Abstract
Molluscs have evolved the capacity to fabricate a wide variety of shells over their 540+ million-year history. While modern sequencing and proteomic technologies continue to expand the catalog of molluscan shell-forming proteins, a complete functional understanding of how any mollusc constructs its shell remains an ambitious goal. This lack of understanding also constrains our understanding of how evolution has generated a plethora of molluscan shell morphologies. Taking advantage of a previous expression atlas for shell-forming genes in Lymnaea stagnalis, I have characterized the spatial expression patterns of seven shell-forming genes in the terrestrial gastropod Cepaea nemoralis, with the aim of comparing and contrasting their expression patterns between the two species. Four of these genes were selected from a previous proteomic screen of the C. nemoralis shell, two were targeted by bioinformatics criteria designed to identify likely shell-forming gene products, and the final one was a clear homolog of a peroxidase sequence in the L. stagnalis dataset. While the spatial expression patterns of all seven C. nemoralis genes could be recognized as falling into distinct zones within the mantle tissue similar to those established in L. stagnalis, some zones have apparently been modified. These similarities and differences hint at a modularity to the molluscan mantle that may provide a mechanistic explanation as to how evolution has efficiently generated a diversity of molluscan shells.
Highlights
Animals fabricate a spectacular variety of biomineralized structures that serve almost all conceivable biological functions
Complementary DNA was synthesized by first combining 1 μg of total RNA with 5 μL of 10 μM oligodT primer in a 10 μL volume and heating to 70◦C for 10 min. To this mixture 5 μL of MMLV-RT buffer, 1 μL of 10 mM dNTPs, 8 μL of nuclease-free water and 1 μL Promega’s MMLV-RT H− point mutant (#M3682) was added, mixed and incubated at 42◦C for 90 min. This cDNA was used as template DNA in PCRs with primers designed to amplify 4 shell-forming genes previously identified in Mann and Jackson (2014), and 2 Glycine-rich shell forming genes, similar to the Shematrin gene family known to play a role in shell formation in oysters (Yano et al, 2006) and an “animal heme dependent peroxidase” gene product that is a likely ortholog to Lstag-sfc-5 that we previously studied in L. stagnalis (Herlitze et al, 2018)
The remaining three genes were selected from an assembly of C. nemoralis transcriptome data (Mann and Jackson, 2014) because they either had features indicative of a role in shell-formation with a distinctive expression pattern or provided a clear example of an ortholog to a shell-forming gene previously spatially characterized in the mantle tissue of L. stagnalis (Herlitze et al, 2018)
Summary
Animals fabricate a spectacular variety of biomineralized structures that serve almost all conceivable biological functions. From a molecular and cellular perspective, a complete understanding of the biomineralization process in any animal model remains elusive. Related to this incomplete functional understanding is a dearth of knowledge regarding the way in which evolution modifies. Plastic Molluscan Mantles the mechanisms of biomineralization to generate structures that fulfill different biological requirements. This is perhaps exemplified no better than within the phylum Mollusca. The long-standing scientific and cultural fascination we have for molluscan shells (Sakalauskaite et al, 2019; Marin, 2020), a plausible and widely accepted hypothesis that can explain how evolution has generated this shelled diversity remains elusive
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