Sunday, October 21, 2007

Russian scientific pioneers neglected outside of Russia Konstantin Mereschkowsky

“Let us imagine a palm tree, growing peacefully near a spring, and a lion hiding in the bush nearby, all of its muscles taut, with blood thirsty eyes, prepared to jump upon an antelope and to strangle it. The symbiotic theory, and it alone, lays bare the deepest mysteries of this scene, unravels and illuminates the fundamental principle that could bring forth two such utterly different entities as a palm tree and a lion. The palm behaves so peacefully, so passively, because it is a symbiosis, because it contains a plethora of little workers, green slaves(chromatophores) that work for it and nourish it. The lion must nourish itself. Let us imagine each cell of the lion filled with chromatophores, and I have no doubt that it would immediately lie down peacefully next to the palm, feeling full, or needing at most some water with mineral salts.”

(Konstantin Sergeevich Mereschkowsky (1905)

The theory that chloroplasts are derived from cyanobacteria, which were long ago
taken up by non-photosynthetic organisms is more than one hundred years old. (Mereschkowsky)Complete proof that it is correct has been obtained from molecular biology. By comparisons of DNA sequences the cyanobacterial ancestry of chloroplasts has been established, just as it is now certain that mitochondria are descendents of another bacterial clade.

Among chloroplasts there are two developmental lines, the “green line” (in green
algae and plants) and the “red line” (in red algae and most other algae). Even if some researchers still believe that these two lines start with two separate endosymbiotic events, the contrary view prevails. This means that all chloroplasts are derived from one original chloroplast, which has appeared when a cyanobacterium entered another cell. It is a little surprising that it is so, since we have so many other examples of very intimate symbiotic relationships between a number of algae and a number of other organisms.

That plastids were once free-living cyanobacteria is now taken for granted by many, and for good reasons, for there is a wealth of data – in particular from the comparison of plastid and cyanobacterial genomes – that support this view. There is currently no seriously entertained alternative hypothesis to the view that plastids descend from cyanobacteria. But that was not always the case. Well into the 1970s there was a generally favoured alternative hypothesis, namely that early in evolution plastids arose de novo from within a non-plastid bearing cell (an autogenous origin) rather than through invasion by a cyanobacterium into a non-plastid-bearing cell with subsequent intracellular coexistence and reduction to an organelle (an endosymbiotic origin). Interestingly, the shift from autogenous to endosymbiotic hypotheses during the 1970s was a reversal of state for during the first two decades of this century, the endosymbiont hypothesis for the origins of plastids (and mitochondria, which will not be further discussed here) was very popular among biologists. It fell into disfavour shortly after the First World War, for reasons that are very difficult to summarize briefly, and remained scorned for 50 years (see Sapp, 1994, for an historical account in English, and Höxtermann, 1998, for a succinct historical account in German). So where did the first version of the endosymbiont hypothesis come from? In a nutshell, it came from Konstantin Sergejewiz Merezkovskij (usually written as Constantin Mereschkowsky), a Russian botanist of little standing who worked at a rather small and by no means prominent university in Kasan and who published a very remarkable paper in 1905. We are not aware of any true precedent for his paper, which draws upon three lines of evidence known at the time.


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