Résumé
Phylogenomics has recently revealed that tunicates represent the
sister-group of vertebrates in the newly defined clade Olfactores.
However, phylogenomic and comparative genomic studies have also suggested
that tunicates are characterized by an elevated rate of molecular
evolution and a high degree of genomic divergence. Despite the recurrent
interest in the group, the picture of tunicate peculiar evolutionary
dynamics is still fragmentary, as it mainly lies in studies focusing on
only a 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 a previously unsampled tunicate, Microcosmus
squamiger. This allowed us to get further insights into
tunicate-accelerated evolution through a comparative analysis based on
pertinent phylogenetic markers, i.e., 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. Finally, an estimation of the d (N)/d (S) ratio in
three pairs of closely related taxa within Olfactores did not reveal
strong differences between the tunicate and vertebrate lineages suggesting
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.