S05 - Session O5 - A major locus controls unreduced pollen formation in diploid and triploid Hydrangea macrophylla

Authors: Conny Tränkner *

Polyploidization through unreduced gametes is a powerful tool in breeding. It allows the rapid and efficient production of polyploid offspring via crossings. Previous systematic crosses of the ornamental crop plant Hydrangea macrophylla resulted in 0, 1-2 or 50-98% polyploid offspring depending on the male parent, and revealed different cultivars that produced unreduced male but reduced female gametes (Tränkner et al. 2020, BMC Genetics). Flow cytometric screening of 89 cultivars identified diploid and triploid cultivars, which produced reduced, unreduced or at contrasting ratios both types of pollen. We found a clear genotypic effect, although cultivars with mixed pollen types showed a slight variation in the ratio of reduced and unreduced pollen between seasons and clonally propagated plants. The inheritance of this trait was studied by crossing individuals that produce reduced pollen with individuals that form unreduced pollen. These crosses resulted in diploid, triploid, aneuploid and pentaploid F 1 plants depending on the parental ploidy and cross direction. Flow cytometric analysis of pollen revealed F 1 plants that produced pollen with 1- to 5-fold DNA content. The populations segregated either 1:1 or 1:0 for reduced and unreduced pollen. These segregation ratios suggest a monogenic, dominant-recessive inheritance, whereby the formation of unreduced pollen is inherited recessively. Subsequent backcrosses and full-sib crosses confirmed this hypothesis. Furthermore, a cross between two cultivars, both of which form unreduced pollen, showed no complementation. Thus, we have identified a major UNREDUCED POLLEN ( UP ) locus that controls the formation of unreduced pollen, while female gametes are reduced. Microscopic analyses showed that pollen mother cells of cultivars with unreduced pollen form dyads instead of tetrads, suggesting meiotic restitution during male meiosis. Furthermore, we will present first results of a bulked DNA NGS analysis based on 157 diploid and triploid F 1 plants producing either reduced or unreduced pollen.