Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis

Abstract

For more than a century, the origin of metazoan animals has been debated. One aspect of this debate has been centered on what the hypothetical “urmetazoon” bauplan might have been. The morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles an intriguing model for one of several “urmetazoon” hypotheses: the placula hypothesis. Clear support for a basal position of Placozoa would aid in resolving several key issues of metazoan-specific inventions (including, for example, head–foot axis, symmetry, and coelom) and would determine a root for unraveling their evolution. Unfortunately, the phylogenetic relationships at the base of Metazoa have been controversial because of conflicting phylogenetic scenarios generated while addressing the question. Here, we analyze the sum of morphological evidence, the secondary structure of mitochondrial ribosomal genes, and molecular sequence data from mitochondrial and nuclear genes that amass over 9,400 phylogenetically informative characters from 24 to 73 taxa. Together with mitochondrial DNA genome structure and sequence analyses and Hox-like gene expression patterns, these data (1) provide evidence that Placozoa are basal relative to all other diploblast phyla and (2) spark a modernized “urmetazoon” hypothesis.

Metazoan opsin evolution reveals a simple route to animal vision

“…Consistently with our results, some of the most thorough analyses to date (20,21) agree that the Placozoa are the sister group of Neuralia, even though some investigators (22,30) found different results. However, Philippe et al (31) have shown the results of Schierwater et al (30) to be invalid. Differently, even though the study of Pick et al (22) is sound, its conclusion that Placozoa is a member of Neuralia is questionable.…”

Assessing the root of bilaterian animals with scalable phylogenomic methods

“…This study placed Porifera as sister to other metazoans, but bootstrap support was low (62% for other animals, Eumetazoa, to the exclusion of sponges). A recent analysis of a manually curated set of mitochondrial and nuclear genes, together with a small morphological matrix, concluded that 'Diploblastica' (including Porifera), not Ctenophora or Porifera, is sister to all other animals (Schierwater et al 2009). This topology, however, was statistically indistinguishable from a tree that placed Ctenophora as sister to all other animals (see table 1 in Schierwater et al 2009).…”

Animals and Their Unicellular Ancestors

“…The morphologically simple 'Placozoa' have also been candidates to be direct descendants from the metazoan last common ancestor (Schierwater et al, 2009). However, molecular phylogenies have shown placozoans in every possible position among the diploblasts, from sister group to Cnidaria (Bridge et al, 1995), to basal diploblasts (Dellaporta et al, 2006;Schierwater et al, 2009), to an intermediate position between sponges and cnidarians (Srivastava et al, 2008).…”