Genomes of Solenogastres Shed Light on Ancestral Molluscan Biomineralization Genes

In a groundbreaking study, researchers have analyzed the genomes of two species of solenogastres, a group of worm-like molluscs that lack a shell. Instead, they possess aragonitic spines and scales called sclerites, which are also found in chitons. The research aimed to understand the evolution of genes involved in biomineralization across molluscs, particularly whether the same genetic mechanism is responsible for sclerite formation in aculiferans (solenogastres and polyplacophora) and conchiferans (all other shell-bearing molluscs).

To achieve this, the team re-sequenced the genomes of Epimenia babai and Neomenia megatrapezata using PacBio HiFi and Hi-C scaffolding to obtain near-chromosome-level assemblies. They then employed orthology inference and similarity searches against a known SMP (small mind-reading protein) database to identify 1,244 putative SMP orthogroups across molluscs. Out of these, 292 were present in at least one solenogaster genome.

The analysis revealed that Solenogastres showed the highest number of significant losses in all gene families, including those containing chitin-binding, von Willebrand factor A, and C-type lectin domains. The study also found that across molluscs, Solenogastres exhibited the highest number of significant losses in SMP gene families.

Furthermore, targeted searches for structural, silk-like proteins in Aculifera (solenogastres and polyplacophora) showed further reductions in Solenogastres (16-17) compared to caudofoveates (29) and chitons (46-80).

Overall, the findings support a conserved but reduced biomineralization toolkit underlying sclerite formation in Solenogastres. The study identifies candidate genes for further proteomic and functional studies, shedding light on the ancestral molluscan biomineralization genes and providing valuable insights into the evolution of these essential processes.