Isozymes of a tunicate and a cephalochordate as a test of polyploidisation in chordate evolution

IN view of the uncertainty regarding early phylogeny of the chordates1,2, Atkin and Ohno3 determined the nuclear DNA contents of representatives of three chordate subphyla, Ciona intestinalis (Tunicata) has a diploid DNA content of 0.42 pg, while Amphioxus lanceolatus (Cephalochordata) contains 1.2 pg—very close to the 1.4 pg considered as the basic vertebrate value4,5. Assuming a tunicate-like ancestry of the cephalochordate-vertebrate stock6–9, Ohno10 proposed that the threefold difference in genome size between Ciona and Amphioxus might reflect the occurrence of at least one round of polyploidisation preceding vertebrate evolution. To test further this hypothesis we determined the number of genes coding for various enzymes in C. intestinalis and Branchiostoma (Amphioxus) lanceolatum. Only few genetically interpretable isozyme data from B. lanceolatum have been reported11,12. An isozyme study on a related species, B. moretonensis has been published recently13. Systematic isozyme investigations in tunicates have not been reported before. We report here that C. intestinalis and B. lanceolatum have identical gene numbers for homologous enzymes with only one exception.