Field of Science

Little whirling photosynthetic (and not so photosynthetic) thingies


I spent long and hard thinking of what I should make the subject of my first post (honestly, it took minutes!) but I eventually decided to write something on a subject I've spent a little time on recently but know precious little about - dinoflagellates (Dinoflagellata).

Dinoflagellates are unicellular protists, generally accepted to form a clade called Alveolata with ciliates and sporozoans (the image at left is a generalised dinoflagellate from Andrew MacRae's Dinoflagellates [http://www.geo.ucalgary.ca/~macrae/palynology/dinoflagellates/anatomy.html]). They are most familiar to the general public as the main culprit behind toxic algal blooms. The main distinguishing feature of dinoflagellates is that they possess two distinct flagella - a fairly straight one that sticks out from the cell, and a wavy one that wraps around the cell, usually in a groove. Most dinoflagellates also have a distinctive nucleus that lacks histones, the proteins that DNA wraps around in other eukaryotes, and with chromosomes that don't decondense between divisions. According to Fensome, Taylor et al. (1993), about half of dinoflagellates are photosynthetic, while the other half are mostly parasitic (some are both).

I got onto the subject of dinoflagellates because I was organising my records on dinoflagellate taxonomy (I try and cover the taxonomy of all organisms, and I am completely happy in the knowledge that this is probably an impossible task for one person - you can see a number of my efforts, varying from some I'm quite proud of to the truly tragic, at http://www.palaeos.org/). Dinoflagellates have perhaps the worst taxonomy of any group of organisms - worse than fossil plants, worse than South American harvestmen, worse than hominins. You may be aware that there are separate taxonomic codes for plants and animals. There are organisms that are neither plants nor animals, but because the codes were developed before this was understood, protists are assigned to either the botanical or zoological codes depending on which they were traditionally regarded as. Photosynthetic protists are covered by the botanical code, mobile protists are zoological. Problem is, some protists are both photosynthetic and mobile. Dinoflagellates are probably the largest group of organisms that have been regarded by different workers as under different taxonomic codes. As a result, the literature is full of names for dinoflagellates that are valid under one code but not under the other, and cases different codes require different names for the same thing. Palaeontologists working on dinoflagellates agreed to use the botanical code after 1961, and Fensome, Taylor et al. (1993) suggested the same thing for neontological taxa.

The other major issue with dinoflagellates is reconciling the fossil and living taxa. Many dinoflagellates form resistant vegetative cysts at some stage in the life-cycle, and these are the only stage that can be fossilised. Fossil taxonomy, therefore, is based on these. Neontological taxonomy, however, is generally based on the motile stage of the cycle. As a result, two separate taxonomies have developed in parallel, and there are relatively few cases where a cyst can be connected with a motile form. The worst case of this problem involves the fossil genus Spiniferites Mantell 1850, which has long been known to represent the cyst of the living genus Gonyaulax Diesing 1866. Unfortunately, because both of these genera are quite large and involve a lot of species, most workers have turned something of a blind eye to this point, and no real solution has been developed.

Coming up later, a few examples of the odder dinoflagellates, and the boundary between unicellularity and multicellularity. Unless, of course, I get distracted and cover something else.

2 comments:

  1. Strangetruther4 June 2007 at 07:38

    Hi Chris -

    I'm glad you've started this blog, and hope to visit it regularly. Like many including myself, you have initially opted for a "light on black" mode of lettering which inevitably makes people's eyes go funny in less than the time it takes to read one day's entry.

    Cheers,

    Strangetruther

    ReplyDelete
  2. Thanks for the encouragement. Re the lettering - I did chose the white on black because I prefer the immediate look of it, but I'm open to critique.

    ReplyDelete

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