Convergent Evolution of Cellular Weaponry

Cellular weaponry includes a diverse array of ballistic structures, generally referred to as extrusomes, found across the tree of life. Extrusomes have evolved multiple times independently; are morphologically diverse, including structures such as coiled filaments, paracrystalline rods, and telescopic tubules; and facilitate prey capture, defense against predation, and host cell invasion. Their widespread distribution reflects extensive convergent evolution driven by similar ecological pressures. Conversely, homologous extrusomes in closely related taxa can diverge markedly in structure and function following lifestyle shifts, including transitions to parasitism. Alveolates, cryptists, heliozoans, euglenozoans, and numerous other protists possess distinct extrusomes with different discharge mechanisms. Comparable projectiles also occur in multicellular eukaryotes, including cnidarian nematocysts, ctenophore colloblasts, and the infectious spores of parasitic fungi and oomycetes, demonstrating that analogous cellular weaponry has evolved at multiple biological scales. In this article, we synthesize the diversity and phylogenetic distribution of cellular weaponry across eukaryotes, exploring novel examples of convergent evolution at different levels of biological organization.