Evolutionary modification of T-brain (tbr) expression patterns in sand dollar.

Area of Science:

• Developmental Biology
• Echinoderm Evolution
• Molecular Genetics

Background:

• Sand dollars are irregular echinoids that evolved from regular sea urchins ~200 million years ago.
• T-brain (tbr) genes play crucial roles in embryonic development across various species.
• Understanding tbr gene expression in sand dollars can illuminate evolutionary developmental patterns in echinoderms.

Purpose of the Study:

• To investigate the expression patterns of T-brain (tbr) orthologs during the early development of two sand dollar species.
• To compare tbr gene expression between indirect and direct developing sand dollars and with other echinoderms.
• To infer the ancestral expression patterns of tbr in echinoderms.

Main Methods:

• Isolation of two tbr orthologs: Smtbr from Scaphechinus mirabilis (indirect developer) and Pjtbr from Peronella japonica (direct developer).
• Whole mount in situ hybridization to visualize Smtbr and Pjtbr expression during early embryonic stages.
• Comparative analysis of tbr expression patterns across different echinoderm species.

Main Results:

• Smtbr expression was observed in micromere descendants early on, expanding to include macromere descendants and endomesodermal tissues (archenteron, primary mesenchyme) later in development.
• Pjtbr exhibited a similar endomesodermal expression pattern, with additional expression detected in the apical organ (ectoderm).
• The broader endomesodermal expression of tbr in sand dollars suggests this is an ancestral characteristic for echinoderms.

Conclusions:

• Sand dollar tbr genes show distinct expression patterns compared to regular sea urchins, particularly in the endomesoderm.
• Broader endomesodermal expression of tbr is likely an ancestral trait inherited from early echinoderms.
• The expression in the apical organ suggests additional roles for Pjtbr in ectodermal patterning during direct development.