Upon paging through a seed catalog, one can’t help but be impressed with the number of times the term “hybrid” is used. More and more vegetables (and flowers) are available as F1 hybrids. The cost of hybrid seed is equally impressive, prompting some frugal growers to attempt to save the seeds of hybrids for next year’s crop. The result usually is very disappointing; the following article will attempt to explain why.
By definition, a hybrid is simply the offspring that results from the mating of two individual with dissimilar genetic makeup. A more restrictive definition of hybrid is an individual that is the result of a cross between two inbred parents. The result, called a F1 hybrid, was created to exploit the phenomenon of hybrid vigor (or heterosis, as it is scientifically termed). The advantages of hybrid vigor include improved vigor, higher yields, earlier maturity, greater uniformity and an increase in the expression of certain traits.
In this day-and-age of being able to map the entire genetic make-up (genome) of plants and animals, scientists still are at a loss to explain why hybrid vigor occurs. A bit of genetics is required to explain this phenomenon further.
Most plants genetically are diploids meaning they have two sets of chromosomes–one from their male (pollen) parent and one from their female (egg) parent. Contained on these chromosomes are the genes responsible for the expression the various traits of the plant. When the gene(s) for a trait are the same on both the chromosome inherited from the pollen parent as that from the egg parent, the individual is said to be homozygous for that trait. Inbred plants are homozygous for all genes on their chromosomes.
Conversely, when the gene(s) are different the individual is termed heterozygous. For example, fruit color in tomato is controlled by the action of a single gene. Every tomato has two genes for the fruit color trait, one from each of its parents. Genetically, if we assume 'R'=red fruit, 'r'=pink fruit and red is dominant over pink, tomatoes with the genetic makeup of 'RR' and 'Rr' would both have red fruit. The former would be called homozygous for the gene (both genes are the same) for fruit color whereas the latter would be heterozygous (the genes differ). Since the (recessive) gene for pink fruit can only express itself in the absence of the (dominant) gene for red, pink-fruited plants genetically would be 'rr' and also homozygous.
We can use the above to illustrate why F1 hybrids do not "breed true". If a homozygous red-fruited breeding (RR) line were to be crossed with a homozygous pink-fruited (rr) breeding line, all of the F1 progeny would be Rr for fruit color and bear red fruit, since red is dominant over pink. However, when the heterozygous F1 pollinates itself, the result will be both red and pink fruited F2 plants in the ratio of 3 red for every 1 pink. The latter ratio was derived in the mid- 1800s by Gregor Mendel, who was one of the first people to study the inheritance of traits in plants.
Most economically important traits (e.g. vigor, yield) are controlled by the action of many genes. Some geneticists believe F1 hybrids are superior because they contain all of the favorable genes for a trait held by both of their parents. But, if this were true then at least some of their progeny should equal their F1 hybrid parents in performance, and this is not the case. Others believe it is the fact that corresponding genes for a trait are in a heterozygous state (differ from each other such as the 'Rr' red-fruited tomato) in the F1 hybrid when compared with either parent, but there are faults with this theory as well.
The method used to develop hybrid parental lines depends on whether the crop in question is self-pollinated or cross-pollinated. Self-pollinated plants such as tomato are high homozygous, as described above. Therefore, crossing any two inbred (e.g. heirloom) tomatoes will result in the production of F1 hybrid seed. However, in an attempt to form inbred breeding lines with numerous good traits, tomatoes are often crossed to form potential breeding lines. Seed is then saved from plants that possess the favorable traits from both parents. This must be done for six generations before the breeding line is considered to be homozygous and a good prospect to serve as a parent in the production of an F1 hybrid.
In contrast, cross-pollinated crops such as sweet corn are highly heterozygous. In order to form parental breeding lines, controlled self-pollination must take place for at least six years to form homozygous individuals. Each and every generation of self-pollination results in the loss of plant vigor making the parents of hybrid cross-pollinated plants unproductive, adding to seed cost. Fortunately, the vigor is more than regained in the F1 hybrid.
Suffice to say however they are produced, F1 are worth the added seed cost. Their development is painstaking and expensive. Many, many crosses must be made and evaluated before the plant breeder is likely to find a combination of parents that lead to an improvement for the trait(s) under improvement. Pollination must be strictly controlled and often is still done by hand in naturally self-pollinated species. Once a favorable combination of parents has been identified, the cross must be made each time seed of the F1 hybrid is wanted, since hybrid vigor only last one generation. Again, if the crop in question is self-pollinated this very tedious, time-consuming process must be done by hand.
When seed is saved from F1 hybrids, the resulting progeny tend (genetically) to revert back to the parents that were used to make the cross. The result is a loss of hybrid vigor and its benefits, along with disappointing performance. Again, it is important to remember that hybrid vigor lasts only one generation and results only when two parental lines are crossed. Therefore, saving the seeds from hybrids is not recommended.
Editor's note. In 2013 I had some ‘volunteer’ tomatoes come up where the F1 Hybrid 'Sunsugar' was growing in 2012. I also had a Sunsugar planted in 2013 elsewhere in the garden, so I could compare the progeny to it (I termed it Sunsugar and 'Son of Sunsugar'). The fruit was yellow, of similar size and really quite good, but not as sweet and not quite as tasty. The Sunsugar fruit had a somewhat translucent shine where ‘Son of Sunsugar’ was a bit opaque. I had these at MU Bradford Farm’ annual 'TomatoFest' and a number of individuals tried them and agreed, the F1 fruit was better than the progeny. I had no ability to evaluate vigor or anything else.
REVISED: November 23, 2015