High Evolutionary Rate Variations in Tunicate Genomes

Phylogenomic and comparative genomic studies have suggested that tunicates are characterized by an elevated rate of molecular evolution and a high degree of genomic divergence. The picture of tunicate peculiar evolutionary dynamics still mainly lies in studies focusing on only few model species. In order to expand the available genomic data for the group, we used the high-throughput 454 technology to sequence the partial transcriptome of Microcosmus squamiger. This allowed to get further insights into tunicate accelerated evolution through a comparative analysis based on a core of 35 housekeeping genes conserved across bilaterians. Our results showed that tunicates evolved on average about two times faster than the other chordates, yet the degree of this acceleration varied extensively upon genes and upon lineages. Appendicularia and Aplousobranchia were detected as the most divergent groups which were also characterized by highly heterogeneous substitution rates across genes. An estimation of the dN/dS ratio in three pairs of closely related taxa within Olfactores did not reveal strong differences between the tunicate and vertebrate lineages. This observation suggests that for this set of housekeeping genes the accelerated evolution of tunicates is plausibly due to an elevated mutation rate rather than to particular selective effects.

Reference: Tsagkogeorga G., Turon X., Galtier N., Douzery E.J.P. & Delsuc F. (2010). Accelerated evolutionary rate of housekeeping genes in tunicates. Journal of Molecular Evolution 71: 153-167.