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.
Highlights
Attempts to explain the origin of metazoan life seek to unravel both the transition from (1) single-celled to multicellular organisms and (2) diploblastic to triploblastic body plans
Following one of the basic principles in evolutionary biology that complex life forms derive from more primitive ancestors, it has long been believed that the higher animals, the Bilateria, arose from simpler organisms such as the cnidarians
The Bilateria and Cnidaria are sister groups: that is, they evolved in parallel from a very simple common ancestor
Summary
Attempts to explain the origin of metazoan life seek to unravel both the transition from (1) single-celled to multicellular organisms and (2) diploblastic to triploblastic body plans. Attempts to unravel the urmetazoon bauplan and to provide support for any of the five hypotheses depends on identifying the most basal extant diploblast group. Two phylogenetic alternatives have remained under discussion; one sees the sponges (Porifera) and the other the placozoans (Placozoa) as basal relative to all other diploblast groups [6,7,8,9,10]. The latter view was accepted for the most part of the last century. The presence of only four somatic cell types, the smallest metazoan genome, and the lack of any foot or head structures, any anterior–posterior organization, or any kind of organs, and both a basal lamina and an extracellular matrix (ECM) places Trichoplax in a basal and isolated position relative to all other metazoan phyla [11,12,13,14,15,16] (cf. [17], )
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