Acoela

Figure 12.3 An abbreviated phylogenetic tree depicting some metazoan clades with, above the nodes, the number of new miRNAs appearing at each cladogenetic event. The number of different miRNAs in Acoela is low (7 miRNAs) whereas that of 'Platyhelminthes' (Catenulida + Rhabdito-phora) is similar (33 miRNAs) to those of other lophotrochozoans like annelids and molluscs. This supports previous work suggesting the poly-phyly of Platyhelminthes and the basal position of Acoelomorpha (Ruiz-Trillo et al. 1999, 2002). Redrawn in a very modified form from Sempere et al. 2006.

Figure 12.3 An abbreviated phylogenetic tree depicting some metazoan clades with, above the nodes, the number of new miRNAs appearing at each cladogenetic event. The number of different miRNAs in Acoela is low (7 miRNAs) whereas that of 'Platyhelminthes' (Catenulida + Rhabdito-phora) is similar (33 miRNAs) to those of other lophotrochozoans like annelids and molluscs. This supports previous work suggesting the poly-phyly of Platyhelminthes and the basal position of Acoelomorpha (Ruiz-Trillo et al. 1999, 2002). Redrawn in a very modified form from Sempere et al. 2006.

by a marine polyclad, had almost all protostome miRNAs excluding the two ecdysozoan-specific miRNAs so far detected, confirming that they are lophotrochozoan protostomes.

If acoels are early-branching bilaterians, they should bear a reduced subset of the 21 miRNAs conserved across protostomes and deu-terostomes. Consistently, only six miRNAs were found in the acoel Childia sp. (Sempere et al. 2006). Additional species of Platyhelminthes (including parasitic species) have most protostome-specific miRNAs as well as those shared by protostomes and deuterostomes (L. F. Sempere, P. Martinez, J. Baguna and K. J. Peterson, unpublished data). Instead, a second acoel examined, Symsagittifera roscoffensis, has the same six miRNAs as Childia sp. Again, these data strongly support the idea that acoels are early-branching bilaterians and not members of the Platyhelminthes.

gene expression and axial homologies between cnidarians and bilaterians

A major breakthrough in biology during the second half of the twentieth century has been the demonstration that, while animal phyla are morphologically very disparate, they are fundamentally similar genetically. While the genetic composition of extinct taxa (e.g. the LCA of bilaterians) cannot be directly determined, we can use the phylogenetic distribution of developmental genes in extant species to infer the 'genetic toolkit' of the bilaterian LCA. Within the framework of the new molecular phylo-geny (Figure 12.1A), the bilaterian LCA is seen as endowed with scores of genes controlling, for example, body axiality, coelom formation and segmentation, photoreception, circulation and body appendages (Carroll et al. 2001). Such a constellation of genes had to be assembled at the dawn of the Bilateria from radial ancestors not bearing them.

The way we look at the origin of bilaterality changed recently when it was found that the morphologically simple and symmetrically 'radial' anthozoan cnidarians possess, besides genes involved in A-P polarity (Hox/ParaHox, otx, ems, gsc), gastrulation (twist [twi], snail [sna], bra-chyury [Bra], forkhead [fkh]), endodermal (GATA) and germ-cell (nanos [nos], vasa [vas]) specification, orthologues to bilaterian gene families previously thought to be absent in 'radial' organisms. Prominent among them are genes involved in mesoderm specification (Nk2, mef2, MyoD), D-V axial polarity (Wnt-ji-catenin, dpplbmp; Chordin/noggin [chd/nog], Gshl ind, Msh, vnd), nerve tissue and sensory-organ formation (Notch/Delta [N/ Dl], AchetelScute [Ac/Sc], Netrin, Pax 3) as well as in other cell signalling pathways (hedgehog [hh]), Receptor tyrosine kinases (Egfr, Fgfr) andJaklStat (for specific references, see Hayward et al. 2002, Finnerty et al. 2004, Martindale et al. 2004, Extavour et al. 2005, Martindale 2005, Matus et al. 2006, Rentzsch et al. 2006). The presence and expression in cnidarians of many of the genes involved in D-V patterning in bilaterians matched ideas (going back to Stephenson 1926, and held by Hyman 1951 and Salvini-Plawen 1978) of a second or directive axis in cnidarians (namely in anthozoans), perpendicular to the oral-aboral (O-AB) axis (Finnerty et al. 2004). Therefore, both cnidarians and bilaterians evolved from an ancestor already bilateral, putting the origin of the bila-terian LCA even further back in time.

Figure 12.4 summarises in a simplified form the A-P and D-V expression of selected developmental genes in cnidarians and bilaterians (for specific details see references above). Despite highly dynamic expressions, some A-P and D-V genes in cnidarians have patterns comparable to those of bilaterians. This seems so for gastrulation or 'posterior' genes such as Wnt, bra, sna, twi, fkh, for 'endodermal' or 'mesoendodermal' genes such as GATA, for 'mesodermal' genes like

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