Cytoplasmic male sterility (CMS) is a maternally inherited trait that fails to produce viable pollens, and has been reported in a large number of plant species. CMS is encoded in the mitochondrial genome and can arise spontaneously due to mutation in the genome (autoplasmy) or can be expressed following cytoplasmic substitutions due to nuclear-mitochondrial incompatibility (alloplasmy).1 Hybrids in various crops are based on only one CMS source. Such overdependence on a single cytosterility source may be disastrous in case there is a sudden outbreak of pests and diseases, and if susceptibility is associated with a CMS-inducing factor, as that observed in the outbreak of Southern corn leaf blight in T-cytoplasm based hybrids of maize in 1970s. Therefore, diversification of CMS sources should be an important component of a strong hybrid breeding program.

Diversification of CMS sources mainly refers to identifying and utilizing novel or alternate male sterility inducing cytoplasm. It also involves correction of defects in existing male sterility inducing cytoplasm or changing the nuclear genetic background of CMS lines.4 Once, a novel CMS source is identified, its characterization is very essential in order to differentiate it from the existing ones. A novel CMS named as inap CMS which occurred due to intertribal somatic hybridization between Brassica napus and Isatis indigotica Fort. (Chinese woad) of the Isatideae tribe within the Brassicaceae family through backcrossing successively to B. napus. The classical, histological and genetical characterizations of inap CMS showed that it is a novel CMS source.1

Effects of alternate male sterility inducing cytoplasms are studied in many crops like sorghum, sunflower, pearl millet, etc. In sunflower, the CMS analog ARG-2-1-2 (Helianthus argophyllus) performed on par to 234 from classical (H. petiolaris) PET-1 for seed yield, oil content and oil yield.3

A2 male sterile cytoplasm in sorghum was observed to be comparable to the widely used A1 cytoplasm in terms of grain yield, plant height and slight advantage over A1 in terms of frequency of hybrids with significant heterosis for days to 50% flowering and plant height.2

Utilization of the novel CMS sources depends on factors such as stability of male sterility, restorer gene frequency in the germplasm, effect of male sterility inducing cytoplasm per se on agronomic traits and the availability of commercially viable heterosis. Therefore, before, large scale exploitation of alternate CMS source, it should at least be comparable, if not superior to the currently used CMS source.3

References:

1KANG, L., LI, P., WANG, A., GE, X. AND LI, Z., 2017, A novel cytoplasmic male sterility in Brassica napus (inap CMS) with carpelloid stamens via protoplast fusion with Chinese woad. Frontiers in Plant Science, 8: 252832. 

2REDDY, B.V., RAMESH, S., REDDY, P.S. AND RAMAIAH, B., 2007, Combining ability and heterosis as influenced by male-sterility inducing cytoplasms in sorghum [Sorghum bicolor (L.) Moench]. Euphytica, 154: 153-164. 

3SHARMA, M. AND SHADAKSHARI, Y.G., 2022, Effect of alien cytoplasm on seed yield and its attributing traits in sunflower (Helianthus annuus L.). Mysore J. Agric. Sci., 56 (1): 213-220 

4Singh, B. D., 2021, Plant Breeding Principles And Methods. Kalyani Publications, New Delhi.