F1 hybrid
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F1 hybrids is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types. The offspring of distinctly different parental types produce a new, uniform variety with specific and/or desirable characteristics from either or both parents. In fish breeding, those parents frequently are two closely related fish species, while in plant and animal genetics those parents usually are two inbred lines. Mules are F1 hybrids between horse and donkey. Crossing specific parent plants produces a hybrid seed (plant) by means of controlled pollination. To produce consistent F1 hybrids, the original cross must be repeated each season. As in the original cross, in plants this is usually done through controlled hand-pollination, and explains why F1-seeds are so expensive.
In agronomy, the term “F1 hybrid” is usually reserved for agricultural cultivars derived from two different parent cultivars, each of which are inbred for a number of generations to the extent that they are almost homozygous. The divergence between the parent lines promotes improved growth and yield characteristics through the phenomenon of heterosis, whilst the homozygosity of the parent lines ensures a phenotypically uniform F1 generation.
Gregor Mendel's groundbreaking work in the 19th century focused on patterns of inheritance and the genetic basis for variation. In his cross-pollination experiments involving two true-breeding, or homozygous, parents, Mendel found that the resulting F1 generation were heterozygous and all phenotypically resembled the dominant parent plant. Mendel’s discoveries involving the F1 and F2 generation lay the foundation for modern genetics. Today, classification of certain domestic hybrid breeds, such as the Savannah (cat), are classified by their filial generation number.
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[edit] Production of F1 hybrids
[edit] In plants
Two populations of breeding stock with desired characteristics are subject to inbreeding until the homozygosity of the population exceeds a certain level, usually 90% or more. Typically this requires more than ten generations. After this happens, both populations must be crossed while avoiding self-fertilization. Normally this happens in plants by deactivating or removing male flowers from one population, taking advantage of time differences between male and female flowering or hand-pollinating[1].
In 1960, 99 percent of all corn planted in the United States, 95 percent of sugar beet, 80 percent of spinach, 80 percent of sunflowers, 62 percent of broccoli, and 60 percent of onions were hybrid. Such figures are probably higher today. Beans and peas are not commercially hybridized because they are automatic pollinators, and hand-pollination is prohibitively expensive.
[edit] In animals
Unlike most plants, commonly bred fish species as well as all mammals and birds are not hermaphrodite, and therefore it is much easier to prevent self-fertilization during an F1 cross. F1 crosses in fish can be between two inbred lines or between two closely related fish species, such as cichlid subspecies.[2]. The cross is usually performed by natural or artificial insemination.
[edit] Advantages
- Homogeneity and predictability - because of the homozygosity of the parent pure lines, there is next to no genetic variation between individual plants or animals. This makes their phenotype extremely uniform and thus attractive for mechanical operations and makes it easier to fine-tune the management of the population. Once the characteristics of the cross are known, repeating this cross will yield exactly the same result.
- Higher performance - As most non-junk DNA alleles code for different versions of a protein or enzyme, having two different versions of this allele amounts to having two different versions of the enzyme. This will increase the likelihood of having an optimal version of the enzyme present and reduce the likelihood of a genetic defect. This effect is referred to in genetics as the genetic hysteresis effect.
[edit] Disadvantages
The main advantage of F1 hybrids is also their drawback. When F1 cultivars are used for the breeding of a new generation, their offspring (F2 generation) will be high in homozygous genes, as found in the weaker parental generation, and thus will have a depression in yield and lack the hybrid vigour. From the point of view of a commercial seed producer which does not wish its customers to produce their own seed, this is a desired characteristic.
- Both inbreeding and crossing both lines requires a lot of work, which translates in a much higher seed cost. In general, the higher yield offsets this disadvantage.
- F1 hybrids usually give higher yields than traditional varieties, but only when they are grown using chemical fertilisers, pesticides, fungicides and herbicides. In contrast, traditional varieties usually have a higher natural resistance to disease. Therefore, large inputs of artificial chemicals are not usually necessary in order for them to grow well.
- F1 hybrids are bred to mature all at once. This is of interest for modern farmers, because all ripen at the same time and can be harvested by machine. Traditional varieties are often more useful to gardeners because they crop over a longer period of time, avoiding gluts and famines.
- F1 hybrids lack genetic diversity, due to the inbred parental lines.
[edit] References
- Hand pollination
- USDA, Seeds: The Yearbook of Agriculture, 1961.
