Cytogenetic analysis of interspecific hybridization in oil-tea (Camellia oleifera)

Oil-tea (Camellia oleifera) is an important woody plant species producing edible oil in southern China, which shows complicated cytogenetics and intraspecific polyploidy. In order to explore the ploidy variation in interspecific hybridization of oil-tea Camellia, the ploidy of ‘Huashuo’, ‘Youza 2’ and 18 superior hybrids of ‘Youza 2’ × ‘Huashuo’ were identified by flow cytometry and chromosome counting. C.oleifera variety ‘Huashuo’ was hexaploid (2n = 90), and Camellia yuhsienensis superior plant ‘Youza 2’ was aneuploid based on hexaploid (2n  = 91). There are not only hexaploid but also aneuploid in the ‘Huashuo’ × ‘Youza 2’ hybrids. Among the offspring plants, there are 11 plants each with chromosome number of 90, 6 plants each with 91, and 1 plants each with 92. To elucidate the causes of ploidy variation in above findings, the pollen characteristics and chromosomal behavior in meiosis of ‘Huashuo’ were examined by microscopy. The chromosome pairing configuration of meiosis terminal stage of microsporocytes of ‘Huashuo’ was 43 II + 2 IV. In all stages of meiosis, numerous abnormal behaviors were found, such as precocious chromosome migration, free chromosomes, lagging chromosomes and micronucleus. The abnormal rates of metaphase I, anaphase I, metaphase II and anaphase II were 67.5%, 88.1%, 58.4% and 78.9%, respectively. In the tetrad stage, abnormal phenomena such as dyads, triads, hexads and micronucleus were observed, with the frequency of 1.4%, 1.0%, 1.0% and 4.2%, respectively. The pollen diameter of ‘Huashuo’ varied widely (16.92–79.13 μm), suggesting a wide range of ploidy level of pollen grains. From pollen diameter distribution, we concluded that ‘Huashuo’ produced a small number of unreduced pollen grains. These findings are valuable for cytogenetics research and selection strategy of ploidy breeding in C. oleifera.