Opisthokont

Lore & Background

The close relationship between animals and fungi was suggested by Thomas Cavalier-Smith in 1987, who used the informal name opisthokonta. This was supported by later genetic studies. Early phylogenies placed fungi near plants, but more recent work shows that holozoa (animals) and holomycota (fungi) are much more closely related to each other than either is to plants, partly because opisthokonts have a triple fusion of carbamoyl phosphate synthetase, dihydroorotase, and aspartate carbamoyltransferase not present in plants. Animals and fungi are also more closely related to amoebas than to plants, and plants are more closely related to the SAR supergroup of protists. Cavalier-Smith and Alexandra Stechmann argued that uniciliate eukaryotes such as opisthokonts and Amoebozoa, collectively called unikonts, split off from biciliate eukaryotes (bikonts) shortly after they evolved.

Reader's Guide

Opisthokonts are significant because they encompass two of the most familiar and diverse kingdoms of life: animals and fungi. Their defining feature—a single posterior flagellum in flagellate cells—distinguishes them from other eukaryotes, which typically have one or more anterior flagella. This group also shares key biosynthetic and metabolic pathways, such as the synthesis of extracellular chitin and osmotrophic absorption of nutrients. The opisthokont clade is divided into Holomycota (fungus-like organisms, including fungi, Fonticula, and Nucleariida) and Holozoa (animal-like organisms, including animals, choanoflagellates, and Mesomycetozoea). No opisthokonts basal to this split have been identified as of 2020. The phylogeny of opisthokonts has been largely resolved using phylogenomics, though some placements remain uncertain, such as that of Tunicaraptor. Understanding opisthokonts helps clarify the evolutionary relationships between animals, fungi, and other eukaryotes, and highlights the shared ancestry of heterotrophic, often sessile, organisms.

Did You Know?

• The name opisthokont comes from Ancient Greek for 'rear, posterior' and 'pole, flagellum'.
• Flagellate cells in opisthokonts, such as animal sperm and chytrid fungal spores, propel themselves with a single posterior flagellum.
• Opisthokonts have a triple fusion of carbamoyl phosphate synthetase, dihydroorotase, and aspartate carbamoyltransferase not present in plants.
• Early opisthokonts were likely amoeboid and phagotrophic.