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The last common bilaterian ancestor

¹ Department of Paleobiology, National Museum of Natural History, Washington, D.C. 20560, USA
² Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, USA

View larger version (25K): [in a new window]   Fig. 1. New and old views of metazoan phylogeny. (A) A representative metazoan phylogeny pre-1995 showing the division of metazoa into the diploblasts, acoelomates and pseudocoelomates, and the coelomate protostomes and deuterostomes; note the intermediate position of the lophophorate phyla. (B) A more recent metazoan phylogeny with the acoelomate and pseudocoelomate taxa distributed among the two great protostome subclades, the lophotrochozoa and the ecdysozoa.

View larger version (162K): [in a new window]   Fig. 2. Kimberella, the oldest generally accepted bilaterian fossil from the Ediacaran assemblage. From the Winter Coast of the White Sea, Russia. The adjacent parallel lines are trace fossils associated with Kimberella, and are believed to represent infilled feeding scratches through a microbial mat. The presence of these feeding traces suggests that Kimberella possessed feeding structure similar to the molluscan radula.

View larger version (21K): [in a new window]   Fig. 3. Schematic of different possible times of protostome-deuterostome divergence (orange circles), in relation to other events in the late Neoprotoerozoic. In scenario 1 there is little incongruence between the fossil record and the time of divergence, which occurs following the second or Marinoan glacial interval. In scenario 2, the divergence occurs between the two glaciations, and in scenario 3, before the Sturtian glaciation. Both scenarios 2 and 3 imply a much longer missing interval of post-PDA history. A, Middle Cambrian Burgess Shale fauna; B, Lower Cambrian Chengjiang fauna; C, Diverse Ediacaran fauna; D, Doushantuo phosphorite assemblage (with poor age constraints shown by uncertainty in position). See text for more discussion.

View larger version (18K): [in a new window]   Fig. 4. Evolution of gene regulatory networks during early bilaterian evolution. Colored boxes are transcriptional domains where the state of the domain is dependent upon the presence of the product of the gene of the same color. Stage 1. Initial pattern, similar to that in a Type 1 embryonic system (developmental process in which embryonic lineages proceed directly to expression of differentiation genes) (Davidson, 1991; Davidson, 2001). (A) The genes in the box to left transduce spatial embryonic cues (thick green arrow) and activate an initial gene (green), which in turn activates two additional genes (red and orange) all of which produce transcription factors; the orange gene also cross-regulates the red gene. These transcription factors in turn regulate the gene battery to right. This gene battery encodes proteins used for a differentiated cell type (a-d); each gene has at least two cis-regulatory inputs, indicated in orange and red with ‘x’ denoting other inputs which may vary from gene to gene. (B) Stage 2. Later evolutionary stage: the cell differentiation battery shown in Stage 1 has now been incorporated into a pattern formation system that controls an evolutionarily new morphogenetic process deriving from the state in Stage 1. The additional boxes (Stage 2 and Stage 3) represent new multicellular spatial transcription domains. Only the red gene from Stage 1 is shown in this figure; the red gene is still activated at its initial embryonic address via the green gene as in the ancestor of Stage 1. A new regulatory linkage has appeared, so that the transcriptional activator from the red gene now controls the purple gene, generating the purple transcriptional domain. A growth circuit has also been added. A second cis-regulatory module has been added to the red gene, allowing it to be activated by the purple gene product or repressed by a signal (S) from the underlying spatial domain (Stage 3). The result at Stage 4 is to mount the differentiation gene battery on morphological structure of which the patterning and growth are dependent on the yellow and purple transcriptional domains. Redrawn with permission from Fig. 5.7 of Davidson (Davidson, 2001).