Developmental Signalling in Marine Sponges
(August 2007 - December 2014)
During animal development, formation of morphologically complex structures from the fertilized egg requires precise cell to cell communication. Surprisingly, only a handful of intercellular signalling pathways are used throughout the animal kingdom: wnt, tgf-ß, hedgehog, receptor tyrosine kinase and notch. To gain insight into evolutionary origin of these important pathways, and establish simple models in which to study embryonic development, it is crucial to include basal animals in the suite of model species. Recent work on Nematostella vectensis and other cnidarians demonstrated that the invention of the major signalling pathways predated divergence of cnidarians and bilaterians. It also indicates that to gain insight into the ancestral state of the developmental signalling, it is necessary to look into animals representing earlier branching clades.
Figure 1: Animal tree of life. Despite the fact that sponge
monophyly remains a disputable issue (indicated by dashed lines), it is accepted that sponges are the earliest branching multicellular animals.
Sponges are considered to be the sister group of “true” animals, the eumetazoa, and thus provide the ultimate outgroup for all comparative studies of animal development. Recently, the genome of Amphimedon queenslandica, a Great Barrier Reef demosponge has been sequenced at the DOE Joint Genome Institute.
Adult Amphimedon queenslandica, the first sponge to have its genome sequenced. This demosponge is found in shallow waters of the Great Barrier Reef and is common around Heron Island.
Parenchymella larva of Amphimedon queenslandica swims using ciliae covering its entire surface except the anterior pole (left). The pigment ring at the posterior pole is a sensory organ and provides steering in response to light.
Analysis of the Amphimedon genome and expression studies throughout embryonic development demonstrate that all major metazoan signalling pathways are used during sponge embryogenesis. Wnt, tgf-ß, and hedgeling – a related to hedgehog transmembrane protein – genes are expressed in dynamic, partially overlapping patterns in Amphimedon embryos.
Signaling molecules that are critical in development of complex animals are also expressed in Amphimedon embryos.
Similarly to higher animals, the wnt pathway appears to be used in establishment of the anterior-posterior axis of the sponge embryo. These results open way to further research that would provide insights into developmental mechanisms of all multicellular animals from both molecular biology and evolutionary perspective.
Sycon ciliatum, a calcareous sponge common in Bergen area fjords. The phylogenetic position of calcareous sponges is currently disputed, with some data suggesting that this group is closer related to eumetazoa than to other sponges.
Amphiblastula larva of Sycon ciliatum swims using ciliae of its anterior half (left). Complete transparency of the larvae and juveniles should allow for relatively easy lineage tracing experiments and experimental manipulations.