EVIDENCE FROM MILK CASEIN GENES THAT CETACEANS ARE CLOSE RELATIVES OFHIPPOPOTAMID ARTIODACTYLS

The inferred transition from terrestrial hoofed mammal to fully aquati c cetacean has been intensively studied with fossil evidence. However, large sections of this remarkable evolutionary sequence are missing. Phylogenetic analysis of extant taxa may help to fill in some of these gaps. In this report, kappa-casein (exon 4) and beta-casein (exon 7) milk protein genes from cetaceans and other placental mammals were PCR -amplified, sequenced, and aligned to previously published sequences. Phylogenetic analyses of the casein data suggest that hippopotamid art iodactyls are more closely related to cetaceans than to other artiodac tyls (even-toed hoofed mammals). An analysis of the nuclear casein seq uences combined with published mitochondrial cytochrome b DNA sequence s also supports the Cetacea/Hippopotamidae sister group. This affinity implies that some of the aquatic traits of cetaceans were derived in the common ancestor of Cetacea and Hippopotamidae. An extant ''missing link'' to Cetacea may have been overlooked by science since the descr iption of the semiaquatic Hippopotamus in 1758. Paleontological inform ation is grossly inconsistent with this hypothesis. If the casein phyl ogeny is accurate, large gaps in the fossil record as well as extensiv e morphological reversals and convergences must be acknowledged.