Inter-sectional hybrids (Paeonia Itoh Group) produced by crossing between tree peonies and herbaceous peonies are important emerging ornamental plants. No previous studies have found ploidy other than triploid (2n=3x=15, ABC), or translocation between A and B genomes in intersectional hybrids. By employing the wider range of materials and the improved karyotyping technology, this research aimed to test whether there is other ploidy or inter-genome translocation in intersectional hybrids, and to explore how the intersectional hybrid traits are related to chromosome compositions, which could lay a theoretical foundation for the generation mechanism of inter-sectional hybrids in genus Paeonia and the genetic rules of their traits.

In this study, we did molecular karyotype analysis on 11 intersectional hybrids from P. lactiflora Fen Yun Fei He (2n=2x=10, CC) × P. × lemoinei ‘L’ Esperance' (2n=2x=10, AB) by bicolor genomic in situ hybridization (GISH) and rDNA fluorescence in situ hybridization (FISH). The relationship between morphology and ploidy of leaves from the inter-sectional hybrid Jing Hua Huan Cai was examined by flow cytometry.

The results showed that 9 of them were triploids (2n=3x=15, ABC), in 2 of which chromosomal translocations between A and B genomes were found, including reciprocal translocations between 2A and 2B, or 3A and 3B chromosomes, and non-reciprocal translocations between 3A and 3B chromosomes. In the rest two intersectional hybrids, Jing Rui Zi was diploid (2n=2x=10, AC), but Jing Hua Huan Cai were diploid (2n=2x=10, AC), or near triploid (2n=3x-1=14, AB^-1C) or in more other ploidy, which were found to exist independently or in chimeric state in different divided seedlings. Therein, the unlobed leaves of Jing Hua Huan Cai were all diploids, while the lobed leaves were near triploid, near diploid, or chimera involving near triploid. The intersectional hybrids with different ploidy had highly consistent traits, including herbaceous life form, leathery flower disc and high sterility of both pistils and stamens. However, leaf morphology, flower color and flowering time were significantly different among different ploidy.

The results above confirm that triploidization is the main pathway for the formation of intersectional hybrids, and the newly discovered few diploids might be formed by the loss of B genome chromosomes during mitosis of somatic cells in triploids or near triploids. The traits of intersectional hybrids are strongly influenced by their ploidy level. The three subgenomes from the parents were all closely related to the trait formation of intersectional hybrids. These findings reveal the diversity of chromosome karyotypes and the formation mechanism of intersectional hybrids in Paeonia.