Evolution of the Pax-Six-Eya-Dach network: the calcisponge case study.

Area of Science:

• Developmental biology
• Evolutionary developmental biology
• Marine biology

Background:

• The Pax-Six-Eya-Dach network (PSEDN) is crucial for sensory organ development and morphogenesis in bilaterians.
• Previous studies in demosponges revealed limited PSEDN components, contrasting with cnidarians.
• Calcisponges, evolutionarily distinct from demosponges, possess different developmental toolkits.

Purpose of the Study:

• To identify PSEDN components in the calcisponge Sycon ciliatum.
• To investigate the expression patterns of PSEDN genes during S. ciliatum development.
• To explore the evolutionary origins of the PSEDN.

Main Methods:

• Gene identification and phylogenetic analysis of Pax, Six, and Eya genes in S. ciliatum.
• Expression analysis using adult and embryonic/postembryonic stages of S. ciliatum.
• Analysis of additional eumetazoan genes (Elav, Msi) in cruciform cells.

Main Results:

• Two Pax, three Six, and one Eya genes were identified in calcisponges, differing from demosponges.
• Pax, Six, and Eya genes are expressed in patterns consistent with morphogenesis of choanocyte chambers.
• Paralogues of Pax and Six genes, along with Elav and Msi, are expressed in putative larval sensory cells (cruciform cells).

Conclusions:

• The role of the PSEDN in morphogenesis likely predates the divergence of animals.
• Cruciform cells in calcisponge larvae are strong candidates for sensory cells, expressing key developmental genes.
• The study highlights the ancient evolutionary conservation of developmental gene networks.