Genomic in situ hybridization identifies genome donors of Camellia reticulata (Theaceae)

Camellia reticulata (Theaceae genus Camellia) is a world-famous, ornamental flowering plant. More interestingly, it has a polyploid series varying from 2n = 2x = 30, 2n = 4x = 60 to 2n = 6x = 90, with a basic chromosome number of x = 15. The hypothetic allopolyploid origin and parental genomes of these polyploid types remains unknown. Genomic in situ hybridization (GISH) was used to study the genome organization and evolution of C. reticulata. Total genomic DNA from closely-related diploid species (C. pitardii and C. saluenensis), with the chromosome number 2n = 2x = 30, were labeled and hybridized in the presence of blocking DNA onto metaphase spreads of C. reticulata. The C. pitartii probe painted part of the tetraploid and hexaploid C. reticulata genomes, whereas the C. saluenensis probe delineated part of the hexaploid C. reticulata genome. The results provide compelling evidence for the allopolyploid origin of C. reticultata genomes and demonstrate that 1) the diploid C. reticultata, C. pitardii and C. saluenensis are the progenitors of polyploid C. reticultata, 2) hybridization between diploid C. reticultata and diploid C. pitardii gave birth to allotetraploid C. reticulata, and 3) subsequent hybridization between allotetraploid C. reticulata and diploid C. saluenensis formed the allohexaploid C. reticulata.