Development of hybrids and hybrid seed production of onion

Gupta A and Singh BK. 2016. Development of hybrids and hybrid seed production of onion. In: Principles and Production Techniques of Hybrid Seeds in Vegetables (Singh B, Pandey S, Singh N, Manimurugan C, Devi J and Singh PM Eds). Training Manual No.  67, ICAR-IIVR, Varanasi, UP, pp 101-111.

Onion (Allium cepa L.) is a high value spice-cum-vegetable crop cultivated in almost all parts of the country for its tunicated bulbs. In India, onion occupies an area of 1.20 mha with the production of 19.42 mt. Globally, India ranks first in area and second in production of onion, but its productivity is low (16.1 t/ha) as compareto world’s productivity (19.3 t/ha) (Anonymous 2015). One option to achieve a quantum jump in yield and productivity is through harnessing the potential of heterotic vigour present among genetic resources. Heterosis breeding provides the opportunities for improvement in productivity; earliness; uniformity for maturity, and colour, shape and size of bulbs; and yield attributing characters. Most of the area under onion cultivation in India is covered by open pollinated varieties (OPVs) whereas the areas under hybrids have been increased significantly in developed countries. Now-a-days, both private and public sectors in India are paying more attention towards development of F₁ hybrids. Onion is a classical crop for exploitation of heterosis or hybrid vigour. Being a cross-pollinated crop, it exhibits wide variability in terms of maturity, bulb shape and size, bulb colour, day length requirement for bulbing, vernalization for flowering, TSS, drymatter content, etc. Hybrid seed production by hand emasculation and pollination can onlybe academic reality; commercially it is neither feasible nor economical. After the discovery of cytoplasmic genic male sterility (CGMS) by Jones and Clarke (1925), the phenomenon of heterosis breeding in onion is being exploited all over the world in long day as well as short day onions.

Use of male sterility in onion

The most important qualitative genes in onion are those that cause male sterility. In male sterile plants pollen fails to develop and they are therefore incapable of self-pollination. Hence, any seed produced result from only cross-pollination. This property has been utilized to produce F₁ hybrids which show hybrid vigour for various traits of economic importance. In the absence of male sterility, controlled cross-pollination without any self-pollination can be achieved by the laborious method of removing anthers from a flower head before they shed pollens followed by hand pollination with desired pollens. Such a procedure is too labour intensive, time consuming which is useful only for experimental breeding purpose.

Male sterility in onion was first exploited by Jones and Clarke using a male sterile genetic stock of cultivar ‘Italian Red’ found in breeding plots at Davis, California in 1925.  Fortunately, when this plant was prevented from being cross-pollinated, bulbils were produced in the flower head and it could be propagated. Jones and Clarke (1943) published this classical work describing the genetics of male sterility and indicating how it could be used to produce hybrid cultivars. On the basis of these techniques male sterility has since been exploited in more than 150 crop species (Kale and Munjal 2005). CGMS system presently used worldwide in onion for commercial exploitation of heterosis was originally derived from a variety ‘Italian Red 13-53’. The second source of CMS (T-cytoplasm) was discovered in a French cultivar ‘Jaunepaille des Venus’. This CMS line was found to be different than that from ‘Italian Red 13-53’ as three independent segregating restorer loci were identified in this line, responsible for its complex inheritance. It has common occurrence of restorers which makes this T-cytoplasm more difficult to use. Later on male sterility has been observed in several other onion populations, mainly in long-day cultivars e.g. Pukekohe Longkeeper, Red Wethersfield, Scott County Globe, Stuttgarter Riesen and Zittauer Glebe. In India, male sterility was identified in a local cultivar Nasik White Globe at IIHR, Bangalore (Pathak et al. 1980). The tapetal abnormalities and histochemical changes were mainly responsible for male sterility in onion and there was no role of meiotic abnormalities (Saraswathi and Veere Gowda 2006).