Choanoflagellata

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates having a funnel shaped collar of interconnected microvilli at the base of a flagellum. Choanoflagellates are capable of both asexual and sexual reproduction. They have a distinctive cell morphology characterized by an ovoid or spherical cell body 3–10 µm in diameter with a single apical flagellum surrounded by a collar of 30–40 microvilli (see figure). Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli, where these foodstuffs are engulfed. This feeding provides a critical link within the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals. From Greek Khoanē meaning 'funnel' (due to the shape of the collar) and the Latin word 'flagellum'. Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). Movement of the flagellum draws water through the collar, and bacteria and detritus are captured by the microvilli and ingested. Water currents generated by the flagellum also push free-swimming cells along, as in animal sperm. In contrast, most other flagellates are pulled by their flagella. In addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant. A flagellar basal body sits at the base of the apical flagellum, and a second, non-flagellar basal body rests at a right angle to the flagellar base. The nucleus occupies an apical-to-central position in the cell, and food vacuoles are positioned in the basal region of the cytoplasm. Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracellular matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. Many choanoflagellates build complex basket-shaped 'houses', called lorica, from several silica strips cemented together. The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid attachment to the substrate. In planktonic organisms, there is speculation that the periplast increases drag, thereby counteracting the force generated by the flagellum and increasing feeding efficiency. Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel. Although choanoflagellates are thought to be strictly free-living and heterotrophic, a number of choanoflagellate relatives, such as members of Ichthyosporea or Mesomycetozoa, follow a parasitic or pathogenic lifestyle. The life histories of choanoflagellates are poorly understood. Many species are thought to be solitary; however coloniality seems to have arisen independently several times within the group and colonial species retain a solitary stage. Choanoflagellates grow vegetatively, with many species undergoing longitudinal fission; however, the reproductive life cycle of choanoflagellates remains to be elucidated. A paper released in August 2017 showed that environmental changes, including the presence of certain bacteria, trigger the swarming and subsequent sexual reproduction of choanoflagellates. The ploidy level is unknown; however, the discovery of both retrotransposons and key genes involved in meiosis previously suggested that they used sexual reproduction as part of their life cycle. Some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall. On transfer to fresh media, excystment occurs; though it remains to be directly observed. Further examination of the choanoflagellate life cycle will be informative about mechanisms of colony formation and attributes present before the evolution of animal multicellularity. Evidence for sexual reproduction has been reported in the choanoflagellate species Salpingoeca rosetta. Also evidence has been reported for the presence of conserved meiotic genes in the choanoflagellates Monosiga brevicollis and Monosiga ovata. The choanoflagellates were included in Chrysophyceae until Hibberd, 1975. Recent molecular phylogenetic reconstruction of the internal relationships of choanoflagellates allows the polarization of character evolution within the clade. Large fragments of the nuclear SSU and LSU ribosomal RNA, alpha tubulin, and heat-shock protein 90 coding genes were used to resolve the internal relationships and character polarity within choanoflagellates. Each of the four genes showed similar results independently and analysis of the combined data set (concatenated) along with sequences from other closely related species (animals and fungi) demonstrate that choanoflagellates are strongly supported as monophyletic and confirm their position as the closest known unicellular living relative of animals.