Highlights d A rove beetle defense gland reveals how evolution builds new organs d Two secretory cell types work together, one making a solid toxin, the second a solvent d New cellular functions evolved by combining ancient transcriptome modules and pathways d Evolution of each cell type was shaped by adaptive coevolution between the cell types
How evolution at the cellular level potentiates change at the macroevolutionary level is a major question in evolutionary biology. With >66,000 described species, rove beetles (Staphylinidae) comprise the largest metazoan family. Their exceptional radiation has been coupled to pervasive biosynthetic innovation whereby numerous lineages bear defensive glands with diverse chemistries. Here, we combine comparative genomic and single cell transcriptomic data from across the largest rove beetle clade, Aleocharinae. We retrace the functional evolution of two novel secretory cell types that together comprise the tergal gland - a putative catalyst behind Aleocharinae's megadiversity. We identify key genomic contingencies that were critical to the assembly of each cell type and their organ-level partnership in manufacturing the beetle's defensive secretion. This process hinged on evolving a mechanism for regulated production of noxious benzoquinones that appears convergent with plant toxin release systems, and synthesis of an effective benzoquinone solvent that weaponized the total secretion. We show that this cooperative biosynthetic system arose at the Jurassic-Cretaceous boundary, and that following its establishment, both cell types underwent ~150 million years of stasis, their chemistry and core molecular architecture maintained almost clade-wide as Aleocharinae radiated globally into tens of thousands of lineages. Despite this deep conservation, we show that the two cell types have acted as substrates for the emergence of adaptive, biochemical novelties - most dramatically in symbiotic lineages that have infiltrated social insect colonies and produce host behavior-manipulating secretions. Our findings uncover genomic and cell type evolutionary processes underlying the origin, functional conservation and evolvability of a chemical innovation in beetles.
A long-standing challenge in biology is explaining how the functions of multicellular organs emerge from the underlying evolution of cell types. We deconstructed evolution of an organ novelty: a rove beetle gland that secretes a defensive cocktail. We show that gland function was pieced together via assembly of two cell types that manufacture distinct compounds. One cell type forms a chemical reservoir in the beetle's abdomen and produces alkane and ester compounds. We demonstrate that this cell type is a hybrid of cuticle cells and ancient pheromone and adipocyte-like cells, and executes its function via a mosaic of enzymes sourced from each parental cell type. The second cell type synthesizes noxious benzoquinones using a chimeric pathway derived from conserved cellular energy and cuticle formation pathways. We present evidence that evolution of each cell type was shaped by coevolution between the two cell types: the benzoquinones produced by the second cell type dissolve in solvents produced by the first, yielding a potent secretion that confers adaptive value onto the gland as a whole. Our findings illustrate how cooperation between cell types can arise, generating new, organ-level behaviors.
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