eukaryotic group comprising animals and their closest unicellular relatives. Choanoflagellates are one of the principal unicellular lineages within Holozoa.
Rosette-inducing factor: A bacterial sulfonolipid that induces rosette formation in Salpingoeca rosetta. This molecule showed that bacterial signals can control a choanoflagellate's multicellular development.
Choanoflagellate colony formation: The process by which some choanoflagellates form multicellular colonies from individual cells. Its mechanisms may clarify which parts of animal-like development predate animals.

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