Chromista: Systematics
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For many years, photosynthetic chromists were classified as plants, while non-photosynthetic chromists were classified with the fungi or animals. The close relationship among chromists was not fully appreciated until the rise of ultrastructural and molecular studies. For instance, it is now known that chlorophyll "c" and a number of other pigments found in the Chromista are not found in any plant group, but occur in a number of photosynthetic chromist taxa. There are also peculiarities of the flagella, plastids, and genetic arrangement which are not found in any other group.
The Chromista appear to represent an independent evolutionary line that diverged from the same common ancestor as plants, fungi, and animals. It was for this reason that the new kingdom Chromista was proposed. The precise relationship of the chromists to the other eukaryotes is not yet known, but they appear to be part of the so-called "crown eukaryotes", which includes not only plants, animals, and fungi, but also Alveolates and posiibly the red algae. There is some evidence that the Haptophyta may not belong to the Chromista. They lack several of the morphological and molecular characters common to this group, but we have included them here since no alternative relationships have been suggested.
There is still not full agreement as to how the chromist taxa are interrelated. The cladogram shown here is a composite of the results of molecular and morphological studies (see list of sources below). Our phylogeny is something of an oversimplification: we have excluded a number of taxa that have no fossil record and are not diverse, common, or well studied. We have also retained the Chrysophyta as a group, even though it is now generally believed that it is a paraphyletic, or possibly polyphyletic, group.
There is also considerable variation in the application of names to this group and the vaious subgroups in the literature. The group, which we here call the Chromista, is sometimes called Stramenopiles, Heterokonta, and Chromobionta, among others. Each of these terms has at times been used for a more restricted group however, so we follow Cavalier-Smith (1997) in using the term Chromista.
Chromist systematics is an area of intense and active research. It is quite likely that new research will greatly add to, or even modify, the relationships depicted in the cladogram above.
You can get information on some of these protists by accessing the Protist Image Database at the University of Montreal. Or visit the Stramenopile section of the Tree of Life for current information about chromist systematics.
T. Cavalier-Smith. 1997. Sagenista and Bigyra, two phyla of heterotrophic heterokont Chromists. Archiv für Protistenkunde 148: 253-267.
Cavalier-Smith, T., Allsopp, M.T.E.P., and Chao, E.E. 1994. Thraustochytrids are chromists, not Fungi: 18S rRNA signatures of Heterokonta. Philosophical Transactions of the Royal Society of London, Series B, 346: 387-397.
D.D. Leipe, P.O. Wainright, J.H. Gunderson, D. Porter, D.J. Patterson, F. Valois, S. Himmerich, & M.L. Sogin. 1994. The stramenopiles from a molecular perspective: 16S-like rRNA sequences from Labyrinthuloides minuta and Cafeteria roenbergensis. Phycologia 33(5): 369-377.
G.W. Saunders, D. Potter, & R.A. Andersen. 1997. Phylogenetic affinities of the Sarcinochrysidales and Chrysomeridales (Heterokonta) based on analyses of molecular and combined data. Journal of Phycology 33: 310-318.
Y. Van de Peer, G. Van der Auwera, & R. De Wachter. 1996. The evolution of stramenopiles and alveolates as derived by "substitution rate calibration" of small ribosomal subunit RNA. Journal of Molecular Evolution 42(2): 201-210.
Williams, D.M. 1991. Cladistic methods and chromophyte phylogeny. BioSystems 25: 101-112.
