Induced Dominant Lethality in Lilium

Abstract

The ovules of L. formosanum are arranged in 2 rows in each of 3 ovules, with about 200 ovules per row. After a massive pollination with untreated pollen, seed is usually set along the entire length of each row, but in some cases the ovules at the bottom or top of each row may not develop. When a limited amt. of untreated pollen is used the seed is concentrated in a "favored zone" about 1/3 of the way from top to base of the capsule. A massive pollination with pollen treated with from 2000 to 4000 r-units of X-rays gives little or no seed set. In order to distinguish between dominant lethal effects and irradiation damage to the pollen grains pollinations were first made using a limited, counted no. of treated pollen, followed by massive repollinations with normal grains after 48 hrs. The resulting capsules had aborted ovules in the favored zone, surrounded by normal seeds at the top and base, indicating that fertilizations involving the treated pollen had occurred and that dominant lethals had been induced. The treated grains were found to function in fertilization at the same rate as untreated grains (90.95%). Control expts. showed that in repeat pollinations the stigma was not made unreceptive by the 1st pollination, and that the undeveloped ovules in the favored zone in the X-ray-repeat-normal pollinations were not due to spoilage from aging during the 48 hr. time lapse. Sections of ovaries fixed several days after massive pollinations with X-rayed pollen showed pollen tubes in all micropyles and 2 sperm nuclei in the embryo sac. Various doses of X-rays resulted in some germless seeds, indicating an independent development of the endosperm. Since the endosperm sometimes showed necrotic changes before the zygote had begun to divide, the induced lethals in the sperm were immediately dominant over the maternal genes. Application of untreated pollen to half the stigma and treated pollen to the other half indicated that the treated pollen grains were not handicapped in forming tubes and effecting fertilization. The occurrence of occasional low seed sets from treated pollen was attributed to pseudogamous development. This theory was supported by applying treated pollen to a semi.sterile type, which gave a few seeds at doses expected to be completely lethal.