Posts Tagged ‘Plants’

Monday Organism (Yes, I’m Aware It’s Sunday) – Cyanobacteria

On most Sundays, I won’t be around to post, except in the evening, half-brain dead from ISL class. Anyhow, I’m a day off to recuperate from last week, so I have time to post my very first “Monday Organism”, and a day early, at that!

Since this is the first weekly organism, I think it’s appropriate to explain why there is, in fact, a weekly organism. Since this blog is about biology, it’d be mighty improper unless it had  periodical items about animals, don’t you think? I mean, come on, it’s no use running a blog about biology without fluffy animals in it (or angry wobbly ones or, well, extremely tiny ones).

Also, the Monday Organism is sometimes going to be about higher taxa as well (usually very high taxa, mainly to illustrate an interesting point about evolutionary biology)

The first Monday Organism is actually not an Organism, but a Phylum: Cyanobacteria.

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Cyanobacteria literally means “blue bacteria”, but they’re actually called “blue algae” in Hebrew. The wiki on Cyanobacteria states that the taxonomy of Cyanobacteria is under revision, which is no surprise. In class, this group was even (I think most appropriately) called “Cyanophyta”, meaning “blue algae”.

Cyanobacteria are a fascinating group, and their existence is sound evidence for various evolutionary theories, the most important one is probably the evolution of the chloroplast organelle, the organelle in plant cells in which photosynthesis occurs.

The truly amazing thing about Cyanobacteria is the fact that they’re actually prokaryotes (having no distinct cell nuclei), and yet, they have photosynthetic pigments in their cells which are used to produce organic material by absorbing light energy from the sun. This means, in effect, that Cyanobacteria are the evolutionary precursor for the eukaryotic plants.

While it is obvious that all algae are commonly related, the truly interesting characteristics of Cyanobacteria are the ones that point out to the evolution of plant organelles. When I first learnt about Endosymbiont theory, I was plainly told that “endosymbiont bacteria eventually became permanent organelles”. Now these endosymbiont bacteria have a name: Cyanobacteria. In fact, the evidence shows that the Cyanobacteria themselves evolved into the chloroplast, and it is quite possible that every plant cell is, in a way, a symbiotic colony of eukaryotes and prokaryotic photosynthetic bacteria!

Obviously, the radiation of photosynthetic taxa is prolific enough to rule out such a simplistic story, but the evidence shows similar genetic and biochemical traits in modern day chloroplasts and in the makeup of Cyanobacteria. Since this isn’t an encyclopedic article and I rather focus only on one interesting concept at the time, I’ll give just one example for “evidence” of the common descent of CB and chloroplasts :  the genetic makeup of chloroplast DNA (yes, they have their own DNA and they replicate on their own!) is similar to Cyanobacteria DNA. This alone is solid evidence for common descent for the two.

There’s lots of special cases of endosymbiosis that show not-so-common descent, but rather “common descents”, but I’ll leave that to the avid reader.

The main point of this post is not so much to tell about CB anatomy (warning: other posts might deal with interesting anatomy and physiology!), rather it is to illustrate classic tools in evolutionary research: genetic, anatomical, biochemical and physiological comparison as instruments for detecting common descent. It’s a crucial way of thinking in all of biology, and it highlights the sometimes elusive practical value in evolutionary theory: knowing the genetic relationship between different taxa can be critical in any biological endeavor. If one seeks to find antibiotic weaponry against infection and disease, knowing the culprit’s phylogeny can be of tremendous use, and phylogeny is best derived from the comparative tools I’ve briefly illustrated here.

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Some thoughts about plant evolution

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Well, I’ve been brewing a post about ISL ethics for a few days now, but besides my chronic procrastination due to school and work, I had another reason to postpone this post, which is that just today I got the first paper from Cocoon about ISL ethics and I think this one really calls for some homework!

Anyhow, as a brief hors d’oeuvre, I would like write a brief post on some nagging thoughts and doubts I had about plant evolution due to this week’s biology class.

First, some background: this semester is “botany-semester”, meaning that all labs and all classes that are specifically about biology (and not, say, math 101, physics 101 etc.) are mainly focused on plants, algae, etc.
Second, before I write a about the nitty-gritty of my argument, let me just say that every single one of my professors, lab chiefs and even the guy who instructs our lab team have either your garden variety yarmulke, or in the case of the PhD student who instructs our lab team, a yarmulke and those curly braids that the hassidic Jews have. I’m really not too privy to the that whole “Hassidic spectrum”, but since he doesn’t wear those penguin suits the Jerusalem Hassidim wear, I can’t say he’s as fundamentalist as those kooks in Jerusalem are.

But anyway,

What I wanted to say is that every single one of my professors, lab chiefs and lab instructors is, well, REALLY JEWISH, really religious and god-fearing, and completely, unabashedly, evolutionist. These guys speak of evolution as if the fact that it’s true is so ho-hum that it doesn’t even worth a second thought. Shiesh. It’s only the major kooks in Israel who have any qualms with it, I guess.

And now, to the batmobile!

The theory of plant evolution goes roughly like this: a great number of yonks ago, prokaryotes endosymbiontly evolved into eukaryotes (something I find totally reasonable), and the variety of prokaryotes that evolved photosynthesis (namely, Cyanobacteria or Cyanophytae, or blue algae), coupled with endosymbiosis, turned into the first eukaryotic algae. So far so good, but the problems I have with plant evolution start here.

A good analysis of algae evolution can be done by looking at the various evolutionary pathways observed in various algae phyla. It’s probably no coincidence that all green algae and all plants have the same preservative polysaccharide (namely starch, unlike our glycogen), all have the same (and rather unsually so for the 7 or so algae phyla) characteristic photopigment (chlorophyll a), all have similar sexual reproduction and all are surrounded by cell walls composing of cellulose (also an “anomaly” among algae phyla)

This is a good and credible explanation for the origin of land plants (plantae or metaphyta). However, things start to get really shaky when you look at the other algae phyla, who have indiscrepant levels of development, which make it rather futile to try to pinpoint who evolved when. For example, the multicellular alga “Chara” has a superficial “stalk” and a complex sexual reproduction system, but it does, however, use isogamy as a means for zygogenesis (the production of zygotes from gametes). Isogamy is rightly considered to be archaic, as it is less efficient, less specialized and is more characteristic of primitive organisms than of evolved ones.

So how come Chara has an unevolved sexual reproduction while Volvox, which is a microscopic colonial alga that has no sexual organs, uses oogeny for zygogenesis, which is strikingly reminiscent of human zygogenesis (the male gamete is small and motile, the female gamete is large and static). The professor merely said that certain things evolve at different paces, and this is a good explanation and a very reasonable one, but I find it hard to accept it while at the same time claiming that this or that phylum evolved before or after based on comparative anatomy.

In cases like this, I prefer to say “I don’t know, but…” rather than to firmly put my finger on a phylogeny (which I can comfortably do regarding green algae and plantae).