Haploid Induction in Hull-less Seed Pumpkin through Parthenogenesis Induced by X-ray-irradiated Pollen

Abstract

Production of hull-less seeds of styrian oil pumpkin (Cucurbita pepo ssp. pepo var. styriaca) is increasing as a result of demand for aromatic seed oil and for other uses. Hybrid cultivars have recently been released but a method for haploid induction has not been established. We focused on the development of a parthenogenetic haploid induction protocol based on pollination with pollen irradiated with X-ray radiation at 0, 50, 100, 150, 200, 300, and 350 Gy. Fruit set largely decreased at 200 Gy, whereas a decline in embryo formation was observed at 100 Gy. Various accessions were tested as the female parent or as the pollen donor and large differences were found. The best parthenogenetic response was found in ‘Turkey #2’ (10.0%), ‘Gleisdorfer Ölkürbis’ (4.4%), and ‘Naked Seed’ (3.9%), whereas ‘GL Opal’ and ‘White Acorn’ were efficient as pollen donors. The ploidy level of 3830 putative parthenogenetic embryos was determined using flow cytometry. Four ploidy levels (n, 2n, 3n, and 4n) were found with the majority being diploid. Interestingly, a significant proportion was determined to be tetraploid and this was clearly correlated with increased radiation delivered to pollen grains. Using selected simple sequence repeat markers on diploid embryos, no spontaneous chromosome doubling could be confirmed. In this study, haploid induction in styrian oil pumpkin was elaborated for the first time. We also showed that X-ray pollen irradiation provides an alternative to gamma radiation treatment, yielding a sufficient percentage of haploid plantlets.