Two-faced diatoms …

Back in March I reflected on the challenges involved in discriminating species of Gomphonema (see “Baffling biodiversity …”).   That there were several species in the sample which prompted the article was indisputable; that some of those species were, individually, quite variable was also clear.  The former issue I resolved, to some extent, by reference back to Hutchinson’s “Paradox of the Plankton” but the latter was harder to explain.

Part of the problem stems, I suspect, from the reliance on morphology to characterise species.  We assume that, because a group of organisms share a set of visible characteristics, then they must also share genes which determine those characteristics and that, in turn, implies a common ancestry.   Turning that assumption on its head, we assume that groups of microscopic algae that appear different to each other belong to different species.   However, a dog lover might point out that Chihuahuas and Great Danes look very different but are, in fact, the same species.   One of the challenges of those of us who study algae is deciding just how much variation in form is typical within a species, and at what point differences are such that they represent more than one species.

Gomphonema sarcophagus from Pitsford Water, Northamptonshire, showing Janus cells.  Photographs by Ingrid Jüttner.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

So what should we make of the diatom valves in the image above?   The valve outlines and breadths are similar but the striae densities are so different that we might think that they belong to two separate species.   However, I recently stumbled, by chance, on a 1998 paper by Stacy McBride and Robert Edgar which discussed the topic of “Janus cells”.  Janus, you may remember, is the Roman god of time and is depicted with two faces, one looking back to the past and the looking to the future. His name has been appropriated, in this context, to describe diatoms that have frustules comprising two valves with different characteristics.   A few genera show consistent differences between the two valves – in Cocconeis and Planothidium, for example, one valve has a raphe whilst the other does not – and there are also differences in striae densities between the raphe and rapheless valves.   The term “Janus cell” is applied to diatoms where there are marked differences between the two valves but this is not a fundamental characteristic of the species or genus.   So, in the example above, we see some forms with much denser striae (11-13 in 10 mm) than others (7-8 in 10 mm).

We don’t know, from just looking at variability in populations, that this is not polymorphism within the species, in much the same way that some humans have attached ear lobes and others do not.   But, as diatom populations grow in number by repeated divisions of single cells, we can assume that most are clones of a small number of genotypes and, therefore, that the differences are due to ontogenetic variation.   What is interesting here is that this variation seems to create two distinct outcomes – coarsely or finely striated valves.  Some have suggested that such variation may be determined by differences in environmental conditions; however, the co-existence in a single population argues against this.

Gomphonema, as I have mentioned in earlier posts, is a genus that challenges taxonomists.  And, because ecologists depend upon taxonomists to give them a means of sorting diatom valves and frustules into meaningful categories, the environmental signals we get from Gomphonema species are often quite confused too.   The possibility of encountering Janus cells just throws one more curve ball into the mix.

Reference

McBride, S.A. & Edgar, B.K. (1998).   Janus cells unveiled: frustular morphometric variability in Gomphonema angustatum.   Diatom Research 13:293-310.

Round, F.E., Crawford, R.M. & Mann, D.G. (1990).  The Diatoms: Biology and Morphology of the Genera.   Cambridge University Press, Cambridge.

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