Evidence for a single loss of mineralized teeth in the common avian ancestor

INTRODUCTION The absence of teeth or edentulism has evolved on multiple occasions within vertebrates, including birds, turtles, and a few groups of mammals (anteaters, baleen whales, and pangolins). There are also mammals with enamelless teeth (aardvarks, sloths, and armadillos). All toothless/enamelless vertebrates are descended from ancestors with enamel-capped teeth. In the case of birds, it is theropod dinosaurs. Instead of teeth, modern birds use a horny beak (rhamphotheca) and part of their digestive tract (muscular gizzard) to grind up and process food. The fossil record of early birds is fragmentary, and it is unclear whether tooth loss evolved in the common ancestor of all modern birds or convergently in two or more independent lineages. Observed shared inactivating mutations in tooth formation. Related genes were mapped onto a time tree depicting evolutionary relationships and times of divergence between modern birds, the closely related extinct taxon Ichthyornis, and the American alligator. The hypothesized loss of mineralized teeth on the modern bird branch at 116 million years ago (Ma) is based on frameshift mutation rates. Observed shared inactivating mutations in tooth formation. Related genes were mapped onto a time tree depicting evolutionary relationships and times of divergence between modern birds, the closely related extinct taxon Ichthyornis, and the American alligator. The hypothesized loss of mineralized teeth on the modern bird branch at 116 million years ago (Ma) is based on frameshift mutation rates. RATIONALE Tooth formation in vertebrates is a complicated process that involves many different genes. Of these genes, six are essential for the proper formation of dentin (DSPP) and enamel (AMTN, AMBN, ENAM, AMELX, and MMP20). We examined these six genes in the genomes of 48 bird species, which represent nearly all living bird orders, as well as the American alligator, a representative of Crocodylia (the closest living relatives of birds), for the presence of inactivating mutations that are shared by all 48 birds. The presence of such shared mutations in dentin and enamel-related genes would suggest a single loss of mineralized teeth in the common ancestor of all living birds. We also queried the genomes of additional toothless/enamelless vertebrates, including three turtles and four mammals, for inactivating mutations in these genes. For comparison, we looked at the genomes of mammalian taxa with enamel-capped teeth. RESULTS All edentulous vertebrate genomes that were examined are characterized by inactivating mutations in DSPP, AMBN, AMELX, AMTN, ENAM, and MMP20, rendering these genes nonfunctional. The dentin-related gene DSPP is functional in vertebrates with enamelless teeth (sloth, aardvark, and armadillo). All six genes are functional in the American alligator and mammalian taxa with enamel-capped teeth. More important, 48 bird species share inactivating mutations in both dentin-related (DSPP) and enamel-related genes (ENAM, AMELX, AMTN, and MMP20), indicating that the genetic machinery necessary for tooth formation was lost in the common ancestor of all modern birds. Furthermore, the frameshift mutation rate in birds suggests that the outer enamel covering of teeth was lost about 116 million years ago. CONCLUSIONS We postulate, on the basis of fossil and molecular evidence, a two-step scenario whereby tooth loss and beak development evolved together in the common ancestor of all modern birds. In the first stage, tooth loss and partial beak development commenced on the anterior portion of both the upper and lower jaws. The second stage involved concurrent progression of tooth loss and beak development from the anterior portion of both jaws to the back of the rostrum. We propose that this progression ultimately resulted in a complete horny beak that effectively replaced the teeth and may have contributed to the diversification of living birds. Edentulism, the absence of teeth, has evolved convergently among vertebrates, including birds, turtles, and several lineages of mammals. Instead of teeth, modern birds (Neornithes) use a horny beak (rhamphotheca) and a muscular gizzard to acquire and process food. We performed comparative genomic analyses representing lineages of nearly all extant bird orders and recovered shared, inactivating mutations within genes expressed in both the enamel and dentin of teeth of other vertebrate species, indicating that the common ancestor of modern birds lacked mineralized teeth. We estimate that tooth loss, or at least the loss of enamel caps that provide the outer layer of mineralized teeth, occurred about 116 million years ago.