Choanoflagellate

Choanoflagellates
Scientific classification
Domain:	Eukaryota
(unranked):	Opisthokonta
Phylum:	Choanozoa
Class:	Choanoflagellatea

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. As the name suggests, choanoflagellates (collared flagellates) 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 the 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 one of the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.

Appearance and Growth

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). The flagellum draws water through the collar, and bacteria and detrital particles are captured by the microvilli and ingested. The flagellum also pushes free-swimming cells along, as in animal sperm — most other flagellates are pulled by their flagella. Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.

n addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant (Leadbeater and Thomsen, 2000). 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 (Leadbeater and Thomsen, 2000; Karpov and Leadbeater, 1998). Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracelluar matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid in 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 (Leadbeater and Kelly, 2001).

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 (Leadbeater, 1983). 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 (Mendoza, 2002). 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 (Leadbeater, 1983).

Choanoflagellates grow vegetatively, with many species undergoing longitudinal fission (Karpov and Leadbeater, 1998); however, the reproductive life cycle of choanoflagellates remains to be elucidated. Currently, it is unclear whether there is a sexual phase to the choanoflagellate life cycle. Interestingly, some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall. Upon transfer to fresh media excystment occurs, though it remains to be directly observed (Leadbeater and Karpov, 2000). Further examination of the choanoflagellate life cycle will be informative about mechanisms of colony formation and attributes present before the transition to multicellularity.

Choanoflagellates resemble the individual choanocyte cells of sponges:

Colonial behaviour

A number of species such as those in the genus Proterospongia form simple colonies, planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.

File:Sphaeroeca wiki.jpg

Sphaeroeca, a colony of choanoflagellates (aprox. 230 individuals)

Ecology

There are over 125 extant species of choanoflagellates. distributed globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water (Thomsen, 1982) and 100 m under Antarctic ice sheets (Buck and Garrison, 1988). Many species are hypothesized to be cosmopolitan on a global scale [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions (Thomsen, et al., 1991). Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.

Comparison and relationship with other taxa

The choanocytes (also known as "collared cells") of sponges (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms.

Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.

The last common ancestor of animals and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the last common ancestor of all animals was a relatively complex multicellular organism, with differentiated tissues, a definite "body plan", and complex embryonic development (including gastrulation). The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >600 million years ago.

The genome of Monosiga brevicollis, with 41.6 million base pairs, is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.

External links

• Tree of Life Webpage for Choanoflagellates
• Choanobase, the Choanoflagellate genetic library, developed and maintained by the Nicole King laboratory at the University of California, Berkeley

References

• Snell, EA, Furlong, RF, and PWH Holland. 2001. Hsp70 sequences indicate that choanoflagellates are closely related to animals. Current Biology. 11:967-970.
• King, N., and S. B. Carroll. 2001. A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution. PNAS 98:15032-7.
• Lang, B. F., C. O'Kelly, T. Nerad, M. W. Gray, and G. Burger. 2002. The closest unicellular relatives of animals. Curr Biol 12:1773-8.
• Philippe, H, Snell, EA, Bapteste, E, Lopez, P, Holland, PWH, and D Casane. 2004. Phylogenomics of eukaryotes: the impact of missing data on alignments. Molecular Biology and Evolution. 21(9):123-135.