Rapid Methods for Onion Breeding

Onion (Allium cepa L.) is an indispensable component of daily food, worldwide. It is one of the most important vegetable crops grown throughout the world since ancient times. To feed the rapidly expanding population, rapid breeding methods are needed to increase the productivity of this crop. Being highly cross-pollinated with 2 years per generation life cycle and large genome size, breeding efforts in this crop are minimal. Further, onion is a thermo-sensitive crop which again complicates the breeding efforts. Hybrid development in onion started way back in 1936 when male sterility was discovered. But isolation and identification of maintainer lines through progeny tests takes 4–5 years. Similarly, inbred development takes 12 years, assuming six generations of inbreeding, to develop inbreds. Inbred development also becomes cumbersome since onions exhibit high inbreeding depression. Doubled haploid technology through in vitro gynogenesis came as a boon for onion breeders. Use of molecular markers for assessing diversity, population structure, and development of linkage maps was a big breakthrough. This was followed by the research efforts on marker development for identification of cytoplasm and nuclear Ms locus. These research efforts have aided breeders in identification and isolation of maintainer lines in most of the onion populations worldwide. Though some discrepancies still remain, marker-aided selection of maintainer lines and S cytoplasm is a great leap toward hybrid development. Genetic transformation holds promise for improvement, but in the present atmosphere towards transgenic crops, this area has been put up in the backburner at present. There are successful reports of marker development for disease resistance. But not many efforts toward marker development for disease resistance have been reported. This is due to the fact that either sources of resistance have not been identified or there are pre- and post-fertilization barriers. In future, emphasis on genome sequencing and genome editing will open new areas of rapid breeding methods in onion.