Exon shuffling potentiates a diverse repertoire of brown algal NB‐ARC‐TPR candidate immune receptor proteins via alternative splicing.

SUMMARY: Like other organisms, brown algae are subject to diseases caused by bacteria, fungi, and viruses. Brown algal immunity mechanisms are not well characterized; however, there is evidence suggesting that pathogen receptors exist in brown algae. One key protein family likely associated with brown algal innate immunity possesses an NB‐ARC domain analogous to innate immune proteins in plants and animals. In this study, we conducted an extensive survey of NB‐ARC genes in brown algae and obtained insights into the domain organization and evolutionary history of the encoded proteins. Our data show that brown algae possess an ancient NB‐ARC‐tetratricopeptide repeat (NB‐TPR) domain architecture. We identified an N‐terminal effector domain, the four‐helix bundle, which was not previously found associated with NB‐ARC domains. The phylogenetic tree including NB‐ARC domains from all kingdoms of life suggests the three clades of brown algal NB‐TPRs are likely monophyletic, whereas their TPRs seem to have distinct origins. One group of TPRs exhibit intense exon shuffling, with various alternative splicing and diversifying selection acting on them, suggesting exon shuffling is an important mechanism for evolving ligand‐binding specificities. The reconciliation of gene duplication and loss events of the NB‐ARC genes reveals that more independent gene gains than losses have occurred during brown algal evolution, and that tandem duplication has played a major role in the expansion of NB‐ARC genes. Our results substantially enhance our understanding of the evolutionary history and exon shuffling mechanisms of the candidate innate immune repertoire of brown algae. Significance Statement: This work explored the evolution of the brown algal immune candidates NB‐ARC‐tetratricopeptide repeat (NB‐ARC‐TPR) genes, constructed separate phylogenies based on the central NB‐ARC, specificity determining TPR, and the N‐terminal four‐helix‐bundle domains. We found exon shuffling potentiates expression of a diverse repertoire of brown algal NB‐ARC‐TPR candidate immune receptor proteins via alternative splicing, and identified the four‐helix‐bundle effector domain in the N‐terminal of NB‐ARC genes for the first time, which suggests a function related to programmed cell death. [ABSTRACT FROM AUTHOR]