Evidence for gene transfer by the use of sublethally irradiated pollen in Zea mays and theory of occurrence by chromosome repair through somatic recombination and gene conversion

Abstract

Gene transfer by the use of sublethally γ-irradiated pollen was investigated in Zea mays using three marker genes. Irradiations at 10 and 15 kr produced rare viable T1 seeds which gave rise, usually, to sterile plants. At 5 kr more viable T1 seeds were produced, the majority of which gave rise to phenotypically normal, fertile plants. About a quarter of the resulting (T1) plants showed recessive maternal characters, instead of being true hybrids showing dominant characters of the pollen parent. In effect, different, partial hybrids were produced in which only a portion, and not all, of the pollen genome was combined with the total egg genome. The above results are similar to those previously obtained in Nicotiana rustica (Jinks et al. 1981) where pollen irradiated at sublethal doses was used. A new hypothesis is proposed to explain gene transfer following use of sublethally irradiated pollen. It is suggested that in divisions during early embryogeny following normal fertilisation by pollen treated with near-lethal doses of radiation, there is selection of cells in which irradiation damaged segment of paternal chromosomes are repaired, through homologous somatic recombination and gene conversion, by segments from the normal maternal chromosomes. The chromosome repair may be specifically facilitated by irradiation-induced sub-chromatid breakage which occurs only in the prophase of mitotic cells—the usual phase of the generative and gametic nuclei in the pollen grains at the time of irradiation. The proposed mechanism highlights the dynamic role of single strands in chromosome recombination and repair and the use of ionising radiations and pollen grains in the timely production of such strands.