Evolution of NLR resistance genes with noncanonical N-terminal domains in wild tomato species

Abstract

Nucleotide-binding and leucine-rich repeat immune receptors (NLRs) provide resistance against diverse pathogens. To create comparative NLR resources, we conducted resistance gene enrichment sequencing (RenSeq) with single-molecule real-time sequencing of PacBio for 18 accessions in Solanaceae, including 15 accessions of five wild tomato species. We investigated the evolution of a class of NLRs, CNLs with extended N-terminal sequences previously named Solanaceae Domain. Through comparative genomic analysis, we revealed that the extended CNLs (exCNLs) anciently emerged in the most recent common ancestor between Asterids and Amaranthaceae, far predating the Solanaceae family. In tomatoes, the exCNLs display exceptional modes of evolution in a clade-specific manner. In the clade G3, exCNLs have substantially elongated their N-termini through tandem duplications of exon segments. In the clade G1, exCNLs have evolved through recent proliferation and sequence diversification. In the clade G6, an ancestral exCNL has lost its N-terminal domains in the course of evolution. Our study provides high-quality NLR gene models for close relatives of domesticated tomatoes that can serve as a useful resource for breeding and molecular engineering for disease resistance. Our findings regarding the exCNLs offer unique backgrounds and insights for future functional studies of the NLRs.