Time for another reflection on my mundane week of teaching. I know this is unexciting, but I’m trying to be self-aware about what I’m doing in the class.

I’ve already summarized some of what I did this week: we explored the meaning of “epigenetics”, and I made a big push to get them to think critically about the papers we’re reading. They’re supposed to be developing a topic they’ll explore independently, so I’ve had them doing library work to find a line of research they find interesting, and master the skill of extracting the key questions the work is trying to address. I’ve got a small stack of short papers that I’m going to read this weekend and we’ll see how well they can do that.

We also discussed symbiotic interactions in development, and next week the topic is other environmental effects. They are getting much, much better at opening up and talking at the miserable hour of 8am.

The other regular highlight of my week is FlyDay, when I have to scrub dead maggots and pupae out of fly bottles. I had to postpone FlyDay this week! Yesterday I was scheduled to meet with students and parents visiting the university to confirm their plans to attend, and I was all spiffed up in a nice suit, which isn’t the best thing to wear when one is flicking bits of chitin and gooey medium around. I went in early this morning to scrub bottles and get them cooking in the autoclave.

By the way, at that student meeting I was the official biology representative, and although biology is currently the largest major on campus, almost no one stopped by to talk to me. It might have been my terrifying glare, or my sciencey reek, but no: it was because there was a separate table for the pre-professional programs (pre-med, pre-vet, pre-dental, etc.). This is a minor peeve of mine: this is not 19th century England. You do not graduate from your public school education and go straight into medical school — no, here in 21st century America you get a broad-based undergraduate education first, and then you apply to med school. You should be thinking about your liberal arts education first, and in a couple of years we’ll start coaching you on how to get into those professional programs.

Oh, well. They ignore me now, but I know that I’ll get my claws on most of them soon — they’ll want all those bio classes to prep them for the MCATs.

I should mention that I am teaching another course beyond ecological development — I’m teaching a lab course on transmission genetics. They’ve been doing crosses with flies all semester long, and we’re getting to an interesting point.

The first half semester we’re doing a mapping cross, using recombination to estimate the distances between a couple of genes on the X chromosome. We’re using flies that are mutant for eye color (white, w), wing length (miniature, m), and bristle morphology (forked, f), and I’ve also got a few groups mapping body color (yellow, y), wing veins (crossveinless, cv) and forked, f; the latter are doing a pilot test to see if I want to add that cross to our regular repertoire.

The way this works is that they are given wild type and triple mutant flies. I first have them raise a new generation of the purebred stock, simply to get a little practice in sexing flies and basic skills in growing them. So they first do these crosses:

♀w^– m^– f^–/w^– m^– f^– x ♂w^– m^– f^–/Y

which produces bottles full of homozygous white-eyed, miniature-winged, forked-bristled flies, and

♀w⁺ m⁺ f⁺/w⁺ m⁺ f⁺ x ♂w⁺ m⁺ f⁺/Y

which produces bottles full of homozygous wild type flies.

Then I have them do a reciprocal cross of flies from the two bottles. These are X-linked traits, so it matters which strain is the mother and which the father, and I want them to see that. That is, they cross wild type females to triple mutant males, like so:

♀w⁺ m⁺ f⁺/w⁺ m⁺ f⁺ x ♂w^– m^– f^–/Y,

which produces progeny that are all wild type, both male and female (they all inherit the dominant wild type allele at all loci from their mothers). After they’ve scored the flies from this cross, we dispose of them all and don’t think any further about them.

They also cross mutant females to wild type males, like this:

♀w^– m^– f^–/w^– m^– f^– x ♂w⁺ m⁺ f⁺/Y.

That has the useful result that all the sons inherit w^– m^– f^– from their mother and a Y chromosome from their father, so they all express the mutant phenotype. The daughters, however, are all heterozygous, inheriting the mutant alleles from their mother and a wild type chromosome from their father, so their genotype is:

♀w^– m^– f^–/w⁺ m⁺ f⁺

Now the fun begins. Meiotic recombination in those flies will rearrange the +’s and -‘s in those chromosomes with a frequency dependent on their distance from one another — you’ll get less recombination between genes that are close to one another.

This week, they completed the reciprocal cross and got their heterozygous females and mutant males. Yay! That worked. They are now setting up a test cross to assess recombination frequencies.

I just want to say that I think I planned everything perfectly. That test cross will be ready to score next week, which is the week before spring break, which means we’ll have the data for all the calculations before they leave, and when they get back, I’ll be able to lead them through all the theory. It also means I’ll be able to purge a lot of fly bottles and get them scrubbed up over the break (you can tell already that I have glamorous plans for my short vacation). Trust me, though, this is good — there have been semesters where, due to student error, the flies haven’t been ready, and then my spring break is spent maintaining 120 bottles of student flies.

It also means we can launch into the next experiment as soon as they get back: we’re going to do a complementation cross between two eye color mutants, brown eye (bw) and scarlet eye (st). If I’ve got this one all timed out correctly, we’ll be getting F2 results of crosses between heterozygotes for both loci a week before the end of classes.

Now you know. I choreograph fly sex for my convenience.

Next up, I have to choreograph my schedule. It turns out I have been summoned to Howard Hughes headquarters on 8 March and 18 April, which punch big holes in my planned lessons, and which I hadn’t accounted for in my syllabi. I’m going to have to juggle lectures and exams and rearrange the order of various things in a big way this coming week.

Today in my class we talked for a while about epigenetics. I used it as an example of a term we’d encountered more than once in our ecological developmental biology course, but that has some complicated ambiguity and fuzziness that has led to all kinds of weird popular confusions about the subject. I was also using it as an example of critical analysis of a paper, as I discussed yesterday, and it was a lead-up to having the students discuss papers on relevant topics they were interested in — so we spent most of our time talking about other things.

But I’m going to talk now about just this one paper I read. You see, Larry Moran and I have been having this long-running disagreement about epigenetics — nothing hostile, just an occasional cocked eyebrow in each other’s direction — which you can see on display in this article by Larry on epigenetics, in which he disagrees with my definition of epigenetics, back in 2008. Here’s my definition:

Epigenetics is the study of heritable traits that are not dependent on the primary sequence of DNA.

And here’s the definition used in Gilbert’s text:

…molecular processes around DNA that regulate genome activity that are independent DNA sequence and are mitotically stable.

And here’s Larry’s objection:

Here’s the problem. If this is epigenetics then what’s the point? When I was growing up we had a perfectly good term for these phenomena—it was regulation of gene expression. Why is there a movement among animal developmental biologists to use “epigenetics” to refer to a well-understood phenomenon?

While I agree that “epigenetics” is a huge, broad, diverse category of phenomena, I think he’s overlooking a key point to claim it is synonymous with gene regulation. It is gene regulation that is heritable and mitotically stable. It’s still far too open-ended, but it’s not just any old example of gene regulation.

It’s also clear and consistent. Larry challenges us with eight instances of regulatory phenomena and asks which ones qualify as epigenetic. Easy. 1, 2, 6, 7, 8. Those are the ones where he specifically mentions multi-generational inheritance of a regulatory state. 3, 4, and 5 describe responses within a single cell in a single generation (5 is sneaky, though: Drosophila oocytes are having gene expression modified in ways that might be transmitted through multiple generations — it’s just that those cells are being loaded with bicoid RNA, not having their bicoid genes being set to a sex-specific state).

I am also comfortable with the idea that inheritance of the regulatory state of the lac operon is an example of epigenetics. It’s arguable whether that’s a useful category, but it does fit the definition.

So one approach that could be taken is to come up with a more specific or more practical definition.

Larry has a more recent article in which he agrees with a new paper by Deans and Maggert that tries to do exactly that. It also takes a much appreciated historical approach, giving the various definitions that have been wafting about since the 1930s. For instance, here’s Waddington’s ancient physiological definition:

the branch of biology that studies the causal interactions between genes and their products which bring the phenotype into being

Yes, I agree — that would simply be gene regulation nowadays. You can’t blame us wicked developmental biologists for promoting that one, though, because we don’t use it anymore.

Now we favor the Holliday definition:

the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail change in DNA sequence.

To me, “heritable” is the magic word that makes all the difference. This, however, is not enough for Deans and Maggert. They want to add more focus, often a good thing, and narrow the definition. I was not happy with their argument, and thought it poorly made, though. See if you can find what was objectionable in this section of their paper (I highlighted it to make it easy, an epigenetic modification that does not change the sequence of the letters in the text.)

We don’t feel that it is possible to reconcile Waddington’s focus on gene regulation with Holliday’s more specific criteria within one field and still maintain the level of clarity needed to produce a useful definition. The efforts to preserve a relationship between these two conceptualizations have been impaired by the fact that there are just too many phenomena, with too few mechanistic connections, to categorize into one field. Also, among the definitions that do maintain the requirement of heritability, we feel that many lack the detail to be functionally useful in directing the testing of specific hypotheses, particularly as it relates to the location or site (cytoplasm or nucleus) of epigenetic phenomena. To mitigate these shortcomings, we advocate defining epigenetics as “the study of phenomena and mechanisms that cause chromosome-bound, heritable changes to gene expression that are not dependent on changes to DNA sequence.”

We feel that this definition makes a strong distinction between gene regulation (Waddington’s definition) and epigenetic inheritance (Holliday’s definition), and also emphasizes that epigenetic phenomena must deal exclusively with chromosome-bound changes. By making these distinctions, we have efficiently separated expressional changes caused by cytoplasmic compounds, which are more closely tied to gene regulation, from those which occur on, or in close association to, the chromosome. Doing so makes the focus of the field much clearer and identifies epigenetic mechanisms more explicitly.

We feel that this definition touches on several important elements not encompassed by other definitions, yet commonly implied in most uses. To further explain the reasoning behind our definition, as well as its utility for improving epigenetic research, we would like to offer a clarification and a test.

Yeesh. I don’t feel that your personall feelings are a strong argument, and I cringed when I hit that page. At least edit it to remove the emphasis on your personal discomfort; just say that the old definitions lack detail, rather than that you feel they lack detail.

So let’s pull out their shiny new definition.

the study of phenomena and mechanisms that cause chromosome-bound, heritable changes to gene expression that are not dependent on changes to DNA sequence

Well. All this fuss for a single change, the addition of the phrase chromosome-bound. That’s it. I agree, it does narrow the topic, but it’s still covering an awful lot of territory. I’m not feelin’ it. I have the impression that the primary virtue of the new definition is that it reduces a class of phenomena to a subset that many people are comfortable studying already, and in part reinforces a gene-centered perspective on cellular behavior.

It also leaves me wondering…what about the inheritance of cytoplasmic or membrane-bound factors that induce consistent changes in gene expression in daughter cells? The gene regulation aspect may be mundane, but it’s the inheritance that is interesting. Under the Deans and Maggert definition, this is no longer under the umbrella of epigenetics — it’s something different for which we have no general name now.

It makes Larry happier, though.

I think this is a useful definition. Nobody cares if dividing E. coli cells inherit molecules of lac repressor and continue to repress the lac operon. That’s a trivial form of epigenetics that never posed a threat to our understanding of evolution.

That’s odd. I do care that the lac repressor is cytoplasmically inherited, but then my primary interests, in the most general form, would be in the patterns of stability and change in cellular properties, rather than the metabolism of sugar. Telling me that I should only pay attention to inherited proteins or methylation states that are directly bound to DNA seems arbitrary.

I also don’t consider “poses a threat to our understanding of evolution” to be a relevant criterion. I agree that lac repressors don’t challenge evolutionary theory, but neither do heritable histone modifications or methylation. I’m one of those people who think epigenetics (even under the old definition!) is important and interesting, but doesn’t affect evolutionary theory much at all.

Larry and I agree.

Methylation is trivial.

Well then, if inheritance of the lac operon is such a trivial form of epigenetics that it should be excluded from the definition, then we apparently need yet another definition that excludes the triviality of methylation.

Or, really, we should recognize that “trivial” is not a good reason to exclude something.

I will still second Larry’s argument that none of this stuff overthrows modern evolutionary theory in any way. It would require extremely persistent inheritance of an epigenetic state over many generations to have those kinds of repercussions.

(The Gilbert text does mention one significant effect: the toadflax plant, Linaria vulgaris, has a radically different flower morph, Peloria, that Linnaeus himself classified as a different species. As it turns out, they only differ in the methylation state of the cycloidea gene, but the DNA sequence is identical. This is a case of an epigenetic change persisting for hundreds of generations. It’s a rare case, though, and also…would still definitely fall under the Deans and Maggert definition.)