Brugia malayi, evolutionary relationship to C. elegans

David H. A. Fitch fitch at ACF2.NYU.EDU
Fri May 10 09:25:32 EST 1996


Thomas,

In partial answer to your query, Brugia malayi is probably at least as
distant phylogenetically from C. elegans as is Ascaris.  I don't think that
appropriate molecular tools have yet addressed the question, but
traditionally, Brugia has been classified into an order that is quite
separate from the Rhabditida.  Maggenti (1981: General Nematology,
Springer-Verlag) even puts it into a different subclass, Spiruria, from
Rhabditia.  (The orders Rhabditida and Spirurida--or subclasses Rhabditia
and Spiruria, whatever taxonomy you wish to apply--are grouped together
within the Secernentea, one of the two major groups of Nematoda--or if you
follow Andrassy, Secernentia, one of the 3 major groups of Nematoda...if
you're not confused yet, read on...)

Because of the systematic problems associated with the special adaptations
of parasites, however, one should take such classifications with a rather
large grain of salt.  Recent molecular data, for example, strongly suggests
that the order of parasites called Strongylida arose from within the family
Rhabditidae, making Rhabditidae a paraphyletic taxon (Fitch and Thomas'
"Evolution" chapter in C. elegans II, Cold Spring Harbor, in
press--although I should mention that the idea that Rhabditidae is probably
paraphyletic is not new).

For filarioids, however, I suspect the traditional view is probably fairly
accurate.  For example, Vanfleteren et al. (1994: Mol. Phylogenet. Evol.
3:92-101) used a vertebrate molecular clock to estimate that Ascaridida
diverged about 500 Myr ago from Rhabditida and Strongylida, which diverged
from each other about 400 Myr ago (but again, I think you have to take the
"molecular clock" data with a grain of salt--are vertebrate molecular
clocks going to apply well to nematode lineages?).  Data of Nadler (1992:
Mol. Biol. Evol. 9:932-944) combined with Fitch et al. (1995: Mol. Biol.
Evol. 12:346-358) appear to support the monophyly of at least family
Rhabditidae and superfamily Ascaridoidea (but these are only parts of
larger orders).

The bottom line--Everybody's guess seems to be that the Spirurida probably
diverged from Rhabditida and Strongylida at least as long ago as did the
Ascaridida--corresponding to the diversification of potential hosts for the
parasitic forms sometime during the late Cambrian to late Ordovician (and
possibly early Devonian) periods.  It's still a guess.

Sorry I couldn't be of more help.  You should also ask parasitic worm
persons like Mark Blaxter, and check out Wormwood references.  I doubt that
the "Tree of Life" www site is of any help in this regard.

Best wishes,
Dave

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