Comparative assessment of morphological, cytological, and reproductive characteristics of the spontaneous mixoploid ‘MM 2×/4×’ and its diploid counterpart ‘Miss Manilla’ in Bougainvillea (Bougainvillea × buttiana Honlttum & Standl.)

Abstract

Polyploidization-triggered chromosome multiplication typically leads to improved agronomic and ornamental characteristics in plants. However, mixoploidy may sometimes occur in spontaneous and artificially induced polyploidization events, and the deficiency of basic information about mixoploidy limits its application in breeding. In this study, the ploidy stability of spontaneous mixoploidy Bougainvillea × buttiana ‘Miss Manila’ (‘MM 2×/4×’) was validated through chromosome counting and flow cytometry analysis. Compared with its diploid counterpart ‘MM 2×’, ‘MM 2×/4×’ exhibited significantly larger size in the morphological characters related to leaf, bract, thorn, and perianth tube. The average length, width, and density of stomata in ‘MM 2×/4×’ were also significantly larger than in ‘MM 2×’, while the stomatal density of mesophyll cells decreased. The anthers of ‘MM 2×/4×’produced both 1n and 2n pollen grains, which in vitro germinated reached 8.92 %, significantly higher than ‘MM 2×’ (0.36 %). The seed setting rate of the crossing combinations ‘MM 2×/4×’ × ‘Chitra’, ‘Chitra’ × ‘MM 2×/4×’ and ‘MM 2×/4×’ × ‘Bixi’ were 65.47 %, 32.36 %, and 53.17 %, respectively, and the ploidy levels among different offspring individuals of the same genetic population are relatively consistent. While ‘MM 2×’ could not be hybrid with either the tetraploid ‘Chitra’ or diploid ‘Bixi’. Therefore, the mixoploid ‘MM 2×/4×’ partially restored the fertility of gametes and might be employed for both ploidy breeding directly and creating hybrid segregation populations in bougainvillea for trait genetic dissection.