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What I taught today: maternal effect genes

You know I teach the 8am courses every term, right? Every semester for years I get my oddball classes that weren’t present in the curriculum 13 years ago (when I started here) stuffed into the cracks of the schedule. I’m slowly getting to be a little pushier and am gradually making my way into wakier hours with other classes, but so far, developmental biology is still in the darkness. Fortunately, this talk was so jam-packed with excitement and action that they couldn’t possibly sleep through it! Right?

Just a word about the presentation slides: I’m a firm believer that less is more. My goal is not to display my lecture notes, or lists of bullet point slides that make my points for me, but to show complex and interesting illustrations that I talk about and explain — whoa, I know, how radical. I’ve sat through too many talks that flash 60-80 slides at me in an hour, and it’s too much. Take your time, people! That said, I used 18 slides in a 65 minute lecture today, which I felt was a little excessive — I aspire to someday do a lecture with half that number. But I am weak and need the crutch now.

Also, I returned exams today. People asked if I’d post their answers. No way in hell! These are exams and have the privilege of privacy. I will say that in general the students answered well. The goal of that kind of exam isn’t to confront students with a question that has a specific answer, but with a problem that they should explore, defend, or criticise.

So the subject today was maternal effect genes in Drosophila, specifically the prepatterning information that specifies the anterior-posterior and dorsal-ventral axes. Yes! I can tell you’re all excited!

So I gave them the precursor observations to the actual molecular biology, all this lovely modeling of gradients and information domains that was rich with Turing elegance, and then I dashed their optimism with the cold water of reality: molecular biology has shown that instead of beautifully designed systems, we’ve got bits and pieces cobbled together in a functional kludge. Any pretty patterns we do see are the product of brute force coding.

So they got the overall picture of A/P patterning in flies: a gradient of the Bicoid protein, high in front and low in back, is read by cells to determine their location — its the GPS signal of the early fly. The Nanos protein, also found in a gradient but from back to front, is a hack: it’s only purpose is to clear away a leaky remnant of another gene, Hunchback, which isn’t supposed to be expressed yet (although Nanos may be the diminished rump of a more elaborate ancestral posterior patterning scheme). And the Torso related genes are specifically involved in ‘capping’ the front and back ends of the fly.

The main point of interest about the terminal genes like Torso is their mechanism: we sometimes talk about maternal genes as like a paint-by-number system in which Mom lays out the lines for different areas of differentiation in Baby, and then the embryo fills in the details. The terminal genes are like a perfect example of that: in the follicle, cells literally paint the vitelline membrane of the fly with different informational molecules during the construction of the egg, and then as the embryo develops, these molecules trickle across the perivitelline space (a gap between the outer membrane and embryo proper) to bind receptors and trigger regional differentiation.

It’s also a nice segue into the dorsal/ventral patterning genes, because flies do something similar there: proteins imbedded in discrete regions of the vitelline membrane diffuse to Toll receptors, where they selective activate the Dorsal protein by freeing it from the Cactus inhibitor. We go from a paint-by-number kit to a restored gradient from back to belly side of localization of free Dorsal protein to the cell nucleus. By the way, in case they were getting bored with flies, Dorsal is homologous to NF-κB in us vertebrates, using the same nuclear exclusion/inclusion mechanism, and NF-κB is a hot molecule in biomedicine and cancer research right now.

That was my hour. I closed by threatening them with talk of zygotic genes, specifically the gap genes, next week.

Also, Wednesday we’re going to try something a little different. We’ve finished chapter 5 of Carroll’s book Endless Forms Most Beautiful so they should be ready to weigh the importance of various mechanisms, so I split the class in two and told half of them to read Wray’s article on the importance of cis-regulatory mutations in evolution, and Hoekstra and Coyne’s article that argues for a more balanced emphasis. I’d love to have a fight break out in the room.


  1. says

    My goal is not to display my lecture notes, or lists of bullet point slides that make my points for me, but to show complex and interesting illustrations that I talk about and explain — whoa, I know, how radical.

    I get that trick!
    For lectures where there is no mandatory attendance, many lecturers only give half the information on their slides and if you want to pass with a good grade then attending the lecture is paramount.

  2. says


    If the message of your talk can be encapsulated in 50 low information density powerpoint slides, you have no reason to be giving a talk in the first place. Send a memo instead.

  3. RFW says

    P-zed: I want to tell you that I appreciate these reports from the behind the lectern, and that I admire your dedication to your teaching and the care you take with it. I hope your students are suitably appreciative.

    Your attitude about the number of slides shown in a lecture and their content, in particular, is absolutely on point. When are you going to write a manual for university lecturers?

    Yes, this is pure, unadulterated fan mail.

  4. barp says

    Kudos on your slide minimalism. Can’t count how many lectures and seminars I’ve sat through that were simply a barrage of powerpoint slides, and they all were awful. It seems sometimes people have the notion that if enough info is on the slides, it exempts them from having to teach at all–the powerpoint does it for them.

    I took a course on the Twin Cities campus last semester on teaching in post-secondary settings, and this emphasis on “less is more” in terms of both number of slides, and information density on those slides, was one of the main sticking points. Also, though I often find the methods involved in education research to be a little questionable, and the conclusions typically overstated, there’s a fair amount of research to support the idea that beating your students over the head with a thousand slides doesn’t work.


    Though anyone who’s sat though enough terrible lectures could tell you that.

  5. evodevo says

    Yessss ! Patterning has always fascinated me – my questions are about the pattern setup in the follicle. How do the follicle cells set up the gradient? What keeps bicoid and nanos confined to a gradient instead of diffusing out into a uniform field over time? Etc. etc. My last information inputs were in the middle to late aughts, so I’m not really up on these questions.

  6. Menyambal --- son of a son of a bachelor says

    I agree fully on the lecture/visual presentation ratio, and applaud your efforts to improve it. Spread the word!

    I also applaud the biology in this post. I understood enough of it to feel proud of myself, and enough to know how much more fascinating knowledge is out there that I don’t know. And never will.

    You don’t get as many comments on your scientific posts, possibly because there is noting to argue about. So I’m commenting to say thanks for the science.

    Thank you, PZ.

  7. ChasCPeterson says

    People asked if I’d post their answers. No way in hell!

    I think somebody meant to ask if you’d post the answers, not students’ attempts.

  8. thecalmone says

    Just out of interest (from an ex student teacher) – where do you get the images that you use on your slides? Are they yours, or maybe lifted from the course textbook?

  9. says

    The egg shapes the embryo by different molecules on the membrane? That is cool!

    No questions this time. Though I’m still curious how a cell determines it’s an anchor cell, and what happens if two decide that.

  10. chrislawson says

    I’ve used a lot of slides in the past, but they were more as an aid to my peculiarly digressive style rather than a lot of high-density information. And in those slides, I’ve clearly marked the ones that do have the critical information for students who go back to the slides for revision, plus I usually add a 10-12 slide addendum that is a mini-quiz, again for revision purposes.

    Having said that, it’s a technique I’m losing interest in partly because of the amount of work involved, and partly because our uni is moving away from traditional lectures and using the PowerPoint slides plus voice-over that the students can play in their own time. Frankly, I’m starting to favour abandoning the lecture altogether except for (i) key presentations of major new findings, (ii) special interest lectures, e.g. a visiting lecturer with something special to say, and (iii) course introductions, where getting the students and teachers into the same space together serves an important social function.

    I’m even going off the VOPP (voice over powerpoint) approach and thinking of doing future “lectures” as web pages that can be read cross-platform with links to key papers/guidelines, low-res video, and small embedded HTML5 animations. Again the problem is workload, but I see this as the most effective and relatively obsolescence-proof method of disseminating didactic information.

  11. chrislawson says

    Giliell, I besnark lecturers who do that. My feeling (as both a past student and a current lecturer) is that if students don’t like turning up to your lectures, the problem is the lecture and not the students.

  12. Sunday Afternoon says

    The CEO at my previous company once spoke to us, an audience of 300+, for 90 minutes in a rented movie theater with just one slide on the overhead projector. He had a very clear idea of what he wanted to say and cued off the single slide to discuss them.

    I learned a lot about public speaking that day.