Question 19M.2.HL.TZ2.f
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.HL.TZ2.f |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Deduce | Question number | f | Adapted from | N/A |
Organisms often release chemicals when attacked as part of their defence system. Scientists studied lima bean plants (Phaseolus lunatus) infested with either an armyworm, Spodoptera exigua, or a herbivorous mite, Tetranychus urticae. Both organisms feed on lima bean leaves, causing the leaves to release chemicals.
[Source: https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png]
![https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png]](https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png%5D){kind=link}
The study was conducted to see which defence chemicals were produced by lima bean leaves when infested by armyworms or herbivorous mites. The scientists identified a mixture of compounds (C) released by the plant when attacked. Two of the chemicals in this mixture were identified (C1 and C2).
The scientists hypothesized that the defence chemicals in C act as signals to produce other chemicals (X, Y and Z) that are also involved in the defence of the plant.
The graphs show the amounts of chemicals X, Y and Z produced when the plants were infested by either one of the two herbivores or treated with the different chemicals C1 or C2.
[Source: R Ozawa and G Arimura, Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the
Production of Specific Herbivore-Induced Volatiles in Plants, Plant and Cell Physiology, 2000, 41, 4, 391–398,
by permission of Oxford University Press]
RNA was collected from leaves of the plants after each treatment (armyworm, herbivorous mite and the chemicals C1 and C2). DNA copies of the extracted RNA were made by a process called reverse transcription. Targeted genes in the DNA were then amplified.
The scientists then used the transcribed DNA of each treatment to study the activation of three genes of the plants’ defence system. The DNA was separated by gel electrophoresis. The activation was tested one hour after treatment and again after 24 hours.
[Source: R Ozawa and G Arimura, Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the
Production of Specific Herbivore-Induced Volatiles in Plants, Plant and Cell Physiology, 2000, 41, 4, 391–398,
by permission of Oxford University Press]
Using the gene activation data, deduce, giving two reasons, whether the armyworm or the herbivorous mite has infested lima bean plants over a longer period of time.
[2]
a. the greater «gene expression» response of the lima bean plant to the mite infestation indicates a longer evolutionary relationship ✔ OWTTE.
b. herbivorous mites cause more genes to be expressed/higher intensity of gene activation ✔
c. herbivorous mites cause a more immediate/earlier response in gene activation ✔
Students were able to identify the mite as the species that had infected the plant for a longer period of time. Fewer understood that the question was referencing evolutionary history. Some students were referencing the time period of data collection for the investigation.
