How to build a bivalve: Lessons from gene expression and single-cell RNA sequencing analyses


David Salamanca

PhD Student
Advisor: Andreas Wanninger

Unit for Integrative Zoology, Department of Evolutionary Biology
university of Vienna


Mollusks are known for their highly diverse repertoire of body plans especially in the Conchifera, one of the two major lineages that comprises those taxa that originated from a uni-shelled ancestor (Monoplacophora, Gastropoda, Cephalopoda, Scaphopoda, Bivalvia). For most clades, and bivalves in particular, data concerning the molecular basis essential to this morphological variation is scarce. In this thesis I initially investigated Hox expression during development of the quagga mussel, Dreissena rostriformis, to elucidate to which degree they might contribute to specific phenotypic traits as in other conchiferans. Hox genes are key developmental regulators that are involved in establishing morphological features during animal ontogeny. Hox expression in Dreissena is first detected in the gastrula with widely overlapping expression domains of most genes, shifting towards more compact, largely mesodermal domains in the trochophore larva. The non-collinear mode of Hox expression in Dreissena might be a result of the low degree of body plan regionalization along the bivalve anterior-posterior axis as exemplified by the lack of key morphological traits such as a distinct head, cephalic tentacles, radula apparatus, and a simplified central nervous system. Secondly, we investigated the transcriptional complexity of a larval stage from Dreissena. For this, we profiled single cell transcriptomes of distinct cell populations from the trochophore larva. To annotate these cell populations, gene sets underlying the clustering were examined and compared to expression data of their orthologs as previously reported from other lophotrochozoans. Genes expected to be specific to certain tissues, such as prototrochal cells, muscle cells, among others, corroborate that the recovered cell clusters reflect the larval body composition of our animal. Lastly, in the third chapter we compared and analyzed genes expressed in the shell field. Our comparative gene analyses in this cluster indicate there is a significant emergence of novel genes shaping the unique transcriptomic signatures of shell field cells from Dreissena. Altogether, this thesis provides a molecular atlas of developmental processes underlying bivalve-specific tissues, whilst establishing a foundation that expands the frontiers of knowledge regarding the plasticity of molluscan body plans.