A new genus, Helgoeca gen. nov., for a nudiform choanoflagellate

Proc. Natl. Acad. Sci. U.S.A.

Hassan

et al. 2008

193

173

Choanoflagellates are single-celled aquatic flagellates with a unique morphology consisting of a cell with a single flagellum surrounded by a ''collar'' of microvilli. They have long interested evolutionary biologists because of their striking resemblance to the collared cells (choanocytes) of sponges. Molecular phylogeny has confirmed a close relationship between choanoflagellates and Metazoa, and the first choanoflagellate genome sequence has recently been published. However, molecular phylogenetic studies within choanoflagellates are still extremely limited. Thus, little is known about choanoflagellate evolution or the exact nature of the relationship between choanoflagellates and Metazoa. We have sequenced four genes from a broad sampling of the morphological diversity of choanoflagellates including most species currently available in culture. Phylogenetic analyses of these sequences, alone and in combination, reject much of the traditional taxonomy of the group. The molecular data also strongly support choanoflagellate monophyly rejecting proposals that Metazoa were derived from a true choanoflagellate ancestor. Mapping of a complementary matrix of morphological and ecological traits onto the phylogeny allows a reinterpretation of choanoflagellate character evolution and predicts the nature of their last common ancestor.evolution ͉ morphology ͉ holozoa ͉ animals ͉ protists

Section: Methodsmentioning

confidence: 99%

“…In nudiform taxa, the motile daughter cell swims away, becomes sedentary, accumulates a complete set of strips, and then produces a lorica (Fig. 1E) (22,25). In tectiform species, the daughter cell (the juvenile) inverts and exits the parent lorica backwards, picking up a complete set of newly synthesized costal strips as it leaves (Fig.…”

mentioning

confidence: 99%

Molecular phylogeny of choanoflagellates, the sister group to Metazoa

Carr

Proc. Natl. Acad. Sci. U.S.A.

Hassan

et al. 2008

193

173

“…The similarity between the two species extends even to minor details such as the clawed anterior ends of the longitudinal costae (Leadbeater et al 2008a). There is a 1 : 1 ratio between helical and longitudinal costae giving the familiar joins between the anterior end of the helical costae and the respective longitudinal costae (figure 4b(iii)(iv) arrowheads).…”

Section: Acanthoeca Spectabilis Ellis ( Figure 3 D(iv))mentioning

confidence: 99%

Three-dimensional images of choanoflagellate loricae

Kent

et al. 2008

Proc. R. Soc. B.

Choanoflagellates are unicellular filter-feeding protozoa distributed universally in aquatic habitats. Cells are ovoid in shape with a single anterior flagellum encircled by a funnel-shaped collar of microvilli. Movement of the flagellum creates water currents from which food particles are entrapped on the outer surface of the collar and ingested by pseudopodia. One group of marine choanoflagellates has evolved an elaborate basket-like exoskeleton, the lorica, comprising two layers of siliceous costae made up of costal strips. A computer graphic model has been developed for generating three-dimensional images of choanoflagellate loricae based on a universal set of 'rules' derived from electron microscopical observations. This model has proved seminal in understanding how complex costal patterns can be assembled in a single continuous movement. The lorica, which provides a rigid framework around the cell, is multifunctional. It resists the locomotory forces generated by flagellar movement, directs and enhances water flow over the collar and, for planktonic species, contributes towards maintaining cells in suspension.Since the functional morphology of choanoflagellate cells is so effective and has been highly conserved within the group, the ecological and evolutionary radiation of choanoflagellates is almost entirely dependent on the ability of the external coverings, particularly the lorica, to diversify.

“…The outer layer comprises longitudinal costae and the inner layer of costae is arranged helically or in transverse rings. Recent ultrastructural work on three nudiform species, Savillea micropora, Helgoeca nana, and Acanthoeca spectabilis, all of which have a prominent layer of left-handed helical costae, has led to the conclusion that a rotational movement must occur during lorica assembly (Leadbeater 2008;Leadbeater, Henouil, and Berovic 2008a;Leadbeater et al 2008bLeadbeater et al , 2009.…”

mentioning

confidence: 99%

“…As with the logistical aspects of lorica production, there appears to be a set of universal ''rules'' that govern the movements necessary to create a lorica from bundles of costal strips on a juvenile cell (Leadbeater et al 2008b). In particular, two movements must be generated: (i) a forwardly directed movement and (ii) a left-handed (clockwise) rotation.…”

mentioning

confidence: 99%

Role of the Cytoskeleton in Choanoflagellate Lorica Assembly

Frösler

J Eukaryotic Microbiology

2009

Cell division in Acanthoeca spectabilis produces a "naked" motile daughter cell (juvenile) that settles onto a surface and deposits siliceous costal strips that are stored extracellularly in bundles. When complete, the bundles of strips are assembled in a single continuous movement to form a basket-like lorica. Assembly can be divided into four overlapping stages. Stage 1 entails the left-handed rotation of strips at the anterior end while the posterior end remains stationary. Stage 2 includes the posterior protrusion of the cell to form a stalk. Stage 3 involves the anterior extension of the spines, and Stage 4 the dilation of the lorica chamber and deposition of the organic investment. Scanning electron microscopic images reveal a one-to-one association between the moving bundles of strips and the anterior ring of lorica-assembling tentacles. Treatment with microtubule inhibitors produces "dwarf" cells that lack stalks, have their spines extended, and possess collars but lack flagella. Treatment with microfilament (actin) inhibitors prevents extension of the anterior spines. These experiments demonstrate that posterior cell extension is primarily mediated by microtubules whereas extension of the spines is controlled by the actin cytoskeleton. The processes of cytoskeletal rotation and extracellular costal strip movement are compared, respectively, with rotation of nuclei in animal embryos and movement of mammalian cells over surfaces.