Transposable elements cause the loss of self-incompatibility in citrus

Area of Science:

• Plant reproductive biology
• Molecular genetics
• Evolutionary biology

Background:

• Self-incompatibility (SI) is crucial for plant genetic diversity, often based on S-RNase systems.
• Citrus species exhibit frequent loss of SI, with prior studies identifying single nucleotide mutations in S-RNase.
• The mechanisms driving the conversion from SI to self-compatibility (SC) in citrus remain largely unexplored.

Purpose of the Study:

• To identify novel mechanisms responsible for the loss of self-incompatibility (SI) in citrus.
• To investigate the role of a miniature inverted-repeat transposable element (MITE) in the FhiS2-RNase gene promoter of Fortunella hindsii.
• To determine if MITE insertions can functionally alter the SI phenotype in citrus.

Main Methods:

• Identification and characterization of a MITE insertion in the FhiS2-RNase promoter region.
• Demonstration of the MITE's impact on FhiS2-RNase gene expression.
• Utilizing transgenic experiments to confirm the MITE's role by deleting the insertion and assessing SI restoration.

Main Results:

• A 786-bp MITE insertion was discovered in the promoter of FhiS2-RNase in Fortunella hindsii, a self-compatible citrus species.
• This MITE insertion was shown to prevent the expression of FhiS2-RNase, leading to the loss of SI.
• Transgenic experiments successfully restored FhiS2-RNase expression and SI by deleting the MITE insertion.

Conclusions:

• This study presents the first evidence of MITEs at the S-locus influencing the SI phenotype in plants.
• MITE insertions in promoter regions represent a significant mechanism for the evolution of self-compatibility in S-RNase-based systems.
• The frequent occurrence of MITEs near S-RNase alleles across citrus genera suggests this mechanism broadly impacts citrus breeding strategies.