Classification of the Amoebae
Page updated 26/8/03
classification of the amoebae has been fraught with ambiguity and
confusion. This is in part due to the fact that many of the group
lack morphologically helpful features and the fact that so many protists
from diverse origins have taken up the amoeboid way of life.
Fredrick Page (formerly of the Culture Collection of Algae and Protozoa)
has brought some order to the classification of the amoeba in his heroic
life-long study. He has described dozens of amoebal species for the
first time and his keys remain essential reading for those wishing to
understand the amoeba both freshwater (Page,
F.C., 1988) and marine (Page,
F.C., 1983). Ultimately, a thorough
understanding of amoebal classification must include genetic data such as
SSUrDNA sequencing. Progress has been made along these lines for a
variety of amoebal species and in particular Acanthamoeba and Naegleria
(De Jonckheere's group). However, even here difficulties and
ambiguities have arisen, for example.. The assumption that the
SSUrDNA analysis makes is that each protist has one unique sequence and
that this has been inherited directly from its immediate parent with the
occasional mutation. It is known that at least one species of algae
possesses two distinct SSUrDNA genes which of course complicates things!!
Some strains of Acanthamoeba contain at least two genes (Stothard
et al, 1998), and some Neoparamoeba
have more than three! (Fiala & Dykova 2003). It seems that for some groups notably the Vahlkampfiidae and Entamoebidae
these genes are held on a multi-copy circular plasmid. This
introduces a fresh suspicion; can one plasmid (with its itinerant
replication codes and promoters) be transmitted horizontally between
members of these groups? Even given that SSUrDNA analysis may not be
perfect I believe that it is the best option open to us. One
advantage is that significant progress has already been made and many
disparate amoeboid genera have had members sequenced. If it turns
out that there are significant problems with SSUrDNA analysis as a means
to accredit relationships it will still serve at least one very tangible
advantage this is that one determined there would be little future doubt
that amoeba "A" is the same or similar to a fresh isolate.
As things stand in order to establish that a fresh isolate is a strain of
a previous species takes an immense amount of work and considerable
experience. Take for example Rosculus ithacus, isolated from
the rectum of a grass snake, and subsequently isolated from a river and a
ditch in the Cambridge area later. Despite painstaking efforts, the
fact that the fresh strains possessed contractile vacuoles while the
original did not leaves considerable doubt. This would have been
dissipated to a very large degree if the SSUrDNA of the original isolate
had been determined. Of course this would have been very
difficult back in 1963 when the poor snake was originally explored but now
with PCR and sequencing by post this is almost trivial (if a little
expensive for the budget of the average amoeba lab!).
is now being appreciated that some groups of amoebae are emerging from the
Fiala, I. & Dykova, I. (2003) Molecular
characterisation of Neoparamoeba strains isolated from gills of
Scophthalmus maximus. Dis Aquat Organ. 55, 11-16.
Schaeffer, A. (1926) "Taxonomy of the amebas."
Pap.Dept.Mar.Biol.Carnegie Institut. Wash. Vol XXIV, 1-116.
Sogin, M. L., Silberman, J.D, Hinkle, G. &
Morrison, H.G. (1996). "Problems with molecular diversity in the
eukarya.".Society of General Microbiology Symposium: Evolution of microbial
Life ed.Roberts, D.M., Sharp, P., Alderson, G. & Collins, M.A. Cambridge
University Press. pp167-184.
Stothard, D. R., Schroeder-Diedrich, J.
M., Awwad, M. H., Gast, R. J., Ledee, D. R., Rodriguez-Zaragoza, S., Dean,
C. L., Fuerst, P. A. & Byers, T. J. (1998) The evolutionary history of the
genus Acanthamoeba and the identification of eight new 18S rRNA
gene sequence types. J.Eukaryotic Microbiol. 45, 45-54.