Evolution of apoptotic signalling pathways within lophotrochozoans

Apoptosis is the main form of regulated cell death in metazoans. Apoptotic pathways are well characterised in nematode, fly and mammals, leading to a vision of the conservation of apoptotic pathways in metazoans. However, we recently showed that intrinsic apoptosis is in fact divergent among metazoans. In addition, extrinsic apoptosis is poorly studied in non-mammalian animals, making its evolution unclear. Consequently, our understanding of apoptotic signalling pathways evolution is a black-box which must be illuminated by extending research to new biological systems. Lophotrochozoans are a major clade of metazoans which, despite their considerable biological diversity and key phylogenetic position as sister group of ecdysozoans ( i.e. fly, nematode), are poorly explored, especially regarding apoptosis mechanisms. Traditionally each apoptotic signalling pathway was considered to rely on a specific initiator Caspase, associated with an activator. To shed light on apoptosis evolution in animals, we explored the evolutionary history of initiator Caspases, Caspase activators and the BCL-2 family (which control mitochondrial apoptotic pathway) in lophotrochozoans using phylogenetic analysis and protein interaction predictions. We discovered a diversification of initiator Caspases in molluscs, annelids and brachiopods, and the loss of key extrinsic apoptosis components in platyhelminths, along with the emergence of a clade specific Caspase with an ankyrin pro-domain. Taken together, our data show a specific history of apoptotic actors’ evolution in lophotrochozoans, further demonstrating the appearance of distinct apoptotic signalling pathways during metazoan evolution. Significance statement Apoptosis, a form of programmed cell death, has been long studied in model organisms such as fly, mouse, and in humans. The restricted focus on these models has led to an overall view that the evolution of genes involved in apoptosis is highly conserved across all animals. The advent of next generation sequencing has led to a boom in the omics data available across the tree of life. Thanks to this, we explored the evolution of key genes involved in apoptosis in the clade Lophotrochozoa ( i.e. molluscs, annelids, flatworms, brachiopods), one of the three large clades that make up bilaterian animals. We found a complex evolutionary history of apoptosis genes, with multiple losses, gains, divergences and redundancies, highlighting the value of exploring gene evolution and apoptotic mechanisms in Lophotrochozoans.