Stacking up the traits; The role of plant breeding and field testing
Our agronomist recognized that the nature of genetic engineering made it imperative that they pay attention to yield and agronomic performance and not only transgene design when evaluating the attributes of transgenic hybrids. This was because the genetic engineering process had a significant bottleneck in the gene introduction stage and conventional breeding was still an important component of creating a new hybrid.
The gene introduction or genetic transformation process is labor intensive and requires repeated attempts to insure success. In corn, significant progress has been made in recent years to coop the natural genetic engineering capabilities of Agrobacterium tumefaciens. (Read more about agrobacterium tumefaciens.) Furthermore, the genetic engineers have devised strategies that allow this natural genetic engineer to transform cultured tissues and cells of a wider range of corn inbreds, including those used by the plant breeder to make elite hybrids. However, transgenic events commercialized in 2002 were generated more than five years ago when transformation methods were less sophisticated (i.e., the gene gun) and a limited number of inbreds could be cultured for transformation and regeneration. Therefore, the newest hybrids are a product of work started between five to ten years ago. The process of regulatory approval adds additional time to commercialization. Each event is evaluated in the U.S. for its impact on the safety of food (FDA), environment (EPA) and other crops (USDA-APHIS). International approval takes longer. While it may be possible to generate new events with three or more transgenes, it is more feasible for companies today to stack combinations of traits controlled by approved events into a hybrid through conventional breeding. In general, both small and large seed companies use the following procedure in developing a new transgenic hybrid.
1) Make crosses between parent lines that do not have transgenes and evaluate inbreds from these crosses to find parents that make higher yielding hybrids.
2) Decide which hybrids would benefit from the characteristic controlled by a specific transgenic event.
3) Use backcrossing for three or more generations to create a converted inbred, one that possesses the transgene and as much of the inbred’s genetic identity as possible.
4) Self the converted inbred to establish a pure line and generate parent seed for hybrid production and evaluation.
Stacking two events into the same hybrid is accomplished with the following alternatives.
5) Identify two converted inbreds that make a high yielding hybrid. Each converted inbred must have different events that are desired in combination.
6) Identify a converted inbred that you think will still make a commercially viable hybrid and backcross a second event into this inbred.
Stacking a combination of three or more events requires the use of combinations of steps 5 and 6.
Prior to the use of transgenics, plant breeders spent a vast majority of their time on step 1. Occasionally, a single gene trait would be identified as a result of mutations that provided the hybrid with traits such as disease resistance, herbicide resistance, seed color or cob color. In those instances backcrossing (steps2 - 4) would be used if the goal was to get the quickest and most predictable result. These backcrossing steps have become a larger proportion of a plant breeding company’s efforts as more transgenes are introduced into crops.
Understanding where plant breeding fits into the picture of transgenic hybrid development has made our agronomist a more critical evaluator of hybrid performance information. They recognize that step 3 may not result in the plant breeder developing a converted inbred with the same attributes of the original non-transgenic version. Therefore, they make sure that performance data they make decisions on does not combine data from previous yield tests of the non-converted hybrid.
The agronomist also recognizes that steps 3 and 4 take time. Since the transgenes introduced so far are designed to protect yield and not increase yield potential, there is the potential for transgenic hybrids to lag in their yield potential relative to the latest hybrids produced via step 1. Yield lag may not be an issue where insect or weed pressure are a major limitation on yield and the transgenes help manage these problems. Consequently, our agronomist considers carefully the need for each trait in their client’s production environments.
The agronomist likewise recognizes that each transgene alters metabolism in the cells it is expressed to some degree and has the potential to cause a drag on yield potential. Transgenes that direct the plant to encode more copies of a protein will have the biggest impact on metabolism. Yield drag may not be an issue with a single event. However, stacking events compounds their overall impact on metabolism and the plant’s yield potential. Again, yield drag or the outcome of gene stacking is not easy to predict or to measure. For our agronomist, there is no substitute for careful evaluation of the yield data and they work hard to obtain good information for their clients.