Protist

A protist ( PROH-tist) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic group encompassing the entire eukaryote tree of life, from which land plants, animals, and fungi evolved. They are primarily single-celled, exhibiting a wide range of forms such as amoebae, ciliates, thick-walled microalgae and, more commonly, flagellates. Several transitions to multicellularity have occurred among protists, from colonies with alternating cell types to giant slime molds, fungus-like organisms, and seaweeds with differentiated tissues. Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista, or were lumped together as part of the traditional plant and animal kingdoms as algae and protozoa, respectively. With the advent of molecular phylogenetics and electron microscopy studies, some protists were shown to be more closely related to animals or plants than to other protists, and algae were found to be intermixed with protozoa. The classification suffered major revisions, as seemingly unrelated forms were found to be evolutionarily related, and vice versa. In modern classifications, protists are spread across several large clades known as supergroups, many of them containing disparate forms. For example, the Archaeplastida includes mostly phototrophs like red and green algae, from which land plants evolved. Opisthokonta groups fungi, animals, and their single-celled relatives. Amoebozoa and Rhizaria harbor the majority of amoeboid organisms, such as testate amoebae, foraminifers and radiolarians. Stramenopiles and Alveolata are diverse groups of flagellates, many of which have evolved into major parasites (e.g., oomycetes, apicomplexans) or phototrophs (diatoms, brown algae, dinoflagellates). The earliest diverging groups, collectively known as Excavata (e.g., euglenids, metamonads), are flagellates that represent the ancestral traits of the last eukaryotic common ancestor (LECA). Despite the comparatively low number of described species, protists compose the...