Pan, D.; Rajapaksha, D.; Caglar, C.; Rathjen, R.; Adamski, M.; Adamska, M.

Sponges are widely recognized as important model organisms for studying animal evolution, due to their phylogenetic position at the base of the animal tree of life, as well as similarities to the nearest animal relatives, the choanoflagellates. A critical aspect of animal evolution concerns the origin of germ layers, the embryonic structures which give rise to all tissues and organs of animal bodies. Haeckel's hypothesis suggested a germ layer homology between sponges and corals, and thus all eumetazoans (complex animals including cnidarians and bilaterians). According to this hypothesis, sponge choanoderm (composed of the feeding cells, choanocytes) and sponge pinacoderm (the outer epithelium) would be homologous to eumetazoan endoderm (from which the digestive system originates) and the ectoderm (giving rise to the epidermis), respectively. We addressed this hypothesis comparing tissue-specific transcriptomes derived from single-cell transcriptome datasets of sponges and cnidarians. We have sequenced single cell transcriptomes of Australian calcareous sponge, Sycon capricorn, and identified its cell types using a combination of in silico annotation of the cell clusters and in situ hybridization with marker genes. Single-cell transcriptome datasets for two demosponge species and two cnidarian species were extracted from recent literature. Homology was assessed using the SAMap algorithm, which has been designed to identify homologous cell types across vast evolutionary distances by detection of shared expression profiles. Our results are fully consistent with Haeckel's hypothesis, supporting homology between the innermost layers of sponges and cnidarians (choanoderm and endoderm/gastrodermis) as well as the outermost layers of sponges and cnidarians (pinacoderm and ectoderm/epidermis). Thus, sponge body plan appears to represent an intermediate step between single cell protists (choanoflagellates) and complex animals, rather than being independent experiment in animal multicellularity as suggested by alternative hypotheses.