Haploid induction via unfertilized ovary culture in watermelon

Unfertilized ovary culture constitutes an effective method for haploid breeding and can greatly shorten the breeding time. This method has been successfully used for breeding in many species, but reports of this method for breeding watermelon are scarce. Therefore, we performed an experiment to induce haploid plantlets. We evaluated the effects of several important factors on unfertilized ovary cultures of watermelon, including genotype, medium, the duration of induction and the development stage of the ovaries. The results revealed that the genotype of donor plants was a key factor for in vitro gynogenesis. The induction rate of eight watermelon cultivars varied from 0.00 to 15.14 ELSs per100 ovary slices. The most effective induction medium and maturation medium were MS medium supplemented with 3 mg L^??1 2,4-D, 2 mg L^??1 BAP, 0.5 mg L^?1 NAA and MS supplemented with 0.8 mg L^??1 BAP and 0.2 mg L^??1 NAA, respectively. The duration of induction significantly influenced ELS formation. The optimum duration was 13 days, and unfertilized ovaries collected at anthesis had the higher induction rate. We obtained more than 100 plantlets and used chromosome counting and flow cytometry to determine the ploidy levels of 50 of them, among which 48 were haploid and 2 were diploid. Our results demonstrated that in vitro gynogenesis can be induced in watermelon by unfertilized ovaries culture.