Desirable Events are Rare
Desirable events used by the plant breeder are those that have the transgene inserted in a part of the chromosome that allows for expression of the gene without having a disruptive influence on other genes in the plant (resident genes). However, most events are discarded or eliminated during the process of developing a genetically engineered crop variety or hybrid. A large percentage of the events produced by the genetic engineer never even leave the lab or greenhouse. Others are rejected early in the plant breeding and field evaluation stages leaving relatively few for the plant breeder and agronomist to work with.
For example all the Bt corn hybrids marketed in Nebraska originated from only three or four events. The elimination of events may be for many reasons, such as a mutation or low gene expression. Consequently, the genetic engineer must be able to produce a large number of events before finding a winner.
Often times, people envision that during transformation the transgene pushes another gene out so it can take the original gene’s place. Actually, when a transgene is inserted into the DNA of a chromosome, it does so in a fashion similar to us cutting in line at the supermarket. The chromosome’s DNA breaks and the transgene inserts. One copy or many copies, including the marker gene, will insert into this site. A problem can occur, though, if the new gene inserts itself right in the middle of a resident gene. In this case, the DNA code of the resident gene will be disrupted and the resident gene will be unable to encode the correct RNA message.
It can be compared to adding extra letters in the middle of a word. The word will no longer make sense. If this gene is critical for plant growth or development, the plant will die. If the gene is important for a trait, such as root development in low pH soils, the mutation will not be noticed until the plant is placed in those conditions. It is the job of the plant breeder to screen transgenic crops against negative mutations.
A new transgene inserts into the chromosome without replacing any of the resident DNA. This transgene is inserting into the middle of a resident photosynthetic enzyme gene. This will disrupt the resident gene preventing its expression and possibly causing a reduction in yield potential or even death to the plant. (Image credit: P. Hain)
Another potential problem is having a transgenic plant with only a low level of transgene expression. Chromosomes have segments of DNA called ’junk DNA’. These areas may code for genes that are not used often or may be lengths of DNA with an unknown purpose. When the transgene inserts into one of these less used areas, its expression may be very low or even non-existent. Therefore, transgenic plants must also be screened for the correct level of expression. Low expression can also happen if the transgene’s code is somehow altered or disrupted during the transformation steps leading up to this point.
The following table summarizes examples of possible events that are rejected by genetic engineers.
| Type of Event | Effect of Event | Event Rejection |
|---|---|---|
| 1) The transgene is inserted into a part of the chromosome that contains a 'resident gene' that is critical for cell survival or development | 'Knocks out' the critical resident gene and the cell dies or cannot divide | By the genetic engineer during the selection process imposed on cells in tissue culture |
| 2) The transgene inserts into a part of the chromosome that does not allow for any expression of the gene. | Plants derived from this event may contain the gene but never expresses it. | By the genetic engineer during the selection process imposed on plants coming out of tissue culture |
| 3) The transgene inserts into a resident gene that is necessary for reproduction. | The transgenic plant cannot pass this event on to offspring. | By the geneticist in the early stages of selfing or backcrossing |
| 4) The transgene inserts into the chromosome, is expressed and passed on but is 'silenced' in the next generation or in later generations. | The offspring of transformed plants fail to express the transgene. | By the plant breeder in the early stages of selfing or backcrossing |
| 5) The transgene inserts into a part of the chromosome that genes are not normally found, or has a alteration in the sequence of the inserted DNA. | Expression occurs but is not optimal to control the desired trait. | Discovered during field evaluations by the plant breeder or agronomist. |
| 6) The transgene inserts in a resident gene that controls plant development in a more subtle fashion. This eventually influences agronomic traits and yield. The influence may be observed in some but not all environments or in all genetic backgrounds. | The plant with this event may have all the desired characteristics the genetic engineer is looking for but also has the potential for yield drag. | Discovered during field evaluations by the plant breeder, agronomist or farmer. |