The Research Story, Part 2 (Continued)

One of the reasons Javier collected kochia seed from around the state (Figure 5) was to make a more comprehensive assessment of kochia populations response to dicamba.  Sampling in only one part of the state may not have provided as accurate an assessment.  Second, if kochia in the western part of the state was not as affected (less susceptible) by dicamba compared to the eastern part of the state, then that would suggest that the more frequent use of dicamba on wheat production in the west had already resulted in evolution.  If that were the case, then the risk of it happening in dicamba-resistant soybean would be high if the technology were not managed appropriately.    

How does a change in a plant population take place?  There are two main factors that are required for a change to take place:  selection pressure and time.  When there is a high amount of selection pressure placed on a population, (i.e., dicamba applied repeatedly to a particular population of kochia) over a period of time it creates an opportunity for populations of plants to change or evolve genetically. 

Understanding the biology of a plant is important to understand how a population evolves.  Kochia flowers can be either perfect or imperfect.  Perfect flowers contain both male and female reproductive structures.  Imperfect flowers on kochia are pistillate, which means they contain only female reproductive structures (Friesen et al. 2009).  Perfect flowers enable kochia to potentially self-pollinate, but imperfect flowers require cross pollination, either from perfect flowers on the same plant, or from perfect flowers on nearby plants. Cross-pollination allows the genes from one plant to combine with the genes of another plant.  More genetic variability occurs when plants cross-pollinate.

Genetic variability exists within a population and between populations.  For example, in a kochia population there may be some plants that are susceptible, and other plants that are resistant to a given herbicide.  In the case of kochia treated with dicamba we would expect that the number of susceptible plants compared to resistant individuals would be very large if the population had not previously been treated with dicamba.  When the number of susceptible individuals is greater than resistant individuals the herbicide works.  The goal is to help ensure that there are more susceptible plants than resistant plants, so the proper use of herbicides should preserve this in the population.  For more details on the evolution of a population see the following reading on “Evolution."

Maintaining genetic variability in the population keeps the susceptible allele (version of a gene) present.  Herbicide resistance traits may be passed on a number of ways.  However, if resistance is passed on by a single, dominant allele, then we can expect the following result:

The allele passing on resistance is represented by “R,” and the susceptible allele is represented by “r.”  Plants with the genetic makeup of RR or Rr will be resistant to the herbicide.  Only plants with a genetic makeup of “rr” will be susceptible.  If two susceptible (rr) plants cross-pollinate then only susceptible offspring (seed and ultimately plants) will be produced.  If two RR resistant plants cross, then all offspring will be RR and resistant to the herbicide.  When RR crosses with Rr all offspring will be resistant but half of them will have the susceptible allele.  When an Rr individual crosses with another Rr individual then ¾ of the offspring will be resistant but ¼ will be susceptible.  The percentage of susceptible offspring produced increases when an Rr plant crosses with an rr susceptible plant to give ½ of the offspring being susceptible.  See Table 1 for a visual depiction of these crosses.

Because we cannot control which kochia plants cross-pollinate it is very beneficial to help ensure many susceptible plants (and susceptible alleles) are present in the population.  How do you maintain susceptible plants in a population?