Methods of Plant Breeding | Botany Optional for UPSC PDF Download

Hybrid Varieties

• The creation of hybrid varieties differs from traditional hybridization in that it doesn't aim to establish a pure-breeding population. Instead, the focus is on producing F1 hybrid plants. These F1 hybrids, resulting from crosses between different genotypes, often display greater vigor compared to their parent plants. This phenomenon, known as hybrid vigor or heterosis, can manifest in various ways, such as accelerated growth, increased uniformity, earlier flowering, and higher yield. Increased yield is particularly significant in the realm of agriculture.
• The development of hybrid varieties has seen its most significant progress in the case of corn (maize). This is primarily because corn has separate male flowers (tassels) and female flowers (incipient ears), making it practical and cost-effective to produce hybrid seeds. In contrast, producing hand-made F1 hybrid seeds for other plants, including ornamental flowers, has been economically viable mainly because greenhouse growers and home gardeners are willing to pay premium prices for these hybrid seeds.
• However, recent advancements in the field have made hybrid varieties feasible for a wide range of plants, even those that are self-pollinating, such as sorghums. This innovation involves a built-in cellular mechanism known as cytoplasmic male sterility, or cytosterility. This mechanism disrupts the normal development and functioning of the male reproductive organs (stamens), resulting in either defective pollen or no pollen at all. 
• As a result, there's no need to manually remove stamens, either by hand or machine. The inheritance of cytosterility depends on the interaction between male sterile genes (R + r) and factors present in the cytoplasm of the female sex cell. Importantly, this cytoplasm and its factors are solely contributed by the female parent. Therefore, the inheritance of cytosterility is determined by the female parent's genetic makeup. In plants with fertile cytoplasm, they produce viable pollen. In contrast, plants with sterile cytoplasm but at least one R gene also produce viable pollen. However, plants with sterile cytoplasm and two r genes are male sterile and produce defective pollen.
• To produce F1 hybrid seeds between two different strains (e.g., A and B), the process involves planting a sterile version of one strain (A) in an isolated field alongside a fertile version of another strain (B). Since strain A cannot produce viable pollen, it is pollinated by strain B. Consequently, all the seeds produced by strain A plants are F1 hybrids originating from the cross between the two strains. These F1 hybrid seeds are then used to cultivate the commercial crop. A significant part of the breeder's efforts in this process involves developing pure-breeding sterile and fertile strains to initiate hybrid seed production.

Breeding cross-pollinated species

Breeding cross-pollinated species involves several important methods: (1) mass selection, (2) the development of hybrid varieties, and (3) the development of synthetic varieties. These methods aim to either maintain or restore heterozygosity in cross-pollinated species, as they tend to be naturally heterozygous for many traits and lose vigor as they become homozygous.

• Mass Selection: This method resembles the one used in self-pollinated species. It involves selecting a large number of superior-looking plants, harvesting them in bulk, and using their seeds to produce the next generation. Mass selection has proven highly effective for improving qualitative traits and, over many generations, can also enhance quantitative traits like yield, despite their low heritability. It has been a significant method for breeding cross-pollinated species, especially those of lesser economic importance.
  
• Hybrid Varieties: A prominent example of exploiting hybrid vigor through F1 hybrid varieties is seen in corn (maize). The process of creating hybrid corn involves three steps: (a) selection of superior plants, (b) several generations of self-pollination to produce different pure-breeding and highly uniform inbred lines, and (c) crossing these selected inbred lines. During the inbreeding process, the vigor of the lines decreases significantly, often dropping to less than half that of field-pollinated varieties. However, when unrelated inbred lines are crossed, vigor is restored, and in some cases, the F1 hybrids between inbred lines are superior to open-pollinated varieties. The homozygosity of inbred lines ensures that the hybrid between any two inbreds will be consistently the same. Once the best inbred lines for hybrids are identified, any desired amount of hybrid seed can be produced.
  • Corn (maize) pollination occurs through wind, which carries pollen from tassels to the silks that protrude from the tops of the ears. Controlled cross-pollination on a field scale can be economically achieved by planting two or three rows of the seed parent inbred with one row of the pollinator inbred and removing the tassels from the seed parent before pollen shedding. Most hybrid corn is produced using "double crosses," where four inbred lines are initially crossed in pairs (A × B and C × D), and then the resulting two F1 hybrids are crossed again (A × B) × (C × D). This approach reduces seed costs as commercial F1 seed is produced from the highly productive single cross A × B. Cytoplasmic male sterility has been used to eliminate the need for detasseling the seed parent, further economizing hybrid seed production.
  • It's important to note that much of the hybrid vigor displayed by F1 hybrid varieties is lost in the next generation. Consequently, seed from hybrid varieties is not used for planting, and farmers purchase new seed each year from seed companies.
• Synthetic Varieties: Synthetic varieties are developed by crossing several genotypes known for their superior combining ability. These genotypes are chosen based on their ability to consistently produce superior hybrids when crossed in various combinations. In contrast, varieties developed through mass selection consist of genotypes bulked together without prior testing for their hybrid performance. Synthetic varieties are known for their hybrid vigor and their ability to produce usable seed for subsequent seasons. These advantages make synthetic varieties increasingly popular, particularly in species like forage crops, where the development or use of hybrid varieties can be cost-prohibitive. The use of synthetic varieties is on the rise, contributing to crop productivity in various agricultural contexts.

Distribution and maintenance of new varieties

• The successful utilization of superior new plant varieties depends on the availability of sufficient seed for commercial production. While the primary role of a plant breeder is to create these new varieties, they typically engage in an initial small-scale seed production process. This initial seed is referred to as "breeder's seed." The subsequent phase involves multiplying the breeder's seed to generate "foundation seed." Foundation seed production is often overseen by seed associations or institutes, with government agencies regulating their operations. 
• The final step is the production of "certified seed," which is the offspring of foundation seed. Certified seed is produced on a larger scale by specialized seed growers and is intended for general sale to farmers and gardeners. To meet the quality standards established by the certifying agency, certified seed must be produced and handled carefully. Seed associations are typically responsible for maintaining the genetic purity of new varieties once they are approved for commercial production.
• Commercial plant-breeding companies often distribute their newly developed varieties through seed associations. However, some reputable companies may market their products without adhering to the official certification process. In certain countries, particularly in Europe, new plant varieties can be patented for extended periods, granting breeders exclusive rights to reproduce and sell the variety during this time.