A 6.49-Mb inversion associated with the purple embryo spot trait in potato

Abstract
The embryo spot trait leads to a deep purple or reddish coloration at the base of the cotyledons of the embryo, visible on both sides of flat potato (Solanum tuberosum) seeds. This trait has long been used by potato researchers and breeders as a morphological marker during dihaploid induction. The formation of embryo spots reflects the accumulation of anthocyanins, but the genetic basis of this trait remains unclear. In this study, we mapped the embryo spot trait to a 6.78-Mb region at the end of chromosome 10 using an F2 population derived from a cross between spotted and spotless plants. The recombination rate in the candidate region is severely suppressed, posing challenges for the map-based cloning of the underlying gene and suggesting large-scale rearrangements in this region. A de novo genome assembly of the spotted individual and a comparative genomic analysis to the reference genome of spotless potato revealed a 6.49-Mb inversion present in the spotted plant genome. The left breakpoint of this inversion occurred in the promoter region of an R2R3 MYB transcription factor gene that is highly expressed in the cotyledon base of spotted embryos but is not expressed in that of spotless embryos. This study elucidated the genetic basis for embryo spot formation in potato and provides a foundation for future cloning of the causative gene.