Transposable elements cause the loss of self‐incompatibility in citrus

Abstract

SummarySelf‐incompatibility (SI) is a widespread prezygotic mechanism for flowering plants to avoid inbreeding depression and promote genetic diversity. Citrus has an S‐RNase‐based SI system, which was frequently lost during evolution. We previously identified a single nucleotide mutation in Sm‐RNase, which is responsible for the loss of SI in mandarin and its hybrids. However, little is known about other mechanisms responsible for conversion of SI to self‐compatibility (SC) and we identify a completely different mechanism widely utilized by citrus. Here, we found a 786‐bp miniature inverted‐repeat transposable element (MITE) insertion in the promoter region of the FhiS2‐RNase in Fortunella hindsii Swingle (a model plant for citrus gene function), which does not contain the Sm‐RNase allele but are still SC. We demonstrate that this MITE plays a pivotal role in the loss of SI in citrus, providing evidence that this MITE insertion prevents expression of the S‐RNase; moreover, transgenic experiments show that deletion of this 786‐bp MITE insertion recovers the expression of FhiS2‐RNase and restores SI. This study identifies the first evidence for a role for MITEs at the S‐locus affecting the SI phenotype. A family‐wide survey of the S‐locus revealed that MITE insertions occur frequently adjacent to S‐RNase alleles in different citrus genera, but only certain MITEs appear to be responsible for the loss of SI. Our study provides evidence that insertion of MITEs into a promoter region can alter a breeding strategy and suggests that this phenomenon may be broadly responsible for SC in species with the S‐RNase system.