David H. A. Fitch
fitch at acf2.nyu.edu
Tue Oct 7 11:36:16 EST 1997
Of course, there is no fossil record for nematodes, and any molecular clock
cannot therefore be appropriately calibrated. However, there are other
ways to estimate the time of divergence between rhabditids and tylenchids.
V. Malakhov (1994, Nematodes: Structure, Development, Classification and
Phylogeny, Smithsonian Institution Press, Washington) makes (I think) the
best argument based on the assumption that terrestrial parasitism must have
been preceded by soil saprobiosis (pp. 196-198); i.e., "no earlier than the
appearance of abundant flora and fauna on land and no earlier than the
appearance of soil as a special specific environment for habitation."
Consequently, "the epoch of parasitism formation in this evolutionary
branch can be assigned to the period when land plants and animals appeared;
that is, the Devonian period, probably even toward the end of that period
(no earlier than 350 million years ago)." Actually, that is near the end
of the Devonian period.
This estimate makes sense in other ways too. Molecular and morphological
evidence suggest ascarid-like lineages diverged from the rhabditid-like
lineages before tylenchids diverged from rhabditid-like ancestors.
Ascarid-like groups include parasites of not only terrestrial hosts, but
also of aquatic hosts (including primitively diverging groups of fish).
Malakhov goes on to write, "If one accepts this point of view, then the
epoch of the formation of parasitism in the spiroascaridid branch of
Rhabditia is no more recent than early Ordovician (about 500 million years
So, the earliest possible divergence for rhabditids and tylenchids is
probably between 360 MYA (end of the Devonian) and 300 MYA, by the time
vascular land plants had already extensively diversified. But the actual
divergence could have been much later. Maybe it would be informative to
see if there are any reviews on the distribution of tylenchids according to
the phylogeny of terrestrial plants!
>Is there a published estimate of how long (in millions of years) since C.
>elegans shared a common ancestor with one of the Tylenchid nematodes (e.g.,
>or in fact specifically, Heterodera glycines)? If not, is anyone able to
>provide a guess that you would be willing to have stand as a pers. comm. in
>an article I am writing for J. Nematol.?
>David McK. Bird
>Department of Plant Pathology
>North Carolina State University
>3415 Gardner Hall
>Raleigh, NC 27695-7616
>Office: (919) 515-6813
>Fax: (919) 515-9500
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