Myzostomida: a link between trochozoans and flatworms?

Area of Science:

• * Marine Biology
• * Evolutionary Biology
• * Invertebrate Zoology

Background:

• * Myzostomida are obligate symbionts of echinoderms with a long fossil record.
• * Their unique anatomy (incompletely segmented, acoelomate, with chaetae and trochophore larva) has historically obscured their evolutionary relationships.
• * Traditionally, myzostomids are classified within annelids, either as aberrant polychaetes or a separate class.

Purpose of the Study:

• * To determine the phylogenetic position of Myzostomida within Metazoa.
• * To clarify the evolutionary history and classification of myzostomids.
• * To reassess the evolution of key metazoan body plan characters.

Main Methods:

• * Phylogenetic analysis using DNA sequences from two nuclear genes: small subunit ribosomal RNA (SSU rRNA) and elongation factor-1alpha (EF-1α).
• * Comparison of molecular data with existing ultrastructural analyses.

Main Results:

• * Molecular phylogenetic analyses consistently placed Myzostomida outside of Annelida.
• * Results indicated a closer relationship between myzostomids and flatworms (Platyhelminthes) than any trochozoan group.
• * The findings suggest that segmentation, coeloms, chaetae, and trochophore larvae may have evolved independently or been lost multiple times across animal phyla.

Conclusions:

• * Myzostomida are not annelids, necessitating a reclassification.
• * Their placement closer to flatworms provides new insights into early metazoan evolution.
• * The study highlights the potential for convergent evolution or loss of major body plan features in invertebrates.