Genetic Links in Skeleton Formation of Sponges and Stony Corals

Greg Howard
11th September, 2025

Genetic Links in Skeleton Formation of Sponges and Stony Corals

In situ hybridization in the calcareous sponge Sycon ciliatum reveals that calcarin genes exhibit distinct spatiotemporal expression patterns, with specific variants restricted to spicule founder cells (a–i, k, l) and others to thickener cells (j), demonstrating the complex genetic regulation underlying spicule biomineralization.

Image adapted from: Voigt et al. / CC BY (Source)

Key Findings

  • Sponges, among the earliest animals to build mineral structures, use a toolkit of genes to form skeletal elements called spicules
  • Researchers identified 829 genes active during spicule formation in Sycon ciliatum, including 17 calcarins—proteins similar to those found in coral skeletons
  • Calcarins and coral galaxins show similar genetic arrangements and expression patterns, suggesting a shared evolutionary origin for skeleton formation in these animals
Many animals build hard structures like shells and skeletons through biomineralization, a process where minerals are deposited with the aid of specialized proteins and genes. While biomineralization evolved independently in various animal groups, it often involves repurposing pre-existing genes[1]. Sponges, among the earliest animals to mineralize, form skeletal elements called spicules, but the underlying mechanisms of spicule formation have remained largely unknown. Researchers at Ludwig Maximilians-Universität München (LMU Munich) have now begun to unravel this process in the calcareous sponge Sycon ciliatum. The team focused on Sycon ciliatum because its spicules are produced by a limited number of specialized cells, simplifying the analysis of gene activity during biomineralization. They employed a combination of transcriptomic, genomic, and proteomic techniques to identify genes and proteins upregulated during calcite spicule formation. Transcriptomics reveals which genes are actively being expressed, genomics maps the entire genome, and proteomics identifies the proteins actually present in the cells. They also used in situ hybridization, a method to pinpoint where specific genes are activated within the sponge tissue. The analysis revealed 829 genes exhibiting increased activity in spicule-forming regions. Notably, 17 of these genes coded for proteins named calcarins, which are similar to galaxins found in corals and localized within the spicule matrix, produced by sclerocytes. This finding is significant because corals, particularly stony corals, have a well-defined set of proteins involved in aragonite skeleton formation, often referred to as a “biomineralization toolkit.” The discovery of analogous proteins in sponges suggests a shared evolutionary heritage of biomineralization mechanisms. Previous research has established that the shape of crystals within biological tissues is often dictated by the spaces in which they grow[2]. However, studies have also shown that proteins can actively control crystal shape by interacting with specific crystal faces, reducing growth rates in certain directions[2]. The identification of calcarins in Sycon ciliatum suggests a similar role for proteins in directing spicule formation. Furthermore, the observation that different spicule types exhibit distinct gene activation patterns indicates a fine-tuned level of control over biomineralization. Interestingly, the researchers found evidence of gene duplication and neofunctionalization – where duplicated genes acquire new functions – in Sycon ciliatum. This suggests that the sponge’s biomineralization toolkit has evolved through the modification of existing genes, a process also observed in corals[3]. This parallels the findings in corals, where researchers have identified a suite of proteins crucial for oriented calcium carbonate crystal precipitation, including carbonic anhydrases[3][4]. The presence of carbonic anhydrase CruCA4 in both aragonitic and calcitic coral skeletons[3] highlights the conserved role of this enzyme in biomineralization, despite differences in crystal polymorphs. The study demonstrates striking parallels between the biomineralization processes in the calcitic spicules of sponges and the aragonitic skeletons of corals, despite their independent evolutionary paths. This suggests that similar genetic mechanisms have been co-opted and refined in both groups, highlighting a fundamental evolutionary strategy for animals to build mineralized structures. The findings contribute to a growing understanding of how organisms control mineral formation, potentially informing future research into biomimetic materials and the response of reef-building organisms to environmental change.

GeneticsEcologyMarine Biology

References

Main Study

1) Genetic parallels in biomineralization of the calcareous sponge Sycon ciliatum and stony corals

Published 9th September, 2025

https://doi.org/10.7554/eLife.106239

Related Studies

2) Morphogenesis of calcitic sponge spicules: a role for specialized proteins interacting with growing crystals.

Journal: FASEB journal : official publication of the Federation of American Societies for Experimental Biology, Issue: Vol 9, Issue 2, Feb 1995

3) Comparative Proteomics of Octocoral and Scleractinian Skeletomes and the Evolution of Coral Calcification.

https://doi.org/10.1093/gbe/evaa162

4) Carbonic anhydrases in anthozoan corals-A review.

https://doi.org/10.1016/j.bmc.2012.10.024


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