Citrus fruits have been introduced to the Mediterranean area from Asia for centuries and spontaneous crosses have generated several hybrid forms, some of which have had agricultural or industrial success while others have remained niche food or ornamental products, or have disappeared. Pompia (C. medica tuberosa Risso & Poiteau) is an old endemic citrus fruit from Sardinia of unknown genetic origin. Initial phenotypic and molecular characterizations revealed a high degree of similarity with lemon (C. limon (L.) Burm.) and citron (C. medica L.). To identify the ancestors of Pompia, 70 citrus species of the Citrus genus were genotyped with 36 codominant molecular markers (SSR and InDel) of nuclear and cytoplasmic genomes. Diversity analysis and allelic comparisons between each citrus species at each locus indicated that Pompia resembles lemon and limonette of Marrakech, i.e. the result of a cross between sour orange (C. aurantium L.) and citron, where citron was the pollinator. Two Italian citron varieties were identified as potential male parents, i.e. Diamante and Common Poncire. However, we were unable to differentiate varieties of sour oranges because varietal diversification in this horticultural group resulted from DNA sequence variations that SSR or InDel markers could not reveal. Rhob el Arsa and Poncire de Collioure were found to be two synonyms of Pompia. Pompia appeared to be equally distinct from citron, lemon and sour orange based on the overall analysis of the fruit, leaf and seed phenotype, and juice chemical composition. At the leaf level, the Pompia essential oil (EO) composition is close to that of citron whereas the zest is much closer to that of sour orange.

Pompia is a citrus fruit endemic of Sardinia, Italy, with an essential oil profile showing outstanding anti-inflammatory and anti-microbic properties. Despite its remarkable pharmaceutical potential, little taxonomic and genetic information is available for this species. We applied flow cytometry and classical cytogenetic techniques to assess the DNA content and to reconstruct the karyotype of several Pompia accessions. Molecular data from plastid DNA barcoding and nuclear DNA sequencing were used to study the genetic distance between Pompia and other citrus species. Flow cytometric estimates of DNA content and somatic chromosome counts suggest that Pompia is a regular diploid Citrus species. DNA polymorphisms of nuclear and chloroplast markers allowed us to investigate the genetic relationships between Pompia accessions and other Citrus species. Based on DNA polymorphism data we propose that Pompia is a very recent interspecific hybrid generated by a cross between C. aurantium (as seed bearer) and C. medica (as pollen donor). Our findings pave the way for further and more specific investigations of local Pompia germplasm resources that may help the preservation and valorisation of this valuable citrus fruit tree.