ObjectiveThe inter-subsectional distant hybridization between subsect. Delacayanae and subsect. Vaginatae of Paeonia sect. Moutan is a significant direction in modern tree peony breeding. Nevertheless, the inter-subsectional hybrids are generally sterile, therefore hard to be used as breeding parents subsequently. Paeonia × lemoinei ‘High Noon’ is an inter-subsectional cultivar with good ornamental and resistant traits. In spite of its sterility in most cases, ‘High Noon’ was still the parent of a few excellent hybrids, showing some degree of fertility. The research on the meiotic chromosome behavior of ‘High Noon’ could provide important messages to uncover the forming mechanisms of its low fertility.

MethodWe employed the pistils from the young flower buds to detect the somatic karyotype, and the anthers to observe the meiotic chromosome behaviors during microsporogenesis in this study.

Result‘High Noon’ was a diploid, whose karyotype formula was 2n = 2x = 10 = 7m + 1sm + 2st (1SAT). The pollen mother cells (PMCs) of ‘High Noon’ could generate tetrads and microspores through regular meiosis. The meiotic abnormalities in the first meiotic division took place in about 70% of PMCs, including univalent, multivalent, chromosome bridge, chromosome segmentation, lagging chromosome, unequal chromosome segregation, etc., among which, the chromosome bridge emerged most frequently. In the second meiotic division, meiotic abnormalities occurred in 70% of PMCs as well, involving abnormal spindle orientation, chromosome segregation asynchronicity, chromosome bridge, chromosome segmentation, lagging chromosome, etc., resulting in the formation of the dyad, triad, micronucleus, and extra microspores.

ConclusionThe vast number of abnormality in meiosis of ‘High Noon’ might attributed to its heterozygous karyotype. The abnormal meiotic chromosome behaviors including the widespread unequal chromosome segregation caused by univalent or multivalent, and the chromosome segment missing due to the chromosome bridge and chromosome segmentation, might be the leading causes for the high sterility of ‘High Noon’. Meanwhile, a fraction of PMCs could accomplish the meiosis process successfully, forming the microspores. Therein some unreduced microspores were produced via dyad or triad formation, indicating its potential for polyploid tree peony breeding.