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Slug spicules: adding insights from histology and transcriptome analyses to the already complex picture of biomineralization in Gastropoda

World Congress of Malacology Azores, Portugal

Authors/Editors: Jörger K M
Brenzinger B
Schrödl M
Andrade S C S
Giribet G
Riesgo A
Publication Date: 2013
Type of Publication: Congress Contributions and Posters
Molluscan shells are remarkable in their variety of forms and shapes, and appropriately receive the attention of biomineralization research. Beside shells, other biomineralization products occur among gastropods, although yet of unclear evolutionary origins. Among heterobranch gastropods, intra- or extracellular calcareous deposits are especially prominent in lineages of meiofaunal and nudibranch slugs, a morphotype that evolved independently several times. It remains unclear, though, whether these calcareous deposits are homologous to the shell, or whether they develop independently and convergently.

We compare data on the ultrastructure of spicules and the histology of spicule-bearing cells of different (mainly meiofaunal) slug lineages (Acochlidia, Rhodopemorpha, Sacoglossa). Spicules are highly variable in size and shape but all occur intracellularly and are each formed by a single cell. To further investigate the genetic background of biomineralization in slugs, we generated and de novo assembled two adult transcriptomes for different species of Acochlidia, sequencing paired end (150bp) reads using the Illumina Genome Analyzer II. The transcriptomes were screened for a compiled list of proteins previously described as being involved in biomineralization (i.e., shell deposition) in Mollusca. Based on our data, we report the expression of the ‘shell-forming’ genes nacrein and perlustrin in shell-less slugs as well as other proteins directly or indirectly related to the shell deposition.

The distribution of spicules in the investigated taxa, and their intracellular nature and composition, do not support a homology with heterobranch shells and rather present independently evolved products for biomineralization. To date, the lack of a broad comparative dataset on the genetic processes of shell deposition hinders sound interpretation of our transcriptomic data. Nevertheless, some putatively plesiomorphic ‘shell-forming’ genes – e.g., those involved in the formation of the larval shell – might be retained and reactivated later in the evolutionary history for the formation of spicules.