Choanoflagellatea

Abstract

Choanoflagellates are a group of unicellular and colonial heterotrophic flagellates within the Opisthokonta. The characteristic choanoflagellate cell morphology, present in all species, is an ovoid to round protoplast with one apical flagellum surrounded by a collar of microvilli. Phylogenetic studies indicate that choanoflagellates are the closest known living relatives of animals, which has led to a focus on this group to reconstruct the history of animal evolution. Choanoflagellates display a worldwide distribution from the Arctic to Antarctica, in fresh, marine, and brackish water, and they have also been detected in soil ecosystems. In aquatic habitats they play an essential role in the microbial food web as highly efficient filter feeders. Historically, choanoflagellate taxonomy has been based on morpho- logical features, but recent molecular sequence data have refined the taxonomy and revealed several prominent discrepancies. All choanoflagellates belong to one of two sister groups: The Acanthoecida, which construct a siliceous extracellular structure known as a lorica, and the Craspedida, which lack a lorica but may possess an organic extracellular structure called a theca. The loricate Acanthoecida are comprised of two well-described subgroups, distinguished by characteristic differences in how their loricae are constructed: The Acanthoecidae, or nudiforms, and the Stephanoecidae, or tectiforms. To date, two choanoflagellate genomes have been sequenced, both craspedids: Monosiga brevicollis and Salpingoeca rosetta. The life cycle of S. rosetta has been most thoroughly characterized, with sexual and asexual phases, sessile thecate single cells, slow and fast free-swimming cells with distinct morphologies, and both chain and rosette colony types.