Embryological study on gynogenesis in onion (Allium cepa L.)

Haploid induction in onion can, to date, be induced only via gynogenesis by culturing unfertilized flowers, ovaries or ovules. The process of haploid embryo induction has been macroscopically well studied, but only limited da ta exist from microscopic examination of ovule development status at the in oculation stage and of the origin of gynogenic embryos. Microscopic studies were carried out using individual donor plants with relatively high embryo induction frequencies (45.9 embryos formed per 100 flowers, on average, fo r 2 years). Ovaries from flower bud culture were fixed at 1 week intervals up to the 7th week of culture. These were compared with pollinated ovaries at 1 or 2 weeks after pollination. In total, 1428 unfertilized embryo sacs were examined. The results indicate that, at the time of inoculation, ovule s within ovaries 2.0-3.0 mm. in diameter contained two- or four-nucleate em bryo sacs in the smallest ovaries to mature embryo sacs in the largest ovar ies. It seems likely that the embryos are actually induced from ovaries cul tured at the immature stage. After 1 or 2 weeks in culture, the egg apparat us primarily consisted of distinctly enlarged synergids and the egg cell, w hich was often detached from the micropylar pole. But free nuclear endosper m was also formed. From the 2nd to 7th week in culture, formation of haploi d embryos (from globular to the almost mature cylindrical stage) was detect ed in 5.7% of the ovules. Their origin, for several reasons, was most likel y the egg cell. In addition, ovules containing endosperm only (3.6%) and ov ules containing the egg apparatus (0.5%) or both endosperm and embryo (0.4% ) were detected. This observation is probably unique and has not yet been r eported in other species studied.