The genome of a Urochordate, Ciona intestinalis

Sixteen single-copy coding sequences were hybridized on DNA fractions from shallow CsCl gradients. GC levels of the fractions corresponding to hybridization peaks are indicated on the analytical CsCl profile in Fig. 9.2A. The GC and GC3 levels of the coding sequences are shown in Fig. 9.2B. They cover most of the compositional spectrum of the 

In conclusion, the genome of Ciona is characterized by a very symmetrical compositional distribution of DNA molecules and by a gene distribution which is barely shifted to higher GC values compared to the distribution of DNA. This is a situation which is remarkably different from that found in fishes where both distributions trail towards high GC values, (see Part 4). A speculation (modified from De Luca et al., 2002) on the implication of these differences as far as the formation of two gene spaces of vertebrates are concerned follows. 

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