Molecules move Myzostomida

Geoff Read at
Wed Sep 6 18:06:27 EST 2000

AKA -  an Annelid ally alienated:

Eeckhaut, I., D. McHugh, P. Mardulyn, R. Tiedemann, D. Monteyne, M. 
Jangoux, & M. C. Milinkovitch. 2000. Myzostomida: a link between 
trochozoans and flatworms? - Proceedings of the Royal Society London, B 

Abstract: Myzostomids are obligate symbiotic invertebrates associated with 
echinoderms with a fossil record that extends to the Ordovician period. Due 
to their long history as host-specific symbionts, myzostomids have 
acquired a unique anatomy that obscures their phylogenetic affinities to 
other metazoans: they are incompletely segmented, parenchymous, 
acoelomate organisms with chaetae and a trochophore larva. Today, they 
are most often classified within annelids either as an aberrant family of 
polychaetes or as a separate class. We inferred the phylogenetic position 
of the Myzostomida by analysing the DNA sequences of two slowly evolving 
nuclear genes: the small subunit ribosomal RNA and elongation factor- 
1alpha. All our analyses congruently indicated that myzostomids are not 
annelids but suggested instead that they are more closely related to 
flatworms than to any trochozoan taxon. These results, together with recent 
analyses of the myzostomidan ultrastructure, have significant implications 
for understanding the evolution of metazoan body plans, as major 
characters (segmentation, coeloms, chaetae and trochophore larvae) might 
have been independently lost or gained in different animal phyla.  

  Geoff Read < at>

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