An Intricate Hybrid Breeding System
Cytoplasmic male sterility (CMS) is an intricate breeding system designed to combine good genetic traits from “parent” seeds that is used in the development of many hybrid crops, including rice, maize, sunflower, canola and numerous vegetable crops.
A female inbred (“male sterile”) line that does not produce pollen is cross-pollinated with a male inbred line that produces fertile pollen, resulting in the production of a hybrid seed strengthened by attributes from both parents. A critical first step is the production of the male-sterile female inbreds: they are first developed from germplasm with normal cytoplasm that produce viable pollen. This “maintainer” version of the female line, which can self-propagate since it has normal pollen, is then converted to a male sterile version—the cytoplasmic genetic male-sterile (CMS) line—that does not produce pollen by crossing the maintainer version to a rice line that has a sterility factor in the cytoplasm of its cells. Through a series of backcrossing, an identical male sterile version of the female line can be developed.
To propagate the male-sterile inbred, a parent seed production step must be undertaken: the original fertile maintainer line is cross-pollinated with the genetically identical, yet sterile, female version to produce seed on the sterile version. The resulting parent seed is used as the sterile female line in seed production fields. There, the sterile female inbred is grown adjacent to a fertile male inbred line (i.e., an inbred with different genetics that complements the female inbred) and allowed to cross-pollinate, resulting in the production of hybrid seed. In the case of rice, farmers grow hybrid seeds to produce high-yielding crops with improved stress-tolerance.