Retrotransposons: From "Junk DNA" to Drivers of Fruit Crop Improvement

Historically viewed as "junk DNA," retrotransposons are now recognized as powerful, dynamic agents of genome evolution and a burgeoning frontier in the field of fruit crop breeding. These Class I mobile genetic elements utilize a "copy-and-paste" mechanism, which allows them to amplify and create novel polymorphisms that can alter gene function and regulation. This review article synthesizes the latest research to provide a comprehensive overview of the current and potential applications of retrotransposons in fruit crop improvement. We highlight their profound impact as drivers of natural variation, examining specific examples of retrotransposon-induced mutations that control agronomically important traits such as fruit ripening in guava, flesh coloration in Japanese plum, and the compact spur growth habit in apples. Furthermore, we discuss how their unique insertional polymorphisms have been leveraged as a new generation of molecular markers for genetic diversity analysis, cultivar fingerprinting, and marker-assisted selection, as demonstrated in grapes and avocado. By integrating insights into their mutagenic, regulatory, and epigenetic roles, this review underscores the immense value of retrotransposons as tools for accelerating the development of new, resilient fruit varieties with enhanced quality to address the challenges of modern agriculture.