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Development as a Metabolic Regulator: How Molting Controls Cholesteryl Ester Metabolism in the Somatic Stem Cells of C. elegans Raj Rani1, Or Ben-Hemo1, Benjamin Trabelcy1, Agam Bar1, Hans-Joachim Knölker2, Yoram Gerchman1,3,4, and Amir Sapir1*1Department of Biology and the Environment, Faculty of Natural Sciences, University of Haifa, Oranim, Tivon, 36006 Israel2 Fakultät Chemie, Technische Universität Dresden, Bergstrasse 66, 01069 Dresden, Germany3Institute of Evolution, Faculty of Natural Sciences, University of Haifa, Haifa, Israel4Oranim Academic College, Kiryat Tivon, Israel The metabolism of steroids, such as cholesterol, is critical for mammalian physiology and human health, yet its function in invertebrates remains poorly understood. Using Caenorhabditis elegans, we constructed the first comprehensive homology-based enzymatic atlas of steroid metabolism in invertebrates, identifying 159 candidate genes. We performed a two-dimensional genetic and metabolic screen, knocking down the atlas genes under varying cholesterol conditions to identify those functioning in steroid metabolism. Among the screen hits, we focused on mboa-1, an ortholog of mammalian SOAT1/2 enzymes that synthesize cholesteryl esters from sterols and fatty acids. Surprisingly, mboa-1 knockdown and knockout disrupt hypodermis and cuticle integrity. Consistent with its predicted enzymatic function, bacterially expressed C. elegans MBOA-1 generates cholesteryl esters when supplemented with the steroid 4,3-cholesta and fatty acids. Moreover, 4,3-cholesta—but not steroid hormones—rescued the mboa-1 RNAi phenotype, suggesting a new branch of steroid metabolism in C. elegans. mboa-1 is expressed specifically in the somatic stem cells of C. elegans, the seam cells, which contribute to the hypodermis and cuticle. Expression begins in mid-embryogenesis, persists throughout larval development, but declines sharply in adults. Underscoring its role in cuticle dynamics, mboa-1 expression oscillates with the molting cycle and is regulated by lin-29–mediated heterochronic control during the larval-to-adult transition, a stage when seam cells terminally differentiate. Our functional studies in Clade IV and V nematodes, along with insect expression data, suggest that during evolution, mboa-1 regulation was rewired to support a structural role for cholesteryl esters in cuticle formation, diverging from their primarily metabolic functions in mammals and insects. Our findings reveal how, during evolution, steroid metabolism was repurposed for a novel function in nematodes through the mechanistic reconfiguring of developmental regulation and stem cell biology.