Gut-like ectodermal tissue in a sea anemone challenges germ layer homology

Area of Science:

• Developmental biology
• Comparative genomics
• Evolutionary developmental biology

Background:

• Cnidarians possess two germ layers (ectoderm, endoderm), while bilaterians have three (ectoderm, mesoderm, endoderm).
• Cnidarian endoderm is currently hypothesized to be homologous to both bilaterian endoderm and mesoderm.
• Understanding germ layer homologies is crucial for tracing animal evolution.

Purpose of the Study:

• To investigate germ layer homologies between cnidarians and bilaterians.
• To analyze transcription factor expression and cell types in the sea anemone *Nematostella vectensis*.
• To challenge the existing model of cnidarian-bilaterian germ layer homology.

Main Methods:

• Studying germ layer fate mapping in *Nematostella vectensis*.
• Analyzing the localization of gut cell types.
• Examining the expression of conserved 'endodermal' and 'mesodermal' transcription factor orthologues.

Main Results:

• The developing pharyngeal ectoderm of *Nematostella vectensis* exhibits transcription factor profiles and cell types (exocrine, insulinergic) similar to bilaterian midgut and vertebrate pancreas.
• Cnidarian endodermal derivatives show functions and transcription factor profiles akin to bilaterian mesoderm derivatives (e.g., gonads, heart).

Conclusions:

• The data supports an alternative model of germ layer homologies.
• Cnidarian pharyngeal ectoderm may be homologous to bilaterian endoderm.
• Cnidarian endoderm may be homologous to bilaterian mesoderm.