AN ESTIMATE OF DIVERGENCE TIME OF PARAZOA AND EUMETAZOA AND THAT OF CEPHALOCHORDATA AND VERTEBRATA BY ALDOLASE AND TRIOSE PHOSPHATE ISOMERASE CLOCKS

Previously we suggested that four proteins including aldolase and trio se phosphate isomerase (TPI) evolved with approximately constant rates over long periods covering the whole animal phyla. The constant rates of aldolase and TPI evolution were reexamined based on three differen t models for estimating evolutionary distances, It was shown that the evolutionary rates remain essentially unchanged in comparisons not onl y between different classes of vertebrates but also between vertebrate s and arthropods and even between animals and plants, irrespective of the models used, Thus these enzymes might be useful molecular clocks f or inferring divergence times of animal phyla, To know the divergence time of Parazoa and Eumetazoa and that of Cephalochordata and Vertebra ta, the aldolase cDNAs from Ephydatia fluviatilis, a freshwater sponge , and the TPI cDNAs from Ephydatia fluviatilis and Branchiostoma belch eri an amphioxus, have been cloned and sequenced, Comparisons of the d educed amino acid sequences of aldolase and TPI from the freshwater sp onge with known sequences revealed that the Parazoa-Eumetazoa split oc curred about 940 million years ago (Ma) as determined by the average o f two proteins and three models, Similarly, the aldolase and TPI clock s suggest that vertebrates and amphioxus last shared a common ancestor around 700 Ma and they possibly diverged shortly after the divergence of deuterostomes and protostomes.