EVOLUTION - SINGLE ORIGIN OF COELOMATES

MONOPHYLETIC ORIGIN OF COELOMATES

Segmentation as a starting point

Annelids are the animals most like the ancestral form of all coelomates.  Their segmentation enabled the speedy evolution of diversity.  That diversity includes forms that show little evidence of the remnants of segmentation.  Metamerism, of chordates and some other phyla, is the repetition of organs or structure along the length of a body that no longer has segmentation.

The serial repetition of structures enables regional modification of appendages and other structures from a relatively complex base with minimal genetic changes.  This is illustrated most clearly by arthropod appendages.  It is evident in our own pectoral and pelvic appendages, although not so obviously having an early stage of origin in segmentation.  Our early embryo has some of the more convincing evidence in myotomes; adults have vertebrae with paired nerves, blood vessels, and muscles that may also be convincing.

Reduction of segmentation in the chordate line was a result of its loss by pogonophorans in the portion of the body retained in hemichordate evolution.  Most mollusks lost evidence of metamerism as a result of the shell removing benefits of obvious metamerism. A vermiform fossil shows evidence of the annelid origin of mollusks (Sutton et al. 2001); and a fossil, Neopilina, shows a more molluscan intermediate stage (Lemche, 1957).   Arthropods have striking evidence of metamerism externally in most groups, both with skeletal segmentation and appendages

Just as segmentation was modified beyond easy recognition, the coelom was also greatly modified as higher animals diversified.  Coeloms, found in both the protostome line and the deuterostome line, but not in animals earlier than annelids, provide evidence of the annelid origin of coelomates.

Nephridia

Water and salt regulation is an important function of our kidney.  The annelid nephridium, with pairs in most of their segments, was modified by evolution into the nephrons of our kidneys; an intermediate connection is shown by Ruppert and Balser, 1986.  The oviduct also seems to be a nephron modification.  The water-vascular system of echinoderms may be derived from nephridia; the water-vascular system stays open to the exterior, indicating the echinoderms have probably lost the benefits of osmoregulation and thus never were able to survive in fresh water.

Blood

The central position of annelids in the ancestry of coelomates may be illustrated by blood pigments.  Polychaete worms have as many as four different oxygen transporting pigments in the blood.  Higher forms typically have one of those pigments, in our case, hemoglobin.

Other Molecular Evidence

Once I thought the Pogonophora were the deuterostome connection to protostomes (1983), I found a lot of supporting molecular evidence.  One of the first was the finding of Lipman and Pearson (1985) that crayfish Trypsin I is high-scoring for similarity to bovine trypsinogen.

The interpretation of molecular evidence is complicated by the fact that vertebrates tend to have some important genetic features in family clusters of genes whereas other animals typically have only one from a family.  Also, the same molecules may act in somewhat similar but different ways in different groups.  Hobmayer et al. (2000) found the WNT signaling pathway that had been found in nematodes, insects, and vertebrates (all with bilateral symmetry) also acted in axis formation of a radially symmetric cnidarian.

The protostome-deuterostome transition via pogonophorans may be returned to at some later date.  I expect to end the topic with my next post of a few additional references, some annotated, from my reference file that deal in some way with the topic, most about molecular evidence.

In summary, a wide variety of evidence points to the basic truth of the annelid theory of chordate origin.

References

   Hobmayer, Bert, Fabian Rentzsch, and 6 others.  2000.  WNT signaling molecules act in axis formation in the diploblastic metazoan Hydra.  Nature, 407:186-189.

   Lemche, H.  1957.  A new living deep-sea mollusk of the Cambro-Devonian class Monoplacophora.  Nature, 179:413-416.

   Lipman, David J., and William R. Pearson.  1985.  Rapid and sensitive protein similarity searches.  Science, 227:1435-1441.

   Ruppert, Edward E., and Elizabeth J. Balser.  1986.  Nephridia in the larvae of hemichordates and echinoderms.  Biol. Bull., 171:188-196.

   Sutton, Mark D., Derek E. G. Briggs, David J. Siveter and Derek J. Siveter.  2001.  An exceptionally preserved vermiform mollusc from the Silurian of England.  Nature, 410:461-463.

Joseph G. Engemann    June 30, 2013