Field of Science

On the classification of blobs

The first forays into microscopy revealed a whole world of blobs, tiny microscopic organisms that were invisible to the naked eye. These went through a range of different names, from the 'animalcule' denomination given by Anton van Leeuwenheok (which he used to describe everything small he saw under the microscope, including his own sperm) through 'monera' (a more specific name for certain types of blobs, namely those that weren't eukaryotic) to 'prokaryotes', a name that still stands.

Prokeryote means, literally, 'no nucleus', and it's use allows the world of living organisms to be split up into two groups: Prokaryotes and Eukaryotes. Eukaryotes are things with a nucleus, a membrane covered partition to hold DNA, as well as many separate organelles existing within their cells, such as mitochondria, endoplasmic reticulum, the Golgi apparatus...

Prokaryotes are...uh...everything else.

Which means that the label 'prokaryote' was always waiting to fall apart. After all, they may just be blobs but there are a lot of them, and some are very different blobs. Around the 1970's people started noticing that there were a group of the prokaryotes that behaved differently, mainly through studies done by Carl Woese and George E. Fox who created classification tables based on the genetic sequences of ribosomal RNA (the part of the genome most likely to be conserved, this is often used for classification, especially of things in Deep Time). This showed that there was a distinct group of prokaryotes with a mostly separate evolutionary history (more on the mostly later) to the rest of the prokaryotes. They were originally named 'archaebacteria', and together with 'eubacteria' (true-bacteria) were put in the prokaryotes group. They were blobs without a nuclei, and that was where they belonged.

However, things started to get a bit more complicated the more people looked at archaebacteria. They weren't just a group of slightly odd bacteria, they were something else. Something different. Although their metabolic pathways are similar to bacteria, their methods of turning DNA into proteins more resembles eukaryotic processes. Their flagella (tentacle like structures used for movement) have a markedly different structure from bacterial flagella. Like bacteria, they reproduce asexually and (also like bacteria) they can share their DNA around, in fact they can also share there DNA with bacteria, which makes taxonomists tear their hair out. It's very difficult to classify something when it keeps giving its DNA away, and collecting bits from other sources.

It is proposed in the SGM journal (Society for General Microbiology-journal not available on line) that the term 'prokaryote' should be scrapped altogether. As well as being an incorrect label for a large group of organisms it also produces an incorrect evolutionary perspective. The use of the eukaryote/prokaryote terms suggests a very human based linear "One upon a time there were blobs with no nuclei and then they got nuclei and then they were better" sort of story. A more correct view is that of all three superkingdoms; bacteria, archaea and eukaryotes splitting away from each other. Eukaryotes safely packaging their DNA away, allowing a more complex system to build up, yet forfeiting the ability to share bits of DNA. The archaea and bacteria on the other hand, continued to share their genetic material, just became more selective about it as they diverged (hense the 'mostly' seperate history).

Or maybe not. It might be that the archaea/eubacteria formed a very selective group of blobs, which then split further when some developed a nucleus, while the others continued to share their DNA with the bacteria, picking up different metabolic secrets. It's hard to work out; especially given that similarities between the DNA of archaea and bacteria does not necessarily show their relatedness; it might be a gene that has remained conserved in both of them for millions of years, or it might just be one that was exchanged last week.

There a several arguments against removing the 'prokaryote' as a naming system but most of them boil down to the very multicellular-centric argument of: "but they're all just blobs!" The three superkingdoms of archaea, bacteria and eukaryote are a far more accurate, and scientifically and taxonomically correct way of looking at things than the prokaryote/eukaryote model.

My only complaint is that I spent ages in secondary school trying to learn how to spell 'prokaryote'... removing the name means I could have spent that time doing something far more useful...like building paper planes and reading 'Redwall'.

3 comments:

Anonymous said...

Hi!

I live in Washington State, US, and was doing some research for my final biology test tomorrow, and came across your site from a google reference!

Thanks for writing, since your last post has helped sort out the Prok/Euk in my brain, and it was interesting reading your post on why bacteria make antibiotics.

Cheers,
Jen

rhan said...

So, I'm also just catching up, since I still don't have my own internet (won't until I'm back from Japan, really).

My vote always went with Brian Jaques. His books still comprise an entire shelf in my room at home (although I no longer keep up, thanks to things like papers and revision and recreational PubMedding), and after learning how to spell Mattimeo and all the weird mousey names, things like prokaryote really didn't look so bad...

Y'know, someday we're going to have to come up with an official level beyond species. Then it's going to be King Philip came over from Germany singing ********

And if I know 12 year olds, it'll be something lewd. By the time they've reached university, it'll be something that would physically scorch the ears of the lecturer. Combattive learning, here we come! :)

disa said...

I love it ! Very creative ! That's actually really cool Thanks.