The opisthokonts are a broad group of eukaryotes, including both the animals and fungi, together with a few sorts of protists. That these form a monophyletic group is strongly supported by both genetic and ultrastructural studies. One common characteristic is that flagellate cells, such as most animal sperm and chytrid spores, propel themselves with a single posterior flagellum. This gives the groups its name. In contrast, flagellate cells in other eukaryote groups propel themselves with one or more anterior flagella.

Opisthokont groups that are placed among the Protista include:

• Choanoflagellates
• Corallochytrids
• Ichthyosporea
• Nucleariids

These may be grouped as a single phylum, necessarily paraphyletic, called the Choanozoa or Mesomycetozoa. Most appear closer to the animals than to the fungi, and they are of great interest to biologists studying animal origins. The chytrids have also been included among the Protista, but are now more often placed among the Fungi.

The close relationship between animals and fungi was suggested by Cavalier-Smith in 1987, who used the informal name opisthokonta (the formal name has been used for the chytrids), and was confirmed by later genetic studies. Early phylogenies placed them near the plants and other groups that have mitochondria with flat cristae, but this character varies. Cavalier-Smith and Stechmann argue that the opisthokonts and possibly Amoebozoa split off from the other eukaryotes, called anterokonts or bikonts, shortly after they evolved.

References

• Cavalier-Smith, T. (1987). The origin of fungi and pseudofungi. In A.D.M. Rayer et al. eds. Evolutionary biology of Fungi, p. 339-353.
• Wainwright P.O. et al. (1993). Monophyletic origins of the metazoa: an evolutionary link with fungi. Science 260: 340-342.
• Stechmann, A & T Cavalier-Smith (2002). Rooting the eukaryote tree by using a derived gene fusion. Science 297: 89–91.