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  • Author or Editor: Masami Morishita x
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Watermelon fruit that results from pollination with pollen irradiated with soft-X-ray contains only empty seed, although the fruit develops to a normal size. In this study the processes of fertilization and embryo formation were compared between unirradiated and irradiated pollen in order to reveal the mechanisms by which seedless fruits are generated. The use of soft-X-irradiated pollen resulted in normal pollen tube penetration into the synergid and discharge of sperm cells. Two to three days after pollination, the sperm nucleus was present alongside the egg nucleus before fusion. The polar nuclei divided and the endosperm cell spread in the embryo sac before zygote division. A globular embryo was observed on day 7 to 10 after pollination. Subsequently, the embryo failed to differentiate to organ tissue and degenerated. These results indicate that double fertilization occurred after pollination with the irradiated pollen and that abortion of the embryo results from soft-X-ray induced chromosomal abnormalities in generative nucleus.

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We analyzed sequence variation in chloroplast DNA (cpDNA) to investigate the origin of the cultivated strawberry, Fragaria ×ananassa. From analysis of two noncoding regions, trnLtrnF and trnRrrn5, we found three haplotypes (V, C, and X) in F. ×ananassa. Haplotype V corresponded to the haplotype of F. virginiana and was possessed by cultivars bred over a wide geographic range, including North America, Europe, and Japan. Almost all the North American cultivars analyzed in this study possessed haplotype V, suggesting a founder effect. Haplotype C corresponded to the haplotype of F. chiloensis and was detected mainly in Japanese cultivars. Haplotype X was found in only two English cultivars. This haplotype was positioned as intermediate between haplotypes V and C in a median-joining network and was considered to be representative of the process of differentiation between F. virginiana and F. chiloensis. Results of controlled crosses indicate that cpDNA haplotypes of F. ×ananassa are maternally inherited. These results verify that F. ×ananassa is an interspecific hybrid between F. virginiana and F. chiloensis and indicate that traditional cultivars of F. ×ananassa have been derived from at least three maternal lineages. We demonstrate that the cpDNA variation detected in this study can be used to verify parentage and for extending hypotheses about June yellows, a leaf variegation disorder in strawberry.

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