alent stage (E13.5), confirmed that Hoxd10–13, and the paired

related homeobox 1 gene, Prrx1, are up-regulated in the autopods

of the bat wing compared to the bat hindlimb and mouse fore-

limb. Changes in the mRNA transcript abundance of several

genes involved in retinoic acid (RA) synthesis, degradation and

signaling were consistent with the down-regulation of the RA sig-

naling pathway in the developing bat wing autopod. Signal from a

microarray probe targeting the 5 0 UTR of the Meis homeobox 2

(Meis2) mRNA transcript, showed the greatest difference in

expression in the bat wing compared to bat hindlimb and mouse

forelimb. This difference was not seen for a probe complementary

to the Meis2 C-terminal coding region. The up-regulation of a

Meis2 isoform lacking a C-terminal homeodomain may play an

important role in the sculpting of bat wings during development.

doi:10.1016/j.mod.2009.06.1015

S17-04

Acoel flatworms and the origin of bilaterian animals

Pedro Martinez

Universitat de Barcelona, Barcelona, Spain

Genes of the Hox cluster code for transcriptional regulators

that show a collinear expression along the anterio-posterior

(AP) body axis in all bilateral animals. Because cnidarians seem

to rely on different gene systems (Wnt genes) to pattern their

main body axis, question arises as to when in the evolutionary

history of bilaterians the Hox system was first deployed to confer

positional identity along the AP axis. Recent molecular phyloge-

nies have convincingly shown that the acoel flatworms, tradition-

ally classified within the Platyhelminthes, are the sister group of

the remaining Bilateria, branching out before the common ances-

tor of protostomes and deuterostomes (the so-called PDA or

‘‘Urbilateria’’). This key phylogenetic position offers the opportu-

nity to search in acoels the presence and early role of Hox cluster

genes to pattern the AP axis. In the meeting we will report the

cloning, genomic arrangement, and expression domains of Hox

genes in the acoel species Symsagittifera roscoffensis. Three Hox

genes were detected, one of each belonging to the major groups

of Hox genes: anterior, central and posterior. BAC cloning,

sequencing and chromosomal FISH hybridization show them

not to be clustered in a unique genomic region. Despite its dis-

persal within the genome, they are expressed in the juvenile

worm in nested domains along the AP axis. This minimal set of

Hox genes in acoels and its coarse but nested spatial deployment

likely represents the first step in the evolution of the Hox AP pat-

terning systems. Their specific roles in AP patterning have been

analyzed using RNAi technologies.

doi:10.1016/j.mod.2009.06.1016

S17-05

Embryos and ancestors

Philip Donoghue

University of Bristol, Bristol, United Kingdom

Both comparative embryology and palaeontology were central

planks in the argument that Darwin presented in The Origin, pro-

viding evidence for a hierarchy in nature reflecting genealogical

descent, and the nature of extinct ancestors which living lineages

emerged. These perspectives have been drawn together vividly in

the discovery of fossilised animal embryos in rocks of Cambrian

and Ediacaran age, deposited contemporaneously with the evolu-

tionary emergence of metazoan diversity. As such, these fossils

provide a wholly new perspective on the embryology of early ani-

mals. More than a decade has passed since the first discovery of

fossilised embryos, and in the intervening years numerous local-

ities, horizons and types of embryo have been recovered. The

growing database appears so far to be dominated by the embryos

of direct developing organisms. To some, this provides the final

piece of evidence to reject the classical view that metazoans

evolved from ancestors that underwent maximal indirect devel-

opment, achieving adulthood only after undergoing a cata-

strophic metamorphosis from an earlier larval stage. However,

the fossil record of embryos, like that of other groups, should

not be read literally. The message from decay experiments is that

the record should be interpreted with great caution but it does

nothing to diminish the significance of the palaeoembryological

record which remains our only direct insight into developmental

evolution during the emergence of metazoan phyla.

doi:10.1016/j.mod.2009.06.1017

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