Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Molecular Biology

Background:

• Ascidian tadpole larvae possess two brain pigment cells: an otolith cell (gravity receptor) and an ocellus (photosensory structure).
• These pigment cells contain melanin granules, similar to vertebrate pigment cells, suggesting a key role in the evolution of this cell type.

Purpose of the Study:

• To investigate the molecular changes during pigment cell evolution.
• To isolate and characterize the tyrosinase gene in Halocynthia roretzi, a key enzyme in melanin biosynthesis.

Main Methods:

• Isolation and sequencing of the tyrosinase gene cDNA and genomic clones from Halocynthia roretzi.
• Analysis of the gene's sequence, including its open reading frame (ORF) and unusual genomic organization.
• Detection of gene expression patterns during embryonic development using molecular techniques.

Main Results:

• A gene encoding tyrosinase was isolated, with a 596 amino acid ORF showing 36-39% identity to vertebrate tyrosinases.
• The tyrosinase gene exhibits unusual organization, including a large 3' untranslated region with homology to the coding sequence and a single short intron.
• Gene expression was observed in pigment precursor cells during early neurulation and in developing brain pigment cells of late tailbud embryos, correlating with enzyme activity.

Conclusions:

• This study provides the first molecular description of pigment cell differentiation in ascidian embryos.
• The findings contribute to understanding the evolution of chordate pigment cells and the molecular basis of melanin biosynthesis.