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Hybrid seed production: an applied usage of maternal inheritance

Plant breeders have recognized for many years that the progeny from a specific cross can out yield either of the two parents used in the cross. This is called heterosis, the phenomenon were the phenotypic value of the heterozygote is greater than either of the two parents. For plant breeding, the observation of heterosis for yield has lead to the development of inbred lines that exhibit a heterotic yield advantage.

Corn was the first crop species in which heterosis was exploited. The original approach to develop hybrid corn seed required the manual detasseling of the female parent to prevent self-pollination. The field would be planted with the two lines and high school students would walk the field and detassel the female parent. As you can imagine this was a labor intensive proposition.

Because it was realized that manual detasseling of corn plants would not be required if male sterile system could be developed, attempts were made to apply the two types of male sterility systems, genic and cyotplasmic male sterility. Genic male sterility is controlled by nuclear genes. The weakness of each of the genetic systems, though, was that a portion of the F2 were male fertile, and thus a portion of the seed that was developed was not hybrid.

The ultimate solution to this problem was the use of cytoplasmic male sterility (cms). As the name suggests, this type of cytoplasmic male sterility is controlled by a cytoplasmic factor and is maternally inherited. (Current molecular research suggests, but has not conclusively proven, that the sterility is a mitochondrial encoded function.) Thus all the males that contain the appropriate cytoplasm would be sterile.

But this is only part of the solution for hybrid seed production. The seed company sells hybrid seed to the farmer, and the farmer expects this seed to be fertile. If the hybrid seed sold to the farmer was sterile, the seed company would have to provide a pollinator source to be planted along with with the hybrid seed to obtain the seed. The need for this step though is alleviated by the use of restorer of fertility (Rf) genes. These dominant nuclear genes can override the cytoplasmic male sterility factors. Thus plants that have the cms cytoplasm contain a dominant Rf allele will be male fertile.

Taking the above discussion into consideration, the following is a procedure to produce hybrid corn seed without manual detasseling. A line that contains that contains a male sterile cytoplasm and is recessive for the restorer of fertility alleles (rfrf) is the female parent in a cross with a male that has male sterile cytoplasm and is heterozygous for the restorer of fertility alleles (Rfrf). The F1 progeny from this cross will exhibit the heterotic effects for yield. Furthermore, all the plants will be cytoplasmically male sterile because they contain the cms cytoplasm.

The plants will also segregate 1 Rfrf :1 rfrf. Those that are recessive for the restorer factor will still produce seed because the other heterozygous Rfrf plants will produce ample pollen to pollinate those plant Thus, the farmer will realize the advantages of hybrid seed, and the seed company will not have to use resoruces for manual detasseling. The following diagram demonstrates the process.

Although this system is functional, some difficulties have arisen with it practical application. In corn breeding, the cytoplasm that was used initially to provide the male sterility was the Texas or T cytoplasm. Nearly all of the hybrid corn grown until 1970 contained this cytoplasm. During that year a fungal disease (Southern corn leaf blight) appeared that preferentially attacked plants with the T cytoplasm.

This is an example of genetic vulnerability. This term refers to the genetic condition were all of the individuals in a region have the same genotype that makes them all vulnerable to a single disease or pathogen that could destroy the whole population. Thus, all of the hybrid corn was at great risk the following year.

Fortunately, USDA scientists recognized the upcoming problem, and seed stocks were developed that contained other cytoplasms. These seed stocks had to be manually detasseled, and its hybrid performance was not as good as the previous material, but it did provide a source of seed that was resistant to the disease and saved the United States from losing the entire hybrid corn harvest that year. Since that time though sources of the T cytoplasm have been developed that are resistant to the disease, and hybrid seed production utilizing cytoplasmic male sterility has resumed.

Copyright © 1997. Phillip McClean