Citrus trees originated in an extensive area covering Asia (from India to the north of China) and Oceania (Queensland, Australia). The genus Citrus is defined by two different classification systems: Tanaka’s, with 156 species, and Swingle’s, with only 16 species. However, these two systems often contradict each other due to the overall sexual compatibility between the Citrus species and the frequent occurrence of apomixes (due to nucellar polyembryony), which leads many taxonomists to consid...
