[Taxacom] Tardigrades ----> to Nematodes

[Kenneth Kinman]

Dear All,
     Isn't it interesting that tardigrades and nematodes are among the
few animals able to enter cryptobiosis, especially when threatened by
extreme desiccation.  I would suggest that some clade of tardigrades
acquired genes for the production of trehalose sugars from primitive
land plants (such as mosses).  Both groups would have faced the same
problem of desiccation as they invaded land environments during the
early Paleozoic.       
      I propose that tardigrades in turn passed these "trehalose genes"
on to their nematode descendants, thus enabling them to also take
advantage of cryptobiosis as a survival strategy.  Something to look for
when comparing nematode and tardigrade genomes.  
       --------Ken Kinman                
P.S.   If tardigrades evolved from arthropods, as I suspect, did they
evolve from crustaceans or from chelicerates???  Both of these arthropod
groups give rise to terrestrial forms with reduced number of legs.
Modern tardigrades have four pairs of legs like many arachnids, but the
known fossil tardigrades from the Cambrian of Siberia were apparently
hexapodous.  But then again, I suspect hexapody may have arisen
independently more than once from crustaceans.  Therefore, the exact
number of legs may not be definitive, except that perhaps reduction to 3
or 4 pairs of legs may somehow be particularly advantageous on land
given the high diversity of hexapods and arachnids on land.