evolution of nematodes

David H. A. Fitch fitch at ACF2.NYU.EDU
Fri Oct 25 16:02:56 EST 1996


In response to the questions raised by Alain Bernot, 12:48 pm 10/17/96:

>1. Did the Nematodes appear during the Cambrian explosion (about 540 to 560 Ma
>ago) ?

Nematodes must have appeared long before the "Cambrian explosion".  The
Cambrian "explosion" is a geologically sudden appearance of animals with
rather hard bodies.  One hypothesis is that this sudden appearance actually
represents a sudden diversification of new body plans.  However, a more
reasonable hypothesis is that predation evolved at about this time,
selecting hard-body variants in lineages that had already diversified
(i.e., suggesting that Cambrian "explosion" is really a misnomer if applied
to diversification of body plans).  Nematodes have virtually no fossil
record because they don't have hard bodies (there are some amber fossils,
but nothing from Cambrian shales).  Same with annelids, although annelids
assuredly existed at the time of the "Cambrian explosion".

>2. If not, when did the Nematodes appear ?

According to Greg Wray (SUNY Stony Brook, NY, pers. comm.), there is new
data suggesting that animal lineages are far older than the Cambrian
explosion--perhaps another 500 Myr older!  Nematodes probably diverged from
other animals before protostomes diverged from deuterostomes (see Sidow and
Thomas 1994, Current Biology 4:596-603).  (See also Conway Morris, 1993,
Nature 361:219-225.)

>3. Are the Nematodes real ceolomates, or are they pseudocoelomates ?

Some people would argue that there is no such thing as a
"pseudocoelomate"--at least, a "pseudocoelom" makes a terrible character
for testing monophyly.  Certainly ultrastructural studies have suggested
that it is not possible to delimit 3 distinct body cavity types
(acoelomate, pseudocoelomate, eucoelomate), and there are many grades. 
Also, because zoologists do not know the relationships among several
invertebrate groups (at least 8 different phyla) that have a
"pseudocoelomate" body cavity (whatever that is), they have been
traditionally tossed into the same heap, often called the
"pseudocoelomates" or "Aschelminthes".  Using 18S rRNA genes,
Winnepenninckx et al. (1995, Mol. Biol. Evol. 12:1132-1137) suggested that
(1) the "pseudocoelomates" group is not monophyletic and (2) nematodes
diverged before deuterostomes and protostomes diverged from each other. 
Although it is an excellent molecule for resolving relationships within
nematode families (Fitch et al., 1995, Mol. Biol. Evol. 12:346-358), 18S
rRNA has its own set of problems for resolving lineages as deep as the
divergences among animal phyla (see Sidow and Thomas; Philippe et al.,
1994, Development Suppl. 15-25).

>4. Do the Nematodes have a metameric organisation or not ?

Not strictly in the way that annelids, arthropods and vertebrates do. 
However, there are repeated patterns of cell lineages (e.g., the cell
lineages in the lateral hypodermis), and the ray sensilla of the male tail
are serially homologous.  As in "metameric" organisms, the diversification
of these cell lineages or rays depends on differential Hox gene expression
(e.g., see Kenyon 1994, Cell 78:175-180; Salser and Kenyon 1994, Trends in
Genetics 10:159-164; Chow and Emmons 1994, Development 120:2579-2593).

>5. Any references about these subjects ?

A book is due to come out *soon* (1996? 1997?...) from Cold Spring Harbor
Laboratory Press:  "C. elegans II".  The last chapter is about fitting
nematode worms (especially *the* worm) into a phylogenetic context and
framework, and using that framework to reconstruct how evolution has
produced diversity in form and function.  Other chapters will cover Hox
genes, etc.

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