A molecular perspective on the taxonomy and journey of Citrus domestication

The highly-demanded commercial citrus fruits of family Rutaceae arose primarily through sexual hybridization between the four ancestral taxa generating a range of nothospecies. The diversity of phenotypic traits in these cultivable groups was mainly due to somatic mutations fixed either by apomixis present in Citrus species or grafting for clonal propagation, leaving behind very scanty evidence to study the process of citrus domestication apart from its genealogy. Moreover, sexual compatibility between Citrus and its related genera is another broad area of controversy leading to a continuous reformulation of citrus taxonomy and phylogeny. Although advanced genomic studies to clarify the phylogenetic relationships of citrus are in progress, a detailed overview of citrus taxonomy, diversity, origin and domestication would enhance our knowledge not only to get an evolutionary framework of citrus phylogeny but also to unravel the history of citrus domestication. Therefore, the review has been presented comprehensively with recent studies emphasizing the identification of specific reproductive, sensory and morphological markers selected as traits during the course of domestication. Hence, studies on identifying genes related to polyembryony, self-incompatibility (SI) and anthocyanin production between wild and cultivated citrus have been discussed to provide new insights on citrus apomixis, SI and citric acid reduction. Further, the correlation of pummelo introgression with fruit size and palatability in cultivable mandarins has also been focused on understanding mandarin domestication.