The Evolutionary History of Metallothioneins in Horseshoe Crabs, Scorpions, Spiders and other Chelicerate Species

Metallothioneins (MTs) are a heterogenous family of proteins involved in metal homeostasis and detoxification, playing a crucial role in the adaptation of organisms to varying conditions of metal bioavailability across ecosystems. Despite their significance, MT characterization across animal groups remains uneven. While MTs have been extensively studied in vertebrates and gastropods, their presence and function in many arthropods, particularly within the Chelicerata subphylum, remain largely unexplored. Chelicerates, comprising thousands of species of spiders, scorpions and ticks, are a highly diverse group of animals inhabiting a wide range of environments. Identifying and characterizing their MTs is therefore essential for understanding how they manage metal availability of diverse habitats. In this study, we have identified over 400 chelicerate MTs, classifying them into three distinct types –types 1, 2, and 3– each exhibiting significant structural and phylogenetic differences. Our findings have revealed that most chelicerate MTs are bidomain proteins, with notable exceptions of monodomain forms and larger variants. Evolutionary analysis suggests that type 1 MTs represent the ancestral form, while types 2 and 3 likely emerged during the evolution of spiders. These results pave the way of a deeper understanding of the metal homeostasis and detoxification in chelicerates, laying the groundwork for future studies on the origin, evolution and functional diversification of MTs in arthropods and other metazoan lineages.