March 16, 2008

Cordyceps are Cool and So Are Nikoh and Fukatsu

Forgive the rather clumsy title of this entry. I'm taking a break from reading through a pile of papers on research into the evolution and symbiology of Cordyceps and just finished reading a paper that made me stop and go "Hmmmmm."

The paper in question is:

Nikoh N and Fukatsu T. 2001. Evolutionary Dynamics of Multiple Group I Introns in Nuclear Ribosomal RNA Genes of Endoparasitic Fungi of the Genus Cordyceps. Mol Biol Evol 18: 1631-1642.
It's the second of Nikoh and Fukatsu's papers I've read and it incorporates findings from the previous study on Cordyceps. I don't have time to get into a lot of detail about the papers right now; suffice to say that both papers represent some truly meticulous work with results that go a long way in resolving the phylogeny of a rather enigmatic (and, as I've been told, slightly gruesome) group of ascomycetous fungi.

The part that made me stop and think, though, is found in the discussion section of the cited paper, on page 1639:
...Are there any causal relationships between upkeep of the introns and endosymbiotic/parasitic lifestyles? Although speculative, we suggest that the slow-growing nature of symbionts/parasites might be responsible. It is expected that the more group I introns rDNAs carry, the less efficient synthesis of rRNAs becomes. Because the ribosome is a copious cellular component essential for protein synthesis, multiple group I introns in rDNAs may result in reduced cell growth and division. If so, heavily inserted rDNAs may have a negative fitness effect on most free-living organisms in which the ability to grow rapidly under favorable conditions is essential for their survival and reproduction. On the other hand, the growth rate of endosymbionts/parasites must be suppressed under a strict control to cope with limited space inside the host body, to ensure survival of the host (at least for a while), to efficiently utilize resources from the host, to synchronize their life cycle parameters with those of the host, etc. (Tanada and Kaya 1993). Therefore, it is expected that the disadvantages due to heavily inserted rDNAs might be relaxed in slow-growing endosymbionts/parasites.
I find that a rather compelling hypothesis. In fact, one of the key issues for the success of Cordyceps populations parasitic upon arthropods is impeccable timing. Many of them are fine-tuned to produce stromata just as their host enters their pupal stage (cf. C. heteropoda on cicada nymphs or C. militaris on silkworms, etc.) I've always thought of selfish genes as being deleterious in terms of fitness, but in the case of endoparastites like Cordyceps, perhaps they're part of an overall system that has helped the fungi adjust their own growth to that of the host, having evolved through a series of introgressions and selection. That there are so many such introns in the rRNA sequences in this genus (as many as 69 in one species) is rather telling, I think. Also telling is the fact that these introns are capable of "jumping" in the genome but maintain a consistent position in the extant populations. Taken together, there almost has to be some kind of benefit here. This is some powerful evidence for Cordyceps evolution; that synteny is so highly maintained and polymorphism relatively low in flanking sequences and that transposition of the introns themselves hasn't been found speaks volumes.

Of course, Cordyceps are just plain cool — and a bit gruesome — for other reasons as well. They're related to the fungus from which we humans have managed to derive LSD, for one thing. That makes it all the more interesting that some of these fungi, such as the C. unilateralis in the video below, appear to change the behavior of their hosts — and then grow out of their heads! Check it out:

Aren't you glad you're not a bullet ant right about now?

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