A phylogenomic framework and timescale for comparative studies of tunicates

Area of Science:

• Evolutionary Developmental Biology
• Chordate Phylogeny
• Comparative Genomics

Background:

• Tunicates are crucial models for understanding vertebrate evolution.
• Previous tunicate phylogeny relied on limited genetic markers (18S rRNA, mitogenomes).
• Understanding tunicate relationships is key to chordate evolutionary studies.

Purpose of the Study:

• To resolve the evolutionary relationships among major tunicate lineages.
• To establish a timescale for tunicate diversification.
• To provide a robust phylogenetic framework for comparative studies.

Main Methods:

• Transcriptomic approach to generate a large phylogenomic dataset.
• Inclusion of 258 evolutionarily conserved orthologous genes.
• Phylogenetic analyses using site-heterogeneous CAT mixture models.
• Relaxed molecular clock analyses for divergence time estimation.

Main Results:

• A strongly supported phylogenetic tree resolving four major tunicate clades.
• Confirmation of Thaliacea as sister to Phlebobranchia + Aplousobranchia.
• Ancient diversification estimated between 450-350 million years ago.
• Uncertainty remains regarding the precise position of the genus Ciona.

Conclusions:

• The study provides the most comprehensive phylogenomic dataset for tunicates to date.
• Establishes a reference phylogenetic framework and tentative timescale for tunicates.
• Enables direct comparison with vertebrate model species in evolutionary studies.