Sponges (Porifera) are among the earliest evolving metazoans. Their filter-feeding body plan based on choanocyte chambers organized into a complex aquiferous system is so unique among metazoans that it either reflects an early divergence from other animals prior to the evolution of features such as muscles and nerves, or that sponges lost these characters. Analyses of the Amphimedon and Oscarella genomes support this view of uniqueness-many key metazoan genes are absent in these sponges-but whether this is generally true of other sponges remains unknown. We studied the transcriptomes of eight sponge species in four classes (Hexactinellida, Demospongiae, Homoscleromorpha, and Calcarea) specifically seeking genes and pathways considered to be involved in animal complexity. For reference, we also sought these genes in transcriptomes and genomes of three unicellular opisthokonts, two sponges (A. queenslandica and O. carmela), and two bilaterian taxa. Our analyses showed that all sponge classes share an unexpectedly large complement of genes with other metazoans. Interestingly, hexactinellid, calcareous, and homoscleromorph sponges share more genes with bilaterians than with nonbilaterian metazoans. We were surprised to find representatives of most molecules involved in cell-cell communication, signaling, complex epithelia, immune recognition, and germ-lineage/sex, with only a few, but potentially key, absences. A noteworthy finding was that some important genes were absent from all demosponges (transcriptomes and the Amphimedon genome), which might reflect divergence from main-stem lineages including hexactinellids, calcareous sponges, and homoscleromorphs. Our results suggest that genetic complexity arose early in evolution as shown by the presence of these genes in most of the animal lineages, which suggests sponges either possess cryptic physiological and morphological complexity and/or have lost ancestral cell types or physiological processes.

The importance of polarity-the possession of a primary body axis-is evident in the functional features of animals, such as feeding, and therefore must have arisen simultaneously with the evolution of multicellular animal body plans. Sponges are thought to represent the most ancient extant lineage of multicellular animals and whereas adult sponges do not possess obvious polarity, they are useful study organisms in which to examine the origin and evolution of body polarity. We tested the effect of pharmacological agents known to disrupt the polarity of a wide variety of animals on sponge organization during development. Lithium chloride and alsterpaullone, which mimic canonical Wnt signaling in other animals, caused formation of ectopic oscula and disrupted the ability of the sponge to feed. Transplanted oscula were able to attach to and induce canal reorganization in host sponges suggesting that the osulum has inductive capabilities. This work suggests that canonical Wnt signaling is responsible for setting up the aquiferous system, which acts as an organizing center polarizing the sponge.

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