Heterolobosea

Abstract

Heterolobosea is a group of ~150 described species of heterotrophs, almost all free living. Many are "amoeboflagellates" with a three-phase asexual life cycle, centered on trophic amoebae that can reversibly transform into flagellates (some of which undertake phagocytosis and/or division) and cysts. The amoebae are usually lobose, with "eruptive" pseudopodia. Flagellates typically have two or four near-parallel flagella and, if phagocytic, a feeding groove and/or elongate cytostome. Some taxa have simpler lifecycles, for example, Vahlkampfia spp. apparently lack flagellates, while Percolomonas and Stephanopogon lack amoebae. Stephanopogon, uniquely, has numerous flagella in rows like the kineties of ciliates. Acrasids, meanwhile, are terrestrial "slime molds" in which amoebae aggregate to form stalked fruiting bodies. The mitochondria are often enveloped in endoplasmic reticulum and usually have discoidal cristae, while the Golgi apparatus lacks dictyosomal stacking. Most flagellates have a "doubled" flagellar apparatus with two sub-identical halves. The flagellar apparatus typically includes a large "R2" microtubular root (or two) and a striated rhizoplast. Most heteroloboseans are marine, freshwater, or terrestrial aerobes, but the group shows considerable ecological breadth, for example, Psalteriomonadidae and Creneis are anaerobes, three groups are (mostly) obligate halophiles, and many species are thermophiles. The best-known genus is Naegleria. Naegleria gruberi is a cell biology model (e.g., for flagellar apparatus develop- ment). Naegleria fowleri is a facultative human pathogen responsible for primary amoebic meningoencephalitis (PAM). This infection is usually acquired from warm water via the nasal passages; it is extremely rare but almost always fatal.