Question 19M.2.SL.TZ2.j
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.SL.TZ2.j |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Discuss | Question number | j | Adapted from | N/A |
Mice (Mus musculus) have various defence systems against predators such as foxes (Vulpus vulpus) or stoats (Mustela erminea). The mice release specific alarm compounds when under threat that serve as danger signals for other mice. Predators also release scents that the mice can detect.
In one study, mice were exposed to paper soaked in compounds taken from other mice, foxes or stoats in a test chamber. The scientists then measured the reaction of three different groups of neurons used in smelling: G1, G2 and G3, as shown in the diagram.
The percentage of G1, G2 and G3 neurons responding to the mouse, fox and stoat compounds, as well as a control compound, are shown in the chart.
Percentage of response of neurons in mice
[Source: Julien Brechbühl, et al., (2013), PNAS, 110 (12), pages 4762–4767]
The scientists recorded the neural traces (action and resting potentials) of six G1 neurons exposed to the compounds from the different animals. The horizontal bar beneath each trace corresponds to the time of exposure to the test compound.
[Source: adapted from J Brechbühl, et al., (2013), PNAS, 110 (12), pages 4762–4767]
Another study was carried out on the defence systems in aphids (Myzus persicae) which feed on thale cress plants (Arabidopsis thaliana). Individual aphids release an alarm compound containing the chemical (E)-β-farnesene (EBF) when attacked by a predator. Other aphids are repelled and leave the thale cress plant or hide.
The gene for EBF was inserted to produce transgenic (T) thale cress plants to test aphid reaction to EBF over several generations. Aphids were raised on wild type (W) thale cress and then allowed to feed for three generations on either W or T thale cress. Aphids were tested in each generation for their reaction when EBF was added to a choice chamber. The percentages of aphids attracted to or repelled by EBF and those which made no choice were recorded.
[Source: M De Vos, et al., (2010), PNAS, 107 (33), pages 14673–14678]
Discuss whether natural selection would favour the transgenic EBF-producing thale cress plants if they were released into the wild.
[2]
a. mutant aphids/varieties may be indifferent to/attracted to transgenic plants as these do not present a hazard «not favour» ✔
b. initially/for limited time the plants would thrive as the aphids would be «largely»repelled and thus not eat the plants «so natural selection would favour them» ✔
c. over time/in a few generations, the aphids population become more resistant/more attracted/less repelled to EBF and return to feed on the plants so long-term benefit very limited «so natural selection would not favour them» ✔
d. the aphids resistant to EBF would not respond to other aphid alarms and «likely»be more readily eaten by predators «so the long-term benefit to plants could be supported by natural selection» ✔
The answers must indicate whether natural selection would support or not for each statement.
There was some discussion in the G2 forms about the amount of reading before points h, I and j. Good candidates could be seen to have read it carefully, underling the key words. Weaker candidates not so. Very few candidates scored both marks in j as they just wrote about one possibility. Perhaps the answer box should have been larger.
