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Epistasis: Gene Interaction and the Phenotypic

Examples of “Teeth” in Chicken

A photograph shows a rust-colored hen standing outside the wire gate to a chicken coop with a black and brown rooster inside the coop.

Hen’s teeth: as rare as we thought?© 2013 Nature Publishing Group Bajaj, A. Hen’s teeth. British Dental Journal200, 187 (2006). All rights reserved. 

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Even though birds lost teeth as physical structures between 60 and 80 million years ago, several studies have shown that those tissues within birds that would normally produce teeth still retain the potential to do so. For example, in 1821, Geoffrey St. Hilaire was the first scientist to publish the observation that some bird embryos exhibited evidence of tooth formation, but his contemporaries considered his work flawed. Since then, however, many investigators have unearthed molecular evidence that the genes involved in odontogenesis (tooth development) are indeed retained in chickens.

A primary step in reaching this conclusion occurred when researchers exposed chick jaws to certain proteins known to cue tooth development. As a result, toothlike structures grew, and other tooth markers were expressed (Chen et al., 2000). These findings were artificial in the sense that the prompting signal was experimentally administered; nonetheless, they were significant in showing that a chicken’s jaw could produce teeth if specific conditions were present.

Despite this discovery, no one had yet demonstrated that chickens could develop teeth without external cues. This situation soon changed, however, when researchers Matthew Harris (a graduate student at the time) and John Fallon launched a study involving chickens with a particular kind of autosomalrecessive mutation (Harris et al., 2006). These chickens, designated by the abbreviation ta2, displayed signs reminiscent of early tooth development.

The researchers needed a positive control with which to compare their hens’ teeth-that is, a closely related animal in which teeth occur. Typically, the nonmutant or “wild-type” phenotype serves as a control in gene mutation experiments, but this was an exceptional case in that the wild-type chicken doesn’t have teeth. Harris and Fallon specifically needed to compare the structures they believed to be teeth in their tamutant chickens with the next best thing—the closest ancestor to the chicken that still has teeth—which in this case was the archosaur, otherwise known as the common crocodile. Therefore, the researchers examined the expression of several biomarkers in wild-type chicken embryos, ta2 mutant embryos, and crocodile embryos. They found that the ta2 mutant oral cavities appeared developmentally closer to those of the crocodiles than to those of their wild-type siblings. These results thus demonstrated that all the genetic pieces to the tooth-building puzzle exist in chickens, but the directions have evolved to tell those pieces to do something different over the last 80 million years.

Atavism and Human Tails

True examples of atavism, like the tachicken, are data points indicative of common ancestry between species. In the case of human beings, the presence of a tail is a striking example of such ancestry. Many cases of people born with “tails” exist in the medical literature, but it is not always clear whether these appendages are “true” tails or not. In some instances, they are actually “pseudotails,” or malformations that just happen to be located near a person’s tailbone. True tails, however, result from a particular type of error during fetal development.

In order to understand this error, it’s first important to note that all humans briefly possess tails while in the uterus. Specifically, during normal development, certain fetal cells develop into a tail and then regress as a result of programmed cell death, or apoptosis. Investigators have identified a gene called Wnt-3a as a principal regulator of this process, at least in mice (Takada et al., 1994). Researchers have also discovered that humans indeed have an intact Wnt-3a gene, as well as other genes that have been shown to be involved in tail formation. Through gene regulation, we use these genes at different places and different times during development than those organisms that normally have tails at birth. Should this process of gene regulation somehow go wrong, however, the likelihood (albeit rare) exists that a person could indeed be born with a true tail.