Holozoa

Holozoa is a group of organisms that includes animals and their closest single-celled (protist) relatives, but excludes fungi and all other organisms. It is a monophyletic group or clade, a lineage consisting of all descendants of a common ancestor. Among these descendants, the protists are of high interest because of their close relationship to animals: in the search for the genes responsible for animal multicellularity within these protists, they help elucidate the nature of the unicellular ancestor of animals. Holozoa is the most inclusive clade containing Homo sapiens (a metazoan), but not Neurospora crassa (a fungus). It is a clade with a branch-based definition: it contains all the closest relatives to animals that aren't fungi, as well as their common ancestor. The clade was first discovered through phylogenetic analyses in 2002. These mostly unicellular relatives are the protist lineages of choanoflagellates, filastereans, ichthyosporeans, and three independent species Corallochytrium, Syssomonas and Tunicaraptor. Choanoflagellata (>250 species) are the protists most closely related to animals. They are free-living unicellular or colonial flagellates that feed on bacteria using a characteristic “collar” of microvilli. This collar strongly resembles the collar cells of sponges; because of this, choanoflagellates were theorized to be related to sponges even in the 19th century. The mysterious Proterospongia is an example of a colonial choanoflagellate that was thought to be related to the origin of sponges. The affinities of the other single-celled holozoans only began to be recognized in the 1990s. Ichthyosporea or Mesomycetozoea (~40 species) are mostly parasites or commensals of a wide variety of animals, including humans, fish and marine invertebrates. Most reproduce through multinucleated colonies and disperse as flagellates or amoebae. Filasterea is a group composed by the amoeboid genera Ministeria, Pigoraptor and Capsaspora, united by the structure of their thread-like pseudopods.