Spicule formation in calcareous sponges: Coordinated expression of biomineralization genes and spicule-type specific genes

Oliver Voigt,Lukardis Wencker,André Kittelmann,Marcin Adamski,Maja Adamska,Gert Wörheide

doi:10.1038/srep45658

Abstract

The ability to form mineral structures under biological control is widespread among animals. In several species, specific proteins have been shown to be involved in biomineralization, but it is uncertain how they influence the shape of the growing biomineral and the resulting skeleton. Calcareous sponges are the only sponges that form calcitic spicules, which, based on the number of rays (actines) are distinguished in diactines, triactines and tetractines. Each actine is formed by only two cells, called sclerocytes. Little is known about biomineralization proteins in calcareous sponges, other than that specific carbonic anhydrases (CAs) have been identified, and that uncharacterized Asx-rich proteins have been isolated from calcitic spicules. By RNA-Seq and RNA in situ hybridization (ISH), we identified five additional biomineralization genes in Sycon ciliatum: two bicarbonate transporters (BCTs) and three Asx-rich extracellular matrix proteins (ARPs). We show that these biomineralization genes are expressed in a coordinated pattern during spicule formation. Furthermore, two of the ARPs are spicule-type specific for triactines and tetractines (ARP1 or SciTriactinin) or diactines (ARP2 or SciDiactinin). Our results suggest that spicule formation is controlled by defined temporal and spatial expression of spicule-type specific sets of biomineralization genes.

Highlights

Biology and Environment, The Australian National University, Canberra, 46 Sullivans Creek Road, Acton ACT 2601, Australia. 3GeoBio-Center, Ludwig-Maximilians-Universität München, Richard-Wagner-Str. 10, 80333 München, Germany. 4Bayerische Staatssammlung für Paläontologie und Geologie, Richard-Wagner-Str. 10, 80333 München, www.nature.com/scientificreports/
Bicarbonate transporters of the solute carrier 4 (SLC4) family are known to be involved in carbon transport and pH regulation[25], and a specific variant has been shown to be a key biomineralization gene in scleractinian corals[4]
To further interpret the expression patterns in the absence of the calcitic spicules, which dissolve during the in situ hybridisation (ISH) procedure, double ISH was performed with two different colour detections with combinations of probes for the five new genes and the previously studied carbonic anhydrases SciCA1 and SciCA212

Summary

Introduction

Biology and Environment, The Australian National University, Canberra, 46 Sullivans Creek Road, Acton ACT 2601, Australia. 3GeoBio-Center, Ludwig-Maximilians-Universität München, Richard-Wagner-Str. 10, 80333 München, Germany. 4Bayerische Staatssammlung für Paläontologie und Geologie, Richard-Wagner-Str. 10, 80333 München, www.nature.com/scientificreports/. Each spicule is formed by two (diactines), six (triactines) or seven (tetractines) sclerocytes, of which one (termed founder cell) promotes tip growth, and the other, at least in some species, thickens the spicule (the thickener cell)[15,16] (Fig. 1B,C). A previous study found that spicule formation and the expression of two biomineralization genes, the carbonic anhydrases SciCA1 and SciCA2, is increased in the apical part of S. ciliatum sponges, where new radial tubes and the slender diactines of the osculum are built[12]. By RNA-Seq analysis we identified additional key biomineralization genes of calcareous sponges and studied their temporal and spatial expression patterns by RNA in situ hybridisation (ISH) to understand how they interact in the spicule formation process

Methods

Results

Conclusion