Warning: This is a bit of a rant. For the meat, skip to the figure.
One of those key concepts is the haploid-diploid variation found in many, if not most (if not, secretly, all) eukaryotes. You know the whole thing with syngamy and meiosis and gametic vs. zygotic vs. sporic life histories. You may even wish I hadn't reminded you. Click here if you'd like to experience the wonderful feeling of intense confusion again. So basically, eukaryotes can be haploid or diploid. Typically they have ways of switching between the two phases: diploid --> haploid = meiosis (typically), haploid --> diploid = syngamy (again, roughly). To make things more fun, there may also be several distinct diploid and haploid stages, but let's ignore those for now. Now, it logically follows that there may be variation in how 'prevalent' a certain stage is for various organisms. Let's call it the 'dominant' stage, just for kicks.
Now, how do you define 'dominant'? Well, for humans, it's obviously the part of your life you're an 'individual'. Ok this gets weird when said 'individuals' can clone themselves; also, a bit too philosophical. Let's reword that: It's obviously the stage in your life you're multicellular and big and stuff. Baker's yeast, for example... hang on, what's the big multicellular stage in yeast? Errr... scratch that. Ok, the stage an organism spends most of its time in. Great, works so far. Yeast is most usually haploid. What about moss? It's roughly equal (for the sake of the argument) in both haploid and diploid stages. So it's sporic.
I admit to being a little slow at times, but that seriously confused the fuck out of me -- it seemed arbitrary! How exactly do you decide whether an organism has one or multiple "dominant" stages?
We've been told to "look where meiosis happens". Now this is where it becomes absolute and total mindfuck, on steroids and LSD. Remember the 'gametic', 'zygotic' and 'sporic' life histories? You know what else they're officially(!) called? Gametic, zygotic and sporic...MEIOSES. That's right. We have gametic meiosis, zygotic meiosis and sporic meiosis. Now, sit back and savour the absolute chaos that this naturally incites in young minds yet to be protected by the hard-ass defensive shell your brain produces from years of bitter academic cynicism.
Done? Borderline mental abuse, ain't it?
Of course, while none of those terms have a single redeeming quality besides being physically pronounceable, the worst, by far, is 'gametic meiosis'. Last time I checked, there are no documented case of haploid cells consistently/normally undergoing meiosis. (allowing it has somehow been induced artificially in haploids - who knows) So that's absurd. Even speaking from a field where biological "laws" need not apply. I'm happy to know that someone with qualifications agrees with this, and also has a nice rant on the topic. Of course, I'd say we should do away with 'gametic', 'zygotic' and 'sporic' altogether, but more on that later.
We've also been told "the big, obvious stage [presumably, multicellular] is dominant" Again, last time I checked, Chlamydomonas doesn't exactly jump out of the culture medium and grow before you into a giant... SuperChlamy... or something. That would be really cool for a cartoon character, but most life doesn't exactly strive to be visible to the human eye or anything. In fact, it's much better to not be...
A slightly more sensible point was "look where feeding happens". Great, so sperm are now a dominant stage? If I recall, they do absorb nutrients. Are we gonna go as far as define what manner the nutrients must be obtained in? The lesser known life of Dictyostelium involves cuddling up with a mate, fusing, forming a cyst and then baiting unsuspecting haploid dictys with cAMP...to devour them!
How about "the stage that can live freely"? Well, then many parasites now have no life, and are very sad. Or "the stage that lasts the longest". Well, many things can fuck, encyst, and hang out for what is an eternity compared to their mitotic cycles. Some organisms can spend more time in resting stages than in active ones - ever wondered how a puddle can come back to life as quickly as it dries up?
In the end, I figured this was more of a fuzzy philosophical question, with ultimately everything being somewhat sporic-
Salvation at last!
-until randomly wandering across this neat little diagram today:
A sensible summary of a) Haplontic, b) Haplodiplontic and c) Diplontic life histories. ( Houdan et al 2004 Syst Biodiv based on (and greatly improved from, IMO) Valero et al. 1992 TrEE)
Do you see the difference? At last, a clear, crisp definition! The dominant stage is the one where mitosis occurs, duh! Perhaps it'd help to add 'reproductive' meiosis, to take care of those pesky little exceptions (some multicellular lineages). And personally, I prefer 'haplontic' vs. 'zygotic'. Zygotic sounds very diploid to me. That term owes me a nice chunk of my grade for that 'non-vascular plant' course. 'Haplodiplontic' is wonderful too as you don't have to sit there wondering what a 'spore' is. It's straightforward, concise and universally applicable.
Humans? Diplontic - sperm and eggs don't reproduce mitotically. Dictyostelium? Haplontic - diploid stage quickly followed by meiosis without any mitotic divisions. Moss? Haplodiplontic - both haploid and diploid forms divide mitotically, in this case to form large multicellular organisms. Our favourite beer-making Saccharomyces? Haplodiplontic, actually - it can happily reproduce mitotically in haploid and diploid stages! Red algae? Don't ever remind me. But haplodiplontic as well. A very convoluted form thereof. Pfiesteria-aka-lets-cram-every-possible-eukaryotic-way-of-being-into-one-organism? (yup, that was [reportedly; some doubts RE amoebae] 24 distinct life cycle stages) Appears to be haplontic as a typical dino.)
The original source of the above diagram still makes the usual mistakes of skipping stages taken for granted and relying much too heavily on metazoa, fungi and land plants for explanation (and using Margulis' 'protoctists', ewww...) As per usual, a protistologist comes along and makes everything better! =D
Ah the legacies biology's phylogenetically myopic traditions have left us!
Yet another rant about teaching...
I'm slowly beginning to believe in the following principle: If [caring] students don't understand something, it's either wrong or taught poorly. Usually, but not always, the latter. Science is seriously not that complicated. At all. Just that we humans are fucking abysmal at explaining it. And since most teaching seems to be vertically inherited, poor approaches to certain topics are often maintained due to purely historical reasons. All too often it is perpetrated in the same form the teacher once received it as a student; and since those who make it in academia tend to be those who can grasp concepts despite the poor teaching (sigh...doesn't bode well for me =( ), they are perhaps somewhat oblivious to how cumbersome their inherited approach is.
As much as I love research, I still think teaching is a more pressing priority for academic science.
(Personally, I tend to think of everything from a cellular perspective. Furthermore, if you tell me something that only applies to a small polyphyletic assemblage of conspicuously sized organisms, I tend to file it away as an exception and forget. (I like exceptions, but only when aware of the general principles that go along with them) Furthermore, that 'non-vascular plants' course revolved predominantly around terminology, most of which I immediately forgot after the final. Or even before the final. Hell if I remember what an 'archaegonium' is, and how it differs from a 'sporocarp' or whatever. Especially when the same things get different names depending on who studies them. In fact, don't expect me to remember taxon-specific terms for general things even for organisms I actively study (and like!). I won't. Even though everyone claims to 'know' this, students (and conference attendees, etc) tend to take away concepts, not terminology. Seriously. We all have our favourite jargon, but please pity the uninitiated!)
Now, food for thought: how did a student just plow through four years of biology courses without properly understanding eukaryotic life histories? Our education system is truly scary...
Houdan, A., Billard, C., Marie, D., Not, F., Sez, A., Young, J., & Probert, I. (2003). Holococcolithophore-heterococcolithophore (Haptophyta) life cycles: flow cytometric analysis of relative ploidy levels Systematics and Biodiversity, 1 (4), 453-465 DOI: 10.1017/S1477200003001270
Valero, M. (1992). Evolution of alternation of haploid and diploid phases in life cycles Trends in Ecology & Evolution, 7 (1), 25-29 DOI: 10.1016/0169-5347(92)90195-H