Molluscs are among the most successful animal groups, due in part to their strong and durable shells. Within a single group of molluscs, the snails, there is an enormous variety of different shell shapes, but all are variations on the same geometry, the helical spiral. Because of this underlying simplicity, snail shells are an important model for understanding how new biological structures are formed, but the cellular and developmental mechanisms that generate the shell are unknown. These researchers examined the patterns of cell division in the tissue that secretes the shell, and discovered two distinct growth zones. Remarkably, each zone displays cell division patterns that can explain the growth that drives shell shape. The researchers also found surprising fine-scale patterns in cell shape and gene activity that cause shell formation. The researchers will test the ability of these genes to form shells, and examine these patterns in another species with different shell coiling. Since snail shells are a familiar example of the great diversity of life, this work will provide an excellent example of the ways that small differences in genes and cells can create new and different shapes. The project will also enhance training of graduate and undergraduate students, and will create two classroom modules about shell shape to be used for outreach to middle school and high school students in a local high-poverty district.
This project contains three aims to investigate the ability of cell division and gene expression to pattern the helically coiled secreted shells of the snail Ilyanassa. First, the researchers will define patterns of cell division that putatively contribute to shell shape, then functionally test several signaling pathways that may drive these patterns. Cell division will then be tested in another snail, Lymnaea stagnalis, to determine if the cell division patterns are conserved, and if strains with opposite coiling chirality have asymmetric division patterns correlated with the direction of shell coiling. The second aim will discover new candidate regulators of growth, patterning, and mineralization, via sequencing transcriptomes of embryos where shell development has been specifically blocked by cell ablation. Differentially regulated transcripts will be examined by in situ hybridization to validate their expression in the mantle edge. Preliminary data suggests that this aim will reveal intricate spatial patterning of the mantle edge, and recover new genes that might be regulating growth and patterning. Finally, specific gene-knockdown techniques will test the function of putative regulatory factors and mineralization genes. Preliminary results indicate that they can perturb both shell production and shell morphology. This project will provide fundamental insights into the mechanisms that generate the molluscan shell, and provide a cellular-level system for evaluating hypotheses related to mathematical modeling of morphology. The project is also expected to integrate knowledge of biomineralization with developmental biology.
