How fast is the sessile ciona?

Abstract

Genomewide analyses of distances between orthologous gene pairs from the ascidian species Ciona intestinalis and Ciona savignyi were compared with those of vertebrates. Combining this data with a detailed and careful use of vertebrate fossil records, we estimated the time of divergence between the two ascidians nearly 180 My. This estimation was obtained after correcting for the different substitution rates found comparing several groups of chordates; indeed we determine here that on average Ciona species evolve 50% faster than vertebrates.

Figure 1

Regression line of divergence times derived from the fossil record (Table 1) and the correspondent distances (JTT method) in different vertebrate pairs. For the continuous line: 1. M. musculus – R. norvergicus, 2. X. laevis – X. tropicalis, 3. H. sapiens – B. taurus, 4. H. sapiens – M. musculus, 5. H. sapiens – M. domestica, 6. H. sapiens – O. anatinus, 7. H. sapiens – G. gallus, 8. H. sapiens – X. laevis, 9. H. sapiens – T. rubripes. For the dashed line: same pairs, comparisons involving nonplacental mammals (points 5 and 6) were excluded.