Misunderstandings about plant reproduction and breeding are many, and these seem to have multiplied exponentially since GMOs burst upon the scene. Some of the misunderstandings about plants seem to arise due to generalizing from understandings about animals. Other problems arise when people react to the current furor about GMOs, concluding that anything not a “natural” species is a work of the devil.
What follows is an attempt to explain distinctions among several categories, particularly with relevance to the home gardener and nursery customer.
Plants are wildly promiscuous, compared to animals. Plants will mate and give rise to offspring- with themselves, with genetically-identical clone mates, with close relatives, and with plants of different species- sometimes even of different genera. These processes happen spontaneously in nature all the time, and many of today’s wild species arose through spontaneous hybridization, spontaneous mutations and/or spontaneous multiplication of chromosomes.
Over the millennia—perhaps 10,000 years in the case of corn—human farmers have taken advantage of this reproductive and genetic flexibility of plants to breed new agricultural species and varieties. Corn is believed to have arisen as a hybrid of two wild grass species, which ancient farmers may have observed and then deliberately perpetuated.
For over six centuries in Europe, Asia and the Americas, human-mediated selection from naturally-occurring genetic variability has been important in the development of all of our food crops and many of our ornamentals.
Traditional plant breeding has consisted of:
(1.) deliberate propagation of spontaneously-arising mutations,
(2.) selection over multiple generations of a crop for desirable characteristics such as size, taste, color, and disease resistance, and/or
(3.) Deliberate hybridization of different species or subspecies followed by selection for these desired aesthetic or functional characteristics.
(1.) deliberate propagation of spontaneously-arising mutations,
(2.) selection over multiple generations of a crop for desirable characteristics such as size, taste, color, and disease resistance, and/or
(3.) Deliberate hybridization of different species or subspecies followed by selection for these desired aesthetic or functional characteristics.
HYBRIDS
A hybrid is any cross between plants of different species, subspecies, or occasionally genera. It may arise spontaneously in nature or in the greenhouse, or may have been induced through human manipulation (hand pollination). Some wild species are known to hybridize very frequently with closely-related plants. For instance, different species of Aquilegia (Columbine) grown near each other will reliably form fertile hybrids. The same is true of Sidalcea (Checker Mallow) species.
Hybrids may be designated in the form of botanical names, with an "x" indicating hybrid status: Digitalis x mertonensis. In the nursery trade, sometimes hybrid status is subsumed under a cultivar name, such as 'Copper Rose' Mullein (a sterile hybrid cultivar).
Some hybrids may be propagated through seed, but many can only be produced reliably through starts (produced by cuttings). Contrary to what many people believe based on animal biology, hybrids quite often will produce viable seeds, though the seedlings most often will not be identical to the parents.
MUTATIONS
The word “mutation” has a negative connotation in the popular imagination, but in fact most beneficial adaptations among humans, animals, and plants over thousands of years have been due to accidental mutations. Mutations are simply changes in the genetic code—and when these result in traits that are advantageous, they will multiply among populations as individuals bearing these mutations have better survival and reproductive success. Because many plants are extravagantly reproductive compared to animals, there are many more opportunities for such mutations to arise. In addition, plants are less constrained morphologically than are animals—that is, shape doesn’t matter quite so much, so mutations that greatly change a plant’s appearance can persist.
SPECIES, SUBSPECIES, VARIETIES
When you buy a plant at the nursery, its labeling may reflect its status as a wild species, cultivar, or hybrid. The species is given as a botanical name * consisting of two words, such as Ribes sanguineum (red-flowering currant). Wild plants may in addition be identified as varieties or subspecies, as in Ribes cereum var. inebrians or Ribes aureum ssp. gracillinum.
Botanical varieties are not to be confused with cultivars, see discussion below.
Technically, any true native will not be a cultivar.
CULTIVARS
A “cultivar” or “selection” is a plant lineage that has been named and maintained consistently as a horticultural product. Cultivars usually result from deliberate breeding of individuals with desired characteristics (paler or darker than average flower color, larger flower size, etc.). Some cultivars are hybrids; these may have arisen spontaneously in nature or in the greenhouse, or they may have been induced deliberately via hand-pollination. Others may have arisen spontaneously as mutations (or "sports") that were identified as desirable.
In addition, some cultivars have arisen through multiplication of chromosome sets, which may have occurred spontaneously or may have been induced, e.g. through use of colchicine, a very toxic chemical derived from Autumn Crocus. Colchicine was used medically (for dubious reasons) for centuries. Its usefulness in plant breeding was discovered in the 1930s.
Whether planned or serendipitous, cultivars are often propagated vegetatively via cuttings or tissue culture (tiny cuttings done in sterile conditions in a laboratory) to preserve genetic identity. Cultivar names are written in normal font (not italicized) and are capitalized and surrounded by single quote marks. The species name may or may not be included. A variety of red-flowering currant selected for white flowers is Ribes sanguineum ‘White Icicle’.
If the letters PPAF follow any cultivar name, the variety has been patented, and restrictions as to its propagation and sale pertain.
A special case that illustrates some of these issues, often confusing to gardeners, is that of irises. Many people love to grow showy bearded iris. Named cultivars of bearded iris number in the hundreds. Because they are hybridized and highly selected for certain colors and patterns, these are always propagated vegetatively and sold as bareroot rhizomes or potted plants. Yet bearded iris in your garden will often produce seed, and all iris varieties trace their lineage back to a seed at some point.
“Species iris” are not hybridized, and their names are usually given in botanical Latin. One can grow any species iris from seed. However, within popular horticultural species (such as Siberian Iris, Iris sibirica; or Louisiana Iris, Iris louisiana), there are also many named cultivars that are not hybrids, but were selected from successive generations of the wild species for certain characteristics, particularly color.
GMOS
GMOs- plants with gene combinations manipulated in a laboratory- are rare to nonexistent among nursery plants in general, and particularly among ornamental plants. When the genes involved all come from closely related plant species, the result is identical to something that could have arisen-- and somewhere, may have arisen-- as a result of normal plant genetic variability. Due to the expense involved in producing them, GMOs are developed only for use in large-scale agriculture-- so home gardeners will never encounter a GMO except possibly in a food product at the grocery store.
* For a good, clear discussion of botanical nomenclature, see e.g. http://www.santafebotanicalgarden.org/subpages/JT%20Botanical_Names.html
No comments:
Post a Comment