Cristalografía y ultraestructura de capas lamelares cruzadas en el filo mollusca. Inferencias evolutivas

Résumé

ABSTRACT Mollusc shells are made of between 2 and 5 polycristalline layers. Each layer consists of an aggregate of calcium carbonate crystals embedded in an organic matrix, which is produced by the animal. The different ways in which crystals and organic matrix are arranged to build the layers are called microstructures. There are a wide number of microstructures in mollusc shells, which are recurrent among different species. Crossed lamellar microstructures are the most frequent and they are characterized by aragonite crystalline fibres (3er order lamellae) crossing in at least two main directions. A number of 3er order lamellae dipping in the same direction, group together in bigger units (2nd and 1st order lamellae). For that reason, crossed lamellar microstructures are said to have a hierarchical organization into 1st, 2nd and 3er order lamellae. In such organization, 1st order lamellae constitute parallel and more or less irregular blocks, and inside two adjacent 1st order lamellae there is a countles number of fibres or 3er order lamellae dipping in opposite directions, and arranged into sheets of fibres (2nd order lamellae). This pattern is repeated along the structure. In the 19th century, 1st and 2nd order lamellae morphology has been used as a criteria to classify crossed lamellar microstructures into a range of categories. Such classifications are still in use; despite they do not consider another essential aspect in polycrystalline materials, which is their crystallographic texture. The crystallographic texture refers to the crystallographic axes orientations in the structure and their relationships, and it has been supposed to be very homogeneous within the crossed lamellar microstructures, and then very poorly studied. However, later studies based on X-ray diffraction, have revealed awide diversity in the crystallographic texture of the crossed lamellar microstructures. These studies also proposed that the crystallographic texture can be a reliable phylogenetic tool. Despite the textural variability, the crystallographic studies on crossed lamellar microstructures are scarce today and focused on few species. In this thesis a crystallographic study on 50 crossed-lamellar layers within 47 species is performed. The studied species have been chosen with plylogenetic criteria, in order to include as many clades as possible within the Phylum Mollusca. The techniques used are X-ray diffraction (XRD) and electron backscattered diffraction (EBSD), combined with high resolution electron microscopy [Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM)]. The results show that pole figures can be grouped into ten different textural patterns depending on the maxima distribution and relationships. These ten patterns are recurrent within the studied species and they can be classifed into five groups of crystallographic models. Some of them may derive from others. There are basically two main categories: a/b/1 models where the crystal orientations share a common c axis and a/b/2 models, with two c axes. In the latter, each c axis is contained in one of the two sets of 1st order lamellae composed of 3er order lamellae dipping in the same direction. a/b/2 models seem to derive from a/b/1 models by the splitting and rotation of the crystallographic orientations within the 1st order lamellae plane around an a- or b- common axis. a/b/2 models are completely unknown crystallographic relationships so far, either for inorganic and organic aragonite and thus probably a consequence of the action of some protein pools. Between the four classes having crossed lamellar microstructures, (Polyplacophora, Scaphopoda, Bivalvia and Gastropoda), gastropods present the highest textural diversity, having nine out of the ten patterns recognized. There are some gastropods where the textures differ from one layer to another (Littorinimorpha), and others where it remains constant (Heterobranchia y Neogastropoda). Surprisingly, the textural pattern present in neogastropods is the most widespread in bivalves, while the pattern recognized in Heterobranchia is completely absent in bivalves. In one of the two analysed layers in Fissidentalium metivieri (Scaphopoda), the result is the same as in the family Fissurellidae (Vetigastropoda), while the other layer has the texture typical of Vetigastropoda (Trochoidea, Phasianelloidea) and bivalves (Limoida, Veneroida). Those coincidences in the crossed lamellar crystallography between so distant groups haves to be explained by evolutionary convergence. On the contrary, within each class, at the level of order or family, there are some crystallographic relationships which may define particular clades. The results presented in this thesis will constitute a reference to classify the different types of crossed lamellar microstructures in the Phylum Mollusca and will help to solve some uncertain kinship relationships. Furthermore, these results may help to understand the evolutionary mechanisms of these microstructures where the role of the organic matrix is a key point. A research line worthy to be continued is opened here.