Tag: endosymbiosis

The 10 minute in-class essay

When I was a student I hated writing tests and examinations with the intensity of a thousand flaming suns. Now that I get to set the curriculum, I actively look for alternative assignments to tests and exams that I can use in my classes. One assignment that I’ve had a lot of success with is the 10 minute in-class essay. It’s quick to administer, flexible, and doesn’t take a lot of time to mark.

At the beginning of the semester I tell my students that I will be offering 7 opportunities to write an in-class essay during the course and that they should make sure that they always bring a few sheets of paper and a pen with them to class. I ask that my students complete 5 of the in-class essays. Each essay is worth 2% and the total towards their final grade is therefore 10%. The dates of the in-class essays are not disclosed to the students, but at the beginning of the semester I work out which dates I’m going to be offering the essay opportunities and try to spread them out over the semester. This is where the flexibility comes in. Since I only require 5 essays to be completed, a student could miss 2 opportunities completely (due to illness or other life events) or could do poorly on a few and still earn a good mark on this assignment category. A student could complete all 7 and I will then calculate their mark based on the best 5 essays. The assignment also encourages students to come to class because they don’t know when I’ll offer an in-class essay and it forces students to keep up with the material because the essays are based on the content of that class or the class before. The students like the flexibility of the assignment and the fact that each one is low stakes, so it doesn’t cause a lot of stress. It also provides me a quick way to check in with my students to see if an important concept is not being taught or learned effectively.

Prior to a class in which I’m going to do an in-class essay I put together a slide that contains the question that I would like the students to answer. I try to make this question as open ended as possible and ideally it could have several correct answers depending on how the students are interacting with the course content. In class we do our normal activities and lecture and I use 15 minutes of our in-class time to run the assignment. It takes the students a few minutes to get their paper and pen out and to put away their notes and laptops. I put the question up on the screen and give them 10 minutes to write a few paragraphs in order to answer the question. I circulate around the room to address any issues and then collect the essays. Depending on my schedule I run the in-class essays at the beginning of class (this very effectively discourages tardiness), the middle of class, or at the end of class. In the next class period I take 5 minutes to explain several of the possible answers and to clarify concepts if I saw that common mistakes were being made.

So what does one of these questions look like? As an example, in my Endosymbiotic Theory course we do a section on the origin of mitochondria. The prevailing hypothesis is that mitochondria were once free living bacteria until they invaded or were ingested by another cell. We talk in class about the various lines of scientific evidence that support this particular hypothesis. My in-class essay for this section of the course is the following:

 Antibiotics in medicine that are used to treat infections in people are very effective in disrupting metabolism, cell walls, membranes, transcription and/or translation in bacteria. Given the hypothesis that mitochondria are derived from bacteria, explain why antibiotics are not toxic to humans.

An answer that would earn the full 2% might talk about how mitochondria possess an outer membrane derived from the host organism and an inner membrane derived from the bacterium. They might suggest that the antibiotics have no effect on the outer membrane or cannot penetrate it and therefore the mitochondria are impervious to the drug’s mode of action.

An answer that would earn 1% would talk vaguely about membranes, but wouldn’t clearly explain the rationale behind the answer.

An answer that would earn 0% would be one where the student puts seemingly random facts down on the page that don’t form a cohesive answer to the question.

There are alternative answers to this question that do not involve membranes (e.g. gene transfer to the nucleus, loss of drug targets over evolutionary time, etc.) and if the student is able to make a compelling and convincing argument to answer the question I am flexible in the answers that I accept.

I find the 10 minute in-class essay to be a great evaluation tool and an effective way of assessing what my students are taking away from my classroom. I am often impressed by the answers that my students provide which reveal their creativity and problem-solving abilities.

Advertisements

Weevil Knievel: Daring to destroy endosymbionts that are no longer useful

I teach a fourth year undergraduate course where I introduce students to a wide range of bizarre and interesting endosymbiotic relationships. I have often wondered how transient these relationships are and have assumed that one partner or the other might continuously be trying to get the upper hand in the relationship. A recent research paper explores this question in greater detail in insects (h/t to Tristan Long for passing along the article).
Vigneron and co-workers explored the relationship between the weevil Sitophilus and its endosymbiont Sodalis pierantonius . They show that young adult weevils have very high numbers of the endosymbiont in their guts in order to generate the large amounts of tyrosine and phenylalanine required to make dihydroxyphenylalanine (DOPA) which is in turn required to make the cuticle essential for their exoskeleton. Once the cuticle is completed, these high DOPA levels lead to the active elimination of the endosymbionts in gut tissues. The endosymbionts are recycled using a combination of autophagy and programmed cell death. In contrast, the endosymbiont populations in reproductive tissues remain unharmed.
Using a combination of fluorescent in situ hybridization and scanning and transmission electron microscopy the authors clearly show that the weevils can effectively modulate the size and number of bacteriomes (structures that house the endosymbionts) throughout development. The images in the paper are amazing! The authors propose that such co-ordinated and targeted endosymbiont destruction avoids inflammation and the induction of the immune system and that this recycling may allow the weevil to recover some of the metabolites and energy invested in the earlier stages of the relationship. This is a really cool example of co-evolution! At first glance it seems that the weevil has the upper hand in this relationship, but it’s also important to remember that the endosymbiont has still managed to ensure its transmission to the next generation in germ-line tissues which is no small feat.

Citation: Vigneron et al., 2014. Insects Recycle Endosymbionts when the Benefit Is Over. Current Biology. 24: 2267-2273.