Molluscan shell evolution with review of shell calcification hypothesis

Spikey bivalves: intra-periostracal crystal growth in anomalodesmatans

The external shell surfaces of most anomalodesmatan bivalves are studded with small spikes, particularly at the posterior end. We have studied the morphology, mode of growth, and distribution among taxa of these spikes. In this study we found that spikes vary widely in morphology, from acute spikes to flat plaques. Optical and electron microscopy has revealed that the periostraca of Laternula, Myadora, and Thraciopsis consist of an outer dense layer and an inner translucent layer. The dense layer grows at the expense of the inner layer as it progresses toward the shell edge. The spikes begin to grow in the free periostracum, within the translucent periostracal layer, immediately below the dense layer. With growth, they push the dense periostracal layer upward but without penetrating it. Those parts of the spike in contact with this layer cease to grow, which explains the typical conical shape of spikes. When fully grown, spikes reach the base of the translucent layer, becoming incorporated into the outer shell layer. Scanning electron microscopy and electron backscatter diffraction analysis reveal that the spikes of Lyonsia norwegica and Lyonsiella abyssicola are prisms of aragonite composed of twinned crystals, with the c-axis vertical. A survey of the occurrence of spikes within the anomalodesmatans shows that they are present in all but a few families. Elsewhere within the closely related palaeoheterodonts, intra-periostracal calcification is also known in Neotrigonia and unionids, which indicates that this character may be plesiomorphic for these bivalves. The present data do not support the homology of spikes in other bivalve groups (e.g., veneroids) or in the aplacophorans or polyplacophorans.

Transcriptome and proteome analysis of Pinctada margaritifera calcifying mantle and shell: focus on biomineralization

BACKGROUND

The shell of the pearl-producing bivalve Pinctada margaritifera is composed of an organic cell-free matrix that plays a key role in the dynamic process of biologically-controlled biomineralization. In order to increase genomic resources and identify shell matrix proteins implicated in biomineralization in P. margaritifera, high-throughput Expressed Sequence Tag (EST) pyrosequencing was undertaken on the calcifying mantle, combined with a proteomic analysis of the shell.

RESULTS

We report the functional analysis of 276 738 sequences, leading to the constitution of an unprecedented catalog of 82 P. margaritifera biomineralization-related mantle protein sequences. Components of the current "chitin-silk fibroin gel-acidic macromolecule" model of biomineralization processes were found, in particular a homolog of a biomineralization protein (Pif-177) recently discovered in P. fucata. Among these sequences, we could show the localization of two other biomineralization protein transcripts, pmarg-aspein and pmarg-pearlin, in two distinct areas of the outer mantle epithelium, suggesting their implication in calcite and aragonite formation. Finally, by combining the EST approach with a proteomic mass spectrometry analysis of proteins isolated from the P. margaritifera shell organic matrix, we demonstrated the presence of 30 sequences containing almost all of the shell proteins that have been previously described from shell matrix protein analyses of the Pinctada genus. The integration of these two methods allowed the global composition of biomineralizing tissue and calcified structures to be examined in tandem for the first time.

CONCLUSIONS

This EST study made on the calcifying tissue of P. margaritifera is the first description of pyrosequencing on a pearl-producing bivalve species. Our results provide direct evidence that our EST data set covers most of the diversity of the matrix protein of P. margaritifera shell, but also that the mantle transcripts encode proteins present in P. margaritifera shell, hence demonstrating their implication in shell formation. Combining transcriptomic and proteomic approaches is therefore a powerful way to identify proteins involved in biomineralization. Data generated in this study supply the most comprehensive list of biomineralization-related sequences presently available among protostomian species, and represent a major breakthrough in the field of molluskan biomineralization.

Primitive soft-bodied cephalopods from the Cambrian

The exquisite preservation of soft-bodied animals in Burgess Shale-type deposits provides important clues into the early evolution of body plans that emerged during the Cambrian explosion. Until now, such deposits have remained silent regarding the early evolution of extant molluscan lineages-in particular the cephalopods. Nautiloids, traditionally considered basal within the cephalopods, are generally depicted as evolving from a creeping Cambrian ancestor whose dorsal shell afforded protection and buoyancy. Although nautiloid-like shells occur from the Late Cambrian onwards, the fossil record provides little constraint on this model, or indeed on the early evolution of cephalopods. Here, we reinterpret the problematic Middle Cambrian animal Nectocaris pteryx as a primitive (that is, stem-group), non-mineralized cephalopod, based on new material from the Burgess Shale. Together with Nectocaris, the problematic Lower Cambrian taxa Petalilium and (probably) Vetustovermis form a distinctive clade, Nectocarididae, characterized by an open axial cavity with paired gills, wide lateral fins, a single pair of long, prehensile tentacles, a pair of non-faceted eyes on short stalks, and a large, flexible anterior funnel. This clade extends the cephalopods' fossil record by over 30 million years, and indicates that primitive cephalopods lacked a mineralized shell, were hyperbenthic, and were presumably carnivorous. The presence of a funnel suggests that jet propulsion evolved in cephalopods before the acquisition of a shell. The explosive diversification of mineralized cephalopods in the Ordovician may have an understated Cambrian 'fuse'.

Application of spectroscopic methods for structural analysis of chitin and chitosan

Chitin, the second most important natural polymer in the world, and its N-deacetylated derivative chitosan, have been identified as versatile biopolymers for a broad range of applications in medicine, agriculture and the food industry. Two of the main reasons for this are firstly the unique chemical, physicochemical and biological properties of chitin and chitosan, and secondly the unlimited supply of raw materials for their production. These polymers exhibit widely differing physicochemical properties depending on the chitin source and the conditions of chitosan production. The presence of reactive functional groups as well as the polysaccharide nature of these biopolymers enables them to undergo diverse chemical modifications. A complete chemical and physicochemical characterization of chitin, chitosan and their derivatives is not possible without using spectroscopic techniques. This review focuses on the application of spectroscopic methods for the structural analysis of these compounds.