Re: hybridization vs. genetic engineering

I would like to add a note about hybrids to S. Rae Schnapp's explanation.
In her original inquiry, Karen Mundy asked if it is possible to get the same 
yields with hybrid varieties as with "old varieties."  In a strongly outcrossing 
species like corn, hybrid vigor is a major factor.  In the old varieties of corn 
(called open pollinated varieties), it is critical to maintain enough genetic 
diversity within the variety to keep it vigorous.  People who save seed of 
heirloom corn varieties have to make sure they grow and save seed from a large 
enough population of plants in every generation to keep that genetic diversity 
and thus hybrid vigor.  Because every plant of a hybrid benefits from hybrid 
vigor, it is debatable whether we could get the same yield from open pollinated 
varieties as from hybrids in a crop like corn.  

On the other hand, there are many crop plants that are inbreeding (self- 
pollinating) and have been since domestication.  There is generally little or no 
hybrid advantage in these crops.  Self-pollinated crop plants include wheat, 
rice, oats, barley, sorghum, soybeans, common beans, peas, tomatoes, tobacco, 
peppers, lettuce, and a number of other crops.  In addition, there are some 
outcrossing species where there is little or no hybrid vigor (My old plant 
breeding textbook says cucurbits -- squash, pumpkins, melons, cucumbers -- are 
an example).  So why are there hybrid seeds on the market for many of these 
crops?  Because if you save seed from hybrids, you will not get the same variety 
you started with.  The plants will be less uniform, and not every plant will 
have the same resistance to disease, for example, that you purchased the hybrid 
to get.  So, it is to the advantage of seed companies to sell hybrid seeds, 
rather than improving and selling open-pollinated varieties, so that their 
customers will have to buy new seed to get the same variety (and its 
improvements) the next year.

In addition to the cost to the grower of buying new seed each year, there are 
other costs and risks associated with using hybrid seed.  Production of hybrids 
generally involves either hand-labor (e.g. detasseling corn) or finding mutants 
(or, I suppose, creating them through genetic engineering) that cause the plants 
 to be male-sterile.  (Then, for a crop like corn, where we eat the seed, they 
also have to find or create genes to restore fertility).  Especially for 
self-pollinating crops, creating hybrids often involves large amounts of 
intricate hand labor.  Because of the cost of labor, hybrid seed production is 
done in developing countries, which often do not have the right climates for 
producing disease-free seed.  I know of several cases where, according to 
widespread rumor at least, seed-borne diseases of vegetable crops have been 
introduced into this country on a large scale with hybrid seed.  Mutations for 
male sterility also frequently weaken the plant, and, in one well-known case, 
the southern corn leaf blight of the 1970's made the plants vulnerable to a new 
race of plant pathogen.

An excellent book about this is called "First the Seed."  Unfortunately, I don't 
have the book at hand at the moment, and I can't recall the author's name.

I hope this is helpful.  

Kim Stoner
CT Agricultural Experiment Station
P.O. Box 1106
New Haven, CT 06504