To know a plant, grow a plant!
Using Fast Plants to model how selection for a particular trait can affect future generations can be an effective way to introduce students to natural selection and associated concepts.
In the 2012/13 school year, Astrid and Hedi had an ongoing discussion about the new AP Biology Inquiry Lab with Fast Plants and an artificial selection experiment. Last week, Astrid opened a new discussion about that lab by posting the following, and we encourage all of our members with interest in selection experiments to join in on the discussion.
I'm back again after a rather successful first WFP lab experience with my first AP Bio students this past academic year. We chose trichomes as our trait for selection. And based on our statistical analysis, we concluded that the average number of trichomes expressed by our second generation plants were significantly greater compared to our first generation.
I'm currently in the process of grading the introductions my upcoming AP Bio students wrote for this actual lab as part of their summer assignment. There were some interesting alternative trait choices that I would like to run by this discussion group. I'm hoping to get some advice as to what trait(s) is (are) best to choose from besides trichomes.
I would assume that a given trait can only make the cut if it is at least partly dictated by genetics and not merely environment. I'm interested in your opinions and hopefully past experience.
Here are the suggestions my students came up with:
- plant height (judged I assume by stem length starting directly above the cotyledons until the apex of the plant)
- number of leaves
- number of flowers
Any other traits you can think of?
Thank you for your help.
How about the width of the first true leaf?
There was an idea that came in the group I was in during the workshop that I would like to know if you all think it might work. The idea was to select for salt tolerance. The water here in Midland Texas has a lot of dissolved solids and the question would be can you select for fast plants are can thrive on this water.
All I want to say is----Go For It!
Salt Tolerance would be a great trait to select.....
You’ve posted a very good question here; one of the most important considerations in conducting the artificial selection experiment is choosing the trait to be selected. You assume correctly that it is preferable to select on a trait that is more strongly controlled by genetics as opposed to environment. In fact, much of the reason that the AP Bio lab manual advocates trichomes as a trait of selection is because Fast Plants’ expression of trichomes is largely controlled by genetics and minimally affected by environmental variables.
Your students have come up with some interesting suggestions for other potentially selectable traits. The “Artificial Selection Investigation” group in which this discussion is posted was originally investigating the trait of plant height. (See this link for our results: http://fastplants.ning.com/group/artificial-selection-investigation...). We unfortunately did not see a significant increase in plant height between our parent and offspring generations, which we have attributed to different growing environments between the two generations (see the link above for more details as related to this experiment). That being said, a fellow Ning member, Mattia Rossi, conducted the selection experiment with his students (but they selected for the shortest plants) and his data reflects a successful selection. (Mattia’s data can be found at the link above as well, in the comments section.)
As for the number of flowers on a plant, this trait will be heavily influenced by the amount of nutrients that the plant has access to. A well-fed plant will produce many more flowers than a hungry/starving plant.
I would recommend that you steer clear of trying to select for the number of leaves on a plant. In our experience this trait is nearly fixed within a given range and does not have enough variation to be selected upon.
One trait I would suggest as having potential for this experiment is tolerance to salinity. By ensuring that both generations of plants receive approximately the same nutrients and light levels, the saline conditions act as the selective pressure and seed yield can be used as a metric to determine gain from selection. Though our sample sizes were small, a trial run of the selection experiment using salinity tolerance suggests that this trait can indeed be selected upon.
The most important piece of advice I can give to ensure accurate results is to control the environment, especially temperature, nutrition, and light. Firstly, because it can be difficult to control temperature in a classroom, I would recommend that you avoid selecting for any traits that are related to timing of development (temperature has a BIG effect on speed of development). Nutrition can be controlled for by screening/sifting osmocote pellets and using only pellets of a uniform size. Additionally, soluble nutrient solutions can be mixed to specific concentrations to ensure even nutrition across generations. Finally, you can control for light by adjusting the position of the plants (or the lights) daily so that the growing apices are within a consistent distance from the light source.
Phew, that got a bit long winded. Feel free to ask for more detail or clarification on any of the points above. Hope this was helpful!
Nice reply, Jackson--lots of info there to consider.
Thank you very much, Jackson, for your helpful and thorough message. Please allow me to summarize your points so as to make sure I'm getting everything straight. Take note that trichome number remains a fine trait for selection and would be a fine choice to pursue for this upcoming year again. Please focus on my point 5 as it contains several questions regarding salinity.
1) Selecting for the tallest plant did not show a significant increase in plant height for the second generation.
2) Selecting for the shortest plant did show a significant decrease in plant height for the second generation.
3) Selecting for number of flowers is not recommended as the trait is too much influenced by the environment.
4) Selecting for number of leaves is not recommended as the trait does not show a wide enough spread within a given population.
5) Selecting for salinity is recommended and salinity tolerance is based on the number of seeds produced by a given generation under a given salinity percentage.
Regarding salinity: If we don't end up choosing this trait, instead this may be an excellent end of the year project for one or more of my student lab groups. Jackson, would you be able to detail the salinity conditions you used for your plants? In particular, what type of salt (NaCl I would presume) did you use and what salt concentration (molarity) did you use to hydrate your plants with? What is considered to be the optimal physiological salt concentration for WFP? For the artificial selection lab, did you simply choose a set salt concentration for the entire experiment? Or did you instead expose your plants to a range of salt concentrations? Did you consider different salts as well?
Thank you for your help in answering these questions. It certainly makes you think about other options.
I would be interested in the same salinity information Astrid requested. We cannot do salt tolerance with our students because they do a huge salt/Fast Plant experiment in the middle school and are not thrilled about the idea of another one. However, I would like to try it and see what happens.
I would expect that some plants are going to "look healthier" but when you pollinated, what was your criteria for selection?
There has been quite a bit of research done on Arabidopsis (sos1 mutants that are salt overly sensitive) that indicates salt tolerance/intolerance has a genetic basis.
Whitney, when pollinating my salted plants, I mass pollinated all the plants rather than selecting individuals. I've been referring to this as a passive selection due to the environmental pressure of salt; some plants will be unable to produce pollen or ovules as a result of their salt exposure. The plants that are able to produce seed have then passed on their genes and were effectively "selected" for.
If the learning goal for selection experiments is that students come to understand what all is involved in selection and how complex the interplay of genetics and environment are in determining phenotype, then choosing any trait can be effective for students to analyze. Results in a single generation are going to be insufficient to draw firm conclusions about selection, anyway -- let's keep in mind that engaging in inquiry to learn science concepts is a process, and our goal isn't to have perfect results that simply confirm what we've taught. It's the challenge of wrestling with data that is the kind of rigor we're looking for students to engage in, so I encourage us all to avoid being overly cautious and prescriptive about trait selection. There's no harm in students' data showing that the distribution of the trait they selected for may have been caused by environment. In fact, they may learn more about being a good skeptic and the importance of controls if that is the case . . .
1. Jackson, Hedi, Paul and McKaylee are going to be conducting another salinity selection experiment here at UW that Jackson will blog about on a regular basis --starting in the next week or two.
2. No need to rule out height entirely (short or tall), just realize that environment is a big factor and carefully consider both where you will measure height (e.g. soil surface to first flower may help compensate for developmental differences) and how you will control & measure key environmental factors.
Astrid, your summaries are correct. Regarding salinity, my "saline conditions" consisted of 4 salt treatments given to the plants on 3 Days After Planting (DAP), 7DAP, 10DAP, and 14DAP. Each treatment consisted of 5-6mL salt water applied by pipet to each plant. The salt water was mixed as 12g of non-iodized sea salt (NaCl) per liter of H20. This concentration was determined by growing plants in a range of salt concentrations; we chose 12g/L because it significantly affects most plants, but does not outright kill them. We grew our plants in styrofoam quad-cell-packs (~7ccs in volume), but keep in mind that with a larger soil volume, a higher salt concentration may be necessary to observe significant selective pressure. At this point, we have only considered using the sea salt, though investigations with other salts could be interesting!
Thank you both Hedi and Jackson for your careful reply!
Hedi, I very much understand your point of view when you wrote: "....our goal isn't to have perfect results that simply confirm what we've taught. It's the challenge of wrestling with data that is the kind of rigor we're looking for students to engage in....."
Perhaps it's as if teachers wish for their students to get the expected results, because it so nicely fits in the preconceived box we wish to place certain things in. I may have made this point on this forum before - not sure - but I had to leave basic research after about 7 years, because I just could not deal with the inconsistent data anymore. Despite the fact that I got three publications out of my research about a decade ago, two of which involved help from other lab members, I'm so relieved to no longer be full-time in the lab. And yes, this may mean that I may have more of a tendency to "save" my students from aberrant or confusing data that does not fit the going theory. I realize I may be doing a disservice to my students as much discovery is made when data does not fit the status quo and new theories as a result can be developed. But, hey, that's another discussion.....
Thank you, Jackson. That was very helpful the procedures you used for the salinity studies. I will take note of your suggestions for our May end-of-year project:
1) 12g NaCl / Liter H2O - This amounts to about a 0.2 M NaCl solution
2) 5-6mL on 3DAP, 7DAP, 10DAP, 14DAP per plant
I'll be on the look-out for your upcoming salinity blog:-)