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Many corals and sponges form skeletons that support and shape their bodies. Whereas biomineralization—the formation of these skeletons—has been intensively studied in corals, the main ecosystem engineers of today's hyperdiverse coral reefs, the molecular mechanisms of the process had not been fully clarified in the also ecologically important marine sponges.

A team led by LMU geobiologist Professor Gert Wörheide has now investigated the genetic basis of skeleton formation in calcareous sponges—organisms whose skeletons consist entirely of microscopic calcite structures called spicules—and identified a group of proteins that play an important role in biomineralization. The study is in the journal eLife.

"As sponges are among the oldest animals with mineralized structures, they offer valuable insights into the early evolution of biomineralization," says Dr. Oliver Voigt, lead author of the study.

Calcareous sponges are particular in that their spicules come in various shapes and are formed by just a few specialized cells—sometimes only two. In contrast, skeleton formation is more complicated in other animals—in stony corals, for example, a whole cell layer is involved. This makes sponges a particularly interesting model for investigating the cellular foundations of biomineralization.

In an integrative approach combining various genetic methods with proteomic analyses, the researchers were able to fully decipher for the first time the calcite spicule formation process in the sponge species Sycon ciliatum at the cellular and molecular levels.

In the process, they discovered so-called calcarins, a family of proteins that plays an important role in calcification. Calcarins have structural and functional similarities with galaxins, proteins involved in the formation of the calcareous skeleton in stony corals.

"These results suggest that sponges and corals independently developed comparable genetic tools for biomineralization—a striking example of convergent evolution," explains Voigt.