Embryonic development and molecular mechanisms of land snail appendage patterning

Abstract

Mollusks are a deeply divergent and morphologically diverse metazoan clade, offering a valuable yet underutilized framework for studying the evolution of developmental gene networks. While aquatic gastropods have served as foundational models for investigating processes, such as spiral cleavage and biomineralization, terrestrial gastropods, particularly within the Stylommatophora, remain largely unexplored. This gap limits insight into key macroevolutionary transitions, including terrestrialization, body plan patterning, and the emergence of lineage-specific traits. In this thesis, I establish the brown garden snail, Cornu aspersum, as a laboratory model for terrestrial gastropod development and investigate the proximodistal axis patterning of gastropod cephalic tentacles. In Chapter 1, I construct an embryonic staging system, develop protocols for gene expression assays and in ovo cultivation, and assemble a developmental transcriptome. In Chapter 2, I characterize the expression of conserved proximodistal patterning genes—Distal-less (Dll), homothorax (hth), extradenticle (exd), dachshund (dac), and components of the Wnt, Hedgehog, and BMP pathways—during cephalic tentacle development. These data reveal both conserved and lineage-specific expression domains, supporting the hypothesis that stylommatophoran tentacles represent a case of partial gene regulatory network cooption. Together, this work advances C. aspersum as a model for evolutionary developmental research and contributes to broader discussions of deep homology and morphological innovation in molluscan evolution.