Holozoa: the clade containing animals and their closest single-celled relatives

Composition and characteristics

Holozoans range from complex multicellular animals to diverse unicellular and colonial protists. Typical features that unite the group are inferred from molecular data: shared genes and aspects of cell biology rather than a single obvious morphological trait. Well-known holozoan members include choanoflagellates, filastereans, and groups historically called ichthyosporeans or Mesomycetozoea.

• Animals (Metazoa) — the multicellular members of Holozoa.
• Single-celled relatives such as choanoflagellates and filastereans.
• Groups sometimes studied under different names or older classifications; terminology has changed as phylogenetic methods improved.

Phylogeny and origin

Holozoa is defined as a clade, meaning all organisms descended from a shared common ancestor of animals and their kin. It forms one branch of the larger opisthokont assemblage that also contains fungi. Phylogenomic analyses (large-scale gene comparisons) have clarified relationships inside the clade but some branching orders remain debated.

Importance for understanding animal evolution

Researchers study holozoans to reconstruct steps that led to animal multicellularity. For example, choanoflagellates have a cell form similar to sponge feeding cells and can form colonies; the colonial genus tunicates has historically been confused with other names such as Distaplia in older literature. Fossil evidence is sparse for single-celled relatives, so molecular and cell-biological comparisons are essential.

Notable facts and research directions

Holozoa reveals that many genes associated with multicellularity—adhesion molecules, signaling components, transcription factors—evolved before animals and were repurposed during the origin of Metazoa. Ongoing work combines genomics, cell biology and comparative morphology to map how simple relatives like choanoflagellates and other holozoans contributed to animal innovations. For additional summaries and resources see general entries on related organisms and evolutionary summaries (animals, protists).