June 08, 2008

Fungus in Amber Likely Not Carnivorous: A Response in Mycological Research to Schmidt et al. (2007)

Blogging on Peer-Reviewed ResearchBack in December I mentioned the discovery of what was reported as a nematode-trapping fungus in Cretaceous amber from France by Schmidt et al. (first here and then after having read the article in Science Brevia here).

In the current issue of Mycological Research, a response to the Schmidt et al. piece appears that explains in some detail why there is no evidence to support the contention that the fungus in amber was a nematode-trapper.

Since Mycological Research isn't open-access — not even when it comes to things like this — I'll reproduce some of the salient points. The authors of the response are R. Gregg Thorn (University of Western Ontario), Markus Scholler (Staatliches Museum für Naturkunde), and Walter Gams.

Have carnivorous fungi been found in Cretaceous amber?

...None of the photographs shows a ring attached to a sporulating hypha, nor was any trapped nematode documented (those shown are too slender to have been captured by the purported "traps"). In a drawing (fig. 1E) the authors present a reconstructed life-cycle showing a trapped nematode in a ring, rings attached to a hypha, formation of a ring before closure, and a blastosporic yeast. We have tried to obtain the slides deposited in Paris on loan without any success...

Fungi that trap nematodes with adhesive or constricting rings are members of the Orbiliomycetes, one of the oldest classes of phylum Ascomycota, and oldest in subphylum Pezizomycotina. Their segregation must have occurred much earlier than in Cretaceous times (Spatafora et al. 2007). Taylor & Berbee (2007) give a conservative estimate for the radiation of the Pezizomycotina of 215–400 Myr before present... A connection between a "trapping fungus" and a yeast is unlikely, and we consider that the published picture is probably an artefact of the two organisms being closely associated in space (perhaps only as a result of being captured in resin). The more organic connection between short conidiogenous cells and a main hypha shown in their fig. 1C illustrates a structure presently unknown in the Orbiliomycetes and would preclude the identification of these hyphae as belonging to the Orbiliaceae.

Non-constricting ring traps are only known from the genus Dactylellina (Scholler et al. 1999). The rings are three-celled, with three septa, not one as shown by Schmidt et al. (2007), and are stalked. A nematode that enters a loop would be stopped if its body diameter exceeds the loop diameter... Their struggle breaks the ring from the stalk and the nematode glides away with the ring around its body. The attached ring germinates to form an infection peg that penetrates the nematode, and forms an infection bulb, followed by digestion of the body contents. The ring thus not only serves as a trapping device but also as a diaspore... The attachment of detritus to the rings as shown by Schmidt et al. (2007, fig. S1 A) most probably is due to unspecific binding. The rings even of adhesive traps are not sticky all over and do not collect detritus. An adhesive non-constricting ring trap cannot function, because the nematode would get caught on the ring before entering it...

With all due respect for the importance of fossil documents, we are not prepared to accept the unlikely combination of ring-shaped traps and yeast-like growth as compatible with nematode capturing, which in the photographs of Schmidt et al. (2007) is not convincingly proven...
And there you have the gist of the response. I thought it important to reproduce here since I wrote about the original article in Science and also because this demonstrates the sort of checks and balances embodied in scientific progress. One group of scientists finds something they get excited about and puts forward an inference based on what they see. They publish their findings and ideas. Any other scientist can then look the article over and say, "Hey, wait a minute, here's an alternative hypothesis that includes some information that this other scientist overlooked." Evidence accumulates and everyone has the opportunity to evaluate the situation, weighing the evidence for and against the conclusions drawn on the subject under discussion. No matter the personal status of the scientists involved and no matter how exciting a given inference might be, there's always the possibility for new ideas to be put into play and given due consideration.

In an ideal world, Thorn, Scholler and Gams' reply would have appeared in the same publication that publicized Schmidt et al.'s work on the fossil. Mycological Research has, I'm sure, a readership that is a tiny fraction of that commanded by Science. For whatever reasons, that's not the way it happened in this case. I have no idea whether the reply quoted above was submitted to Science or not and I have no idea whether or not that journal will also publish it or even make note of it. I would hope that they would; to do so would be the most responsible course of action.
Carnivorous Fungi from Cretaceous Amber. Alexander R. Schmidt, Heinrich Dörfelt, and Vincent Perrichot (14 December 2007). Science 318 (5857), 1743. [DOI: 10.1126/science.1149947]

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