The evolution of mollusc shells

Abstract

Molluscan shells are externally fabricated by specialized epithelial cells on the dorsal mantle. Although a conserved set of regulatory genes appears to underlie specification of mantle progenitor cells, the genes that contribute to the formation of the mature shell are incredibly diverse. Recent comparative analyses of mantle transcriptomes and shell proteomes of gastropods and bivalves are consistent with shell diversity being underpinned by a rapidly evolving mantle secretome (suite of genes expressed in the mantle that encode secreted proteins) that is the product of (a) high rates of gene co-option into and loss from the mantle gene regulatory network, and (b) the rapid evolution of coding sequences, particular those encoding repetitive low complexity domains. Outside a few conserved genes, such as carbonic anhydrase, a so-called "biomineralization toolkit" has yet to be discovered. Despite this, a common suite of protein domains, which are often associated with the extracellular matrix and immunity, appear to have been independently and often uniquely co-opted into the mantle secretomes of different species. The evolvability of the mantle secretome provides a molecular explanation for the evolution and diversity of molluscan shells. These genomic processes are likely to underlie the evolution of other animal biominerals, including coral and echinoderm skeletons. This article is categorized under: Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Evolutionary Novelties.

Novel genomic resources for shelled pteropods: a draft genome and target capture probes for Limacina bulimoides, tested for cross-species relevance

“…Targets corresponding to candidate biomineralisation genes and mitochondrial genes were less successfully recovered compared to conserved pteropod orthologues and other coding sequences (Table 4). This could be because biomineralisation-related gene families in molluscs are known to evolve rapidly, with modular proteins composed of repetitive, low complexity domains that are more likely to accumulate mutations due to unequal cross-over and replication slippage [60,61]. Surprisingly, only the COI gene was recovered out of the 10 mitochondrial genes included in the set of probes.…”

Growth and morphogenesis of the gastropod shell

“…shell | mollusc | morphogenesis | gastropod M ollusc shells are a remarkable example of the diversity that can arise as natural selection shapes animal morphology to various ecological and functional demands. Their basic functions are structural support and protection, and shells have adapted for these roles in various ways (1)(2)(3). Soft body parts are attached to internal shell surfaces to maintain the organization of the body, and external structures can be withdrawn into shells for protection from predators or desiccation.…”

A Short Review on Cockle Shells as Biomaterials in the Context of Bone Scaffold Fabrication

“…The phylum Mollusca consist of eight classes under two main divisions, the Conchifera and the Aculifera. The Conchiferas are species with shells including bivalves, cephalopods, gastropods, scaphopods and monoplacophorans, while Aculiferas lacks shells and consists of the polyplacophorans, caudofoveafes and solenogastres (McDougall et al 2018;Vinther et al 2012). Since Aculiferan molluscs have no shells, a considerable number of studies on molluscan biomineralization are focused on the shells of Conchiferan molluscs.…”

“…Molluscan shells are produced by the outer fold of the mantle (Budd et al 2014). Shells are defined as a multilayered structure with composition incorporating calcium carbonate (CaCO 3 ) crystals, proteinaceous material and pigments which vary from taxon to taxon (Felipe 2015;McDougall et al 2018). The thin outermost proteinaceous layer of the shell is known as periostracum, a noncalcified layer which is composed of organic components (Felipe 2015;McDougall et al 2018;Szabo 2008).…”

“…Shells are defined as a multilayered structure with composition incorporating calcium carbonate (CaCO 3 ) crystals, proteinaceous material and pigments which vary from taxon to taxon (Felipe 2015;McDougall et al 2018). The thin outermost proteinaceous layer of the shell is known as periostracum, a noncalcified layer which is composed of organic components (Felipe 2015;McDougall et al 2018;Szabo 2008). It acts as the initial site for shell deposition and has a protective function against shell corrosion in some species (Taylor et al 1969;Williams et al 2016).…”