Biosilicification of loricate choanoflagellate: organic composition of the nanotubular siliceous costal strips of Stephanoeca diplocostata

Abstract

SUMMARYLoricate choanoflagellates (unicellular, eukaryotic flagellates; phylum Choanozoa) synthesize a basket-like siliceous lorica reinforced by costal strips (diameter of approximately 100nm and length of 3m). In the present study, the composition of these siliceous costal strips is described, using Stephanoeca diplocostata as a model. Analyses by energy-dispersive X-ray spectroscopy (EDX), coupled with transmission electron microscopy (TEM), indicate that the costal strips comprise inorganic and organic components. The organic, proteinaceous scaffold contained one major polypeptide of mass 14kDa that reacted with wheat germ agglutinin. Polyclonal antibodies were raised that allowed mapping of the proteinaceous scaffold, the (glyco)proteins, within the costal strips. Subsequent in vitro studies revealed that the organic scaffold of the costal strips stimulates polycondensation of ortho-silicic acid in a concentration-and pH-dependent way. Taken together, the data gathered indicate that the siliceous costal strips are formed around a proteinaceous scaffold that supports and maintains biosilicification. A scheme is given that outlines that the organic template guides both the axial and the lateral growth of the strips.

The Evolution of Silicon Transport in Eukaryotes

“…It should be noted that under this hypothesis, Si transport has ancient and deep origins and may be ancestral in eukaryotes; however, biosilicification is not. From molecular comparisons of silica polymerization mechanisms, it does appear that biosilicification has arisen independently in multiple different lineages (Sumper and Kroger 2004; Matsunaga et al 2007; Gong et al 2010; Shimizu et al 2015; Durak et al 2016). …”

A family of diatom-like silicon transporters in the siliceous loricate choanoflagellates

“…Bulk organic analysis of the costal strips from S. diplocostata yielded a glycoprotein that has been proposed to be involved in costal strip silicification [69]. Multiple glycoprotein-encoding genes were identifiable in the S. diplocostata transcriptome dataset; however, no obvious candidates for a costal strip-specific glycoprotein could be identified (data not shown).…”

“…protection, [72]), so once a foreign SIT gene was incorporated into the genome it would confer a strong selective advantage for that organism. Several common biomolecules, such as collagen [73], glycoproteins [69], polyamines [2] and proteases [74] are known to have the capacity to direct silica polymerization. It may be the case that many taxa possess the capacity for silicification but lack the means to concentrate sufficient silicon for polymerization to occur.…”

Competition between Silicifiers and Non-silicifiers in the Past and Present Ocean and Its Evolutionary Impacts

“…Repeatedly we can observe the evolution of similar morphologies in distantly related taxa, such as micropores in the siliceous components of choanoflagellates (Leadbeater, 2015), chrysophytes (Sandgren et al, 1996), diatoms (Finkel and Kotrc, 2010), and haptophytes (Yoshida et al, 2006); spines and spicules in radiolarians (Kunitomo et al, 2006), dictyochophytes (Preisig, 1994), centrohelids (Zlatogursky, 2016), and sponges (Weaver et al, 2007); or tablets and scales in haptophytes (Yoshida et al, 2006), rhizarians (Nomura and Ishida, 2016), synurophytes (Sandgren et al, 1996), amoebozoans (Lahr et al, 2013), and brachiopods (Williams et al, 2001). Though the genes governing the production of these silica patterns are not fully understood, many parallels with the molecular biology of diatom silicification are emerging, such as a role for glycoproteins in choanoflagellates (Gong et al, 2010) and synurophytes (Ludwig et al, 1996), cytoskeleton-mediated shaping of the growing silica structure in multiple taxa (Leadbeater, 2015;Nomura and Ishida, 2016) and the presence of post-translationally modified LCPAs in haptophyte (Durak et al, 2016) and sponge (Matsunaga et al, 2007) silica. These polymerization mechanisms have apparently evolved independently from those in diatoms, suggesting repeated recruitment of similar molecules for silica formation and patterning, and therefore a similar role for silicification-related evolutionary competition and speciation as diatoms.…”