Dr Todd Streelman,
Chris - Also in the news this week, researchers at Georgia institute of Technology in Atlanta have discovered the genetic ‘root’ of all teeth!
By looking at a type of fish called Cichlids, which have teeth both in the mouth and the throat, they noticed that the development of both sets of teeth is controlled by the same set of genes. The same genes could also control the pattern of growth of feathers and hair – so could shed more light on some of the big evolutionary changes of the past.
Dr Todd Streelman, one of the authors on this week’s Public Library of Science paper, joins us now...
Todd - We study a very unique group of fishes from Lake Malawi in East Africa. What’s so fascinating about them is that they express a tremendous diversity of all sorts of things like colour patterns and brain function and also their teeth. We were able to make use of this natural diversity to begin to ask the question about how they actually make teeth in two different places on the body. Once we found the answer to that we put this information together with lots of information in the literature to try to understand how teeth were made a long time ago and also to try to understand the things in common between all teeth that we presently know about.
Chris - What was the genetic clue that tells you where the teeth have come from in the first place?
Todd - This is one of the interesting things that many people don’t know, the co-evolutionary history of teeth and jaws. Teeth first of all: about half a billion years ago they evolved in organisms that did not have jaws. Interestingly enough they evolved first in the pharynx, deep in the throat. Then they also evolved on the oral jaw, the jaw on the front of our face, when that jaw first appeared in vertebrate history. As you imagined in the fishes we studied they had teeth both on their oral jaw but they also have teeth back in this ancestral location for teeth. We used a technique called in situ hybridisation which is just a way to visualise where and when genes are active. We studied a number of molecules that we had some inkling might be involved in dentition. We identified two things. We identified this ancient set of genes and that ancient set of genes is the set of genes that’s on in the pharynx when teeth are made. Secondly we identified a core set of genes. That core set represents the gene at work that’s active in all teeth. From the fishes we study to shark, to mice and in your teeth as well.
Chris - When you study an early human embryo you can see the same vestiges as the development of a fish occurring in us, for instance. We get gills at certain stages of development too, don’t we? We get these bronchial arches which, some of them, turn into things like our ear drums and out tonsils. In fish they would have been gills but superimposed on that is this pattern of genes that gives us teeth.
Todd - That’s right. There’s a very old rule in evolutionary biology called ontogeny recapitulates phylogenies. That just means that, in a very coarse way, if you look at a contemporary organism and look at its development you can learn something about evolutionary vestiges by studying early phases of its development. That’s one of the things we take advantage of. We also were able to identify some differences between this ancestral network active in these throat teeth and the core network that’s active in the oral jaw of most organisms. We think those are probably some of the genes that tell us about some of the things that have changed as dentitions have evolved over half a billion years. For instance, in the fishes we study they replace every single tooth about every 50-100 days. This is one of these things we think – and other people have suggested also- links teeth to other structures like feathers and hairs that also have this capacity for regeneration. Your teeth are replaced a single time but other mammals never replace their teeth. These are aspects of dentition that have been lost as teeth have evolved. Some of those interesting regenerative capacities are still present in both the pharyngeal and oral teeth that we studied.