Plant Hybridization

Hybridization is the process of interbreeding between individuals of differ­ent species (interspecific hybridization) or genetically divergent individuals from the same species (intraspecific hybridization). Offspring produced by hybridization may be fertile, partially fertile, or sterile.

Plants hybridize much more frequently and successfully than animals do. Pollen from flowering plants disperses widely and may land on flowers of other species. Chromosomal doubling (polyploidy) occurs more frequently in plants and facilitates the fertility of the hybrid offspring. Finally, plant forms are less stringently controlled than animal forms, and so the interme­diate form of a plant hybrid is more likely to be physiologically successful.

One of the first persons to study plant hybridization was Josef Kolrueter, who published the results of his experiments on tobacco in 1760. Kolrueter concluded that interspecific hybridization in nature is rare unless humans disturb the habitat. Since that time, many instances of hybridization among various plant species have been documented.

One good example of plant hybridization involves hybridization between the elegant sego lily (Calochortus selwayensis) and a mariposa lily (C. apiculatus) in western Montana. The sego lily, with purple-spotted petals, lives in dry sites at mid-elevations in the Rocky Mountains under the somewhat open canopy of ponderosa pine forests. The mariposa, with its cream-colored petals, lives in moister sites at higher elevations under the more closed Douglas-fir canopies. Interspecific hybrids between the elegant sego and mariposa lilies are found in great abundance on ski slopes where Douglas-fir canopies have been opened and kept clear of trees and tall shrubs.

The ski slope is a habitat that is too dry and too open for the mariposa to thrive and too moist for the elegant sego, but just right for the hybrids.

Such an intermediate habitat is called a hybrid habitat. Human disturbance can generate hybrid habitats of many types and thus can foster interspecific hybridization.

Backcrossing, which is the interbreeding between hybrids and their parental species, can transfer alleles from one parent to the other using the interspecific hybrids as a genetic bridge in a process called introgression. Introgression increases the genetic variation of one or both of the parents. In the previous example, there is extensive backcrossing between the hy­brids and mariposa lily. The result of this introgression is that some mari­posa lilies now display petals with some of the purple spotting characteristic of the elegant sego, and theoretically they can live in slightly drier habitats.

Often interspecific hybrids are sterile or for some other reason cannot interbreed with the parental species. Occasionally sterile interspecific hy­brids can undergo a doubling of their chromosome set and become fertile tetraploids (four sets of chromosomes). For example, the bread wheats that humans use today are a result of two hybridizations each followed by chro­mosome doubling to produce fertile hexaploids (six sets of chromosomes). In such instances the hybrids can become new species with characteristics different from either of the parents.

Humans have used intraspecific hybridization, hybridization between strains of a single species, to develop high-yielding crops. In corn, contin­ually inbred varieties will often exhibit inbreeding depression, which is a reduction in vitality and yield. Hybridization between inbred lines can re­sult in hybrids that exceed one or more of the parents in size and repro­ductive potential. This increased vitality is called hybrid vigor and has been studied since the time of English naturalist Charles Darwin. Crop yields in­crease dramatically (as much as 100 percent) when hybridization is used in this way. In the twenty-first century, over 90 percent of the corn grown is of hybrid origin.

References

Anderson, Edgar. Introgressive Hybridization. New York: John Wiley & Sons, Inc., 1949.

Grant, Verne. Plant Speciation. New York: Columbia University Press, 1971.

Wagner, Warren. “Reticulate Evolution in the Appalachian Aspleniums.” Evolution 8 (1954): 103-118.