Neuronal Transcriptome Analysis of a Widely Recognised Molluscan Model Organism Highlights the Absence of Key Proteins Involved in the De Novo Synthesis and Receptor-Mediation of Sex Steroids in Vertebrates

ABSTRACT Over the last ten years, the interpretation of the presence of vertebrate sex steroids in molluscs has changed dramatically. Evidence has been accumulating that CYP11A and CYP19A genes (encoding cholesterol side-chain cleavage enzyme and aromatase), that are crucial for the biosynthesis of sex steroids in vertebrates, as well as key functional sex steroid receptors, are missing in molluscan genomes. To provide further evidence, we sequenced the whole transcriptome of the central nervous system of the great pond snail (Lymnaea stagnalis) and screened it for sequences homologous to those used in the generally accepted vertebrate sex steroidogenesis pathway as well as the known sex steroid receptor-related genes (such as CYP11A, CYP19A, 3β-HSD, nPR, and nAR). Our screening confirmed the absence of several key sequences that are essential to accomplish a full sex steroid biosynthesis pathway similar to that of vertebrates. There was also no evidence for nuclear sex steroid receptors. Our findings support the contention that molluscan endocrinology differs from the well-characterized vertebrate endocrine system.