Hybrid Seeds and Hybridization

Hybrid Seeds: Advantages in Resistance to Disease Pests & Production

Introduction to Plant Hybridization

In agriculture and gardening, hybrid seed is seed produced by artificially cross-pollinated plants. Hybrids are bred to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved color, disease resistance, and so forth. Today, hybrid seed is predominant in agriculture and home gardening, and is one of the main contributing factors to the dramatic rise in agricultural output during the last half of the 20th century. In the US, the commercial market was launched in the 1920s, with the first hybrid maize. Hybrid seed cannot be saved, as the seed from the first generation of hybrid plants does not reliably produce true copies, therefore, new seed must be purchased for each planting.

Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Animals, being more mobile, have developed complex mating behaviors that maintain the species boundary and when hybrids do occur, natural selection tends to weed them out of the population since these hybrids generally can not find mates that will accept them or they are less adapted and fit for survival in their habitats. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved; seeds, fruits or other plant parts for consumption, or to make a plant more winter or heat hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors.

Many plant genera and species have their origins in polyploidy. Autopolyploidy resulting from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids or plants with four sets of chromosomes are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenthera gigas. Tetraploids can develop into a breeding population within the diploid population and when hybrids are formed with the diploid population the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes) Another form of polyploidy called allopolyploidy occurs when two different species mate and produce hybrids. Usually the typical chromosome number is doubled in successful allopolyploid species, with four sets of chromosomes the genotypes can sort out to form a complete diploid set from the parent species, thus they can produce fertile offspring that can mate and reproduce with each other but can not back-cross with the parent species. Allopolyploidy in plants often gives them a condition called hybrid vigour, which results in plants that are larger and stronger growing than either of the two parent species. Allopolyploids are often more aggressive growing and can be invaders of new habitats. Sterility in a hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes. While this is a negative in a crop such as wheat, when growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Bananas and seedless watermelon, for instance, are intentionally bred to be triploid, so that they will produce no seeds. Many hybrids are created by humans, but natural hybrids occur as well. Plant hybrids, especially, are often stronger than either parent variety, a phenomenon which when present is known as hybrid vigour (heterosis) or heterozygote advantage. Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids.[top]

Examples

Some plant hybrids include:

* Leyland Cypress, [X Cupressocyparis leylandii] hybrid between Monterey Cypress and Nootka Cypress.
* Limequat, lime and kumquat hybrid.
* Loganberry, a hybrid between raspberry and blackberry.
* London Plane, a hybrid between Plantanus orientalis Oriental plane and Platanus occidentalis American plane (American sycamore). Thus forming [Plantanus x acerfolia]
* Peppermint, a hybrid between spearmint and water mint.
* Tangelo, a hybrid of a Mandarin orange and a pomelo or a grapefruit which may have been developed in Asia about 3,500 years ago.
* Triticale, a wheat-rye hybrid.
* Wheat; most modern and ancient wheat breeds are themselves hybrids.

Some natural hybrids are:

* White Flag Iris, a sterile hybrid which spreads by rhizome division
* Evening primrose, a flower which was the subject of famous experiments by Hugo de Vries on polyploidy and diploidy.

Some horticultural hybrids:

* Dianthus ×allwoodii, is a hybrid between Dianthus caryophyllus × Dianthus plumarius. This is an "interspecific hybrid" or hybrid between two species in the same genus.
* ×Heucherella tiarelloides, or Heuchera sanguinea × Tiarella cordifolia is an "intergeneric hybrid" a hybrid between two different genera.
* Quercus x warei [Quercus robur x Quercus bicolor] Kindred Spirit Hybrid Oak
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Effect of Hybridization on Biodiversity and Food Security

In agriculture and animal husbandry, the green revolutions use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity. Since the indigenous breeds are often better adapted to local extremes in climate and have immunity to local pathogens this represents a significant genetic erosion of the gene pool for future breeding. Newer, genetically engineered (GE) varieties are be a problem for local biodiversity. Some of these plants contain designer genes that would be unlikely to evolve in nature, even with conventional hybridization. These may pass into the wild population with unpredictable consequences and may be detrimental for the success of future breeding programs.[top]

Limiting factors with Hybridization

A number of conditions exist that limit the success of hybridization, the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related hybridization barriers include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.

In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.[top]

Pet Peeves - Hybrid Seeds Cannot Be Saved

One of the things many people did as a child was help their parents with the family garden. And year after year seeds from the previous garden would be carefully saved. With hybrid seeds this is no longer possible, so instead of this time honoured tradition, people are practically forced to buy new seeds each year.

If this is something you also don't like, make sure to look into Heirloom Seeds (these are seeds that have not been modified and the seed from these plants can be saved). Keep in mind, however, the Heirloom seeds do not have the production nor resistance advantages the hybrides do. Like so many things in life, there is a trade-off to be made no matter what decision you go with. [top]