Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional correspondences. In contrast, the excretory mechanisms in non-nephrozoans are largely unknown, and therefore the reconstruction of ancestral excretory mechanisms is problematic. Here, we investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. By combining gene expression, inhibitor experiments and exposure to varying environmental ammonia conditions we show that both, Xenacoelomorpha and Cnidaria, are able to excrete across digestive-associated tissues. However, while the cnidarian Nematostella vectensis seems to use diffusion as its main excretory mode, the two xenacoelomorphs use both, active transport and diffusion mechanisms. Based on these results we propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. We conclude that the emergence of a compact, multiple-layered bilaterian body plan necessitated the evolution of active transport mechanisms, which was later recruited into the specialized excretory organs.
Ascoparia_sp_SRX1343822
Assembled transcriptome of Ascoparia sp. from SRA SRX1343822 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki)
Childia_submaculatum_SRX1534054
Assembled transcriptome of Childia submaculatum from SRA SRX1534054 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Convolutriloba_macropyga_SRX1343815
Assembled transcriptome of Convolutriloba macropyga from SRA SRX1343815 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Diopisthoporus_gymnopharyngeus_SRX1534055
Assembled transcriptome of Diopisthoporus gymnopharyngeus from SRA SRX1534055 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Diopisthoporus_longitubus_SRX1534056
Assembled transcriptome of Diopisthoporus longitubus from SRA SRX1534056 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Eumecynostomum_macrobursalium_SRX1534057
Assembled transcriptome of Eumecynostomum macrobursalium from SRA SRX1534057 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Hofstenia_miamia_SRX503688
Assembled transcriptome of Hofstenia miamia from SRA SRX503688 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Isodiametra_pulchra_SRX1343817
Assembled transcriptome of Isodiametra pulchra from SRA SRX1343817 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Meara_stichopi_SRX1343814
Assembled transcriptome of Meara stichopi from SRA SRX1343814 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Nemertoderma_westbladi_SRX1343819
Assembled transcriptome of Nemertoderma westbladi from SRA SRX1343819 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Sterreria_sp_SRX1343821
Assembled transcriptome of Sterreria sp. from SRA SRX1343821 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Xenoturbella_bocki_SRX1343818
Assembled transcriptome of Xenoturbella bocki from SRA SRX1343818 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).
Xenoturbella_profunda_SRX1280426
Assembled transcriptome of Xenoturbella profunda from SRA SRX1280426 using Trinity (https://github.com/trinityrnaseq/trinityrnaseq/wiki).