What can chicken embryos tells us about dinosaurs? You’d be surprised!
Occasionally in nature, animals are born with mutations that cause them to exhibit ancestral traits. For example, snakes can be born with legs, whales with tiny hindlimbs, horses with toes - even humans with tails! These traits, which occur in both plants and animals, are called ‘atavisms’ or ‘evolutionary throwbacks’. Some animals also normally retain ancestral traits in a very reduced form, such as the pelvic structure in whales or the sightless eyes of the blind mole rat. These latter features are called ‘vestigial structures’. Both atavisms and vestigial structures tell us a lot about the evolutionary history of an organism.
Cleared and stained chicken embryo. Credit: Hans Larsson, McGill University
In normal embryonic growth, the genes that control development are expressed (‘switched on’) in a set pattern so that different body parts or body systems develop in sequence. Developmental genes effectively instruct embryonic cells what body parts to grow where, as well as when to start and stop the growth process. The genes that control the basic development of all animals are surprisingly similar, having been conserved over millions of years. For example, the Pax6gene controls the development of eyes across the animal kingdom, from fruit flies to humans1.
However, not all developmental genes are in use in a given organism. Some developmental genes retained from earlier ancestral lineages are no longer expressed (‘turned on’) in the same way during normal development. An atavism or vestigial structure is a clue that an ancestral developmental gene is still present, but whose expression is normally inhibited (turned off) or down-regulated (turned down). This control of expression is called ‘gene regulation’. For example, we know from the fossil record that some ancestral snakes, such as the 95-million-year-old Pachyrhachis, had hindlegs2. The fact that snakes today are very occasionally born with legs suggests that all snakes still have those developmental genes. However, in the normal development of a modern snake, those genes are regulated differently than they were the fossil ancestor.
Atavisms are natural mutations, but developmental genes can also be experimentally manipulated in a lab situation. The newly developing field of evolutionary development, or ‘Evo-Devo’, searches for clues about evolution by exploring how changes in embryonic development produce the variation we see in adult animals. It is now apparent that factors influencing the regulation of developmental genes in embryonic development could lead to much more radical and rapid evolution than changes affecting the adult organism (as in Darwin’s classic model of natural selection). It could partially explain the rapid bursts of evolution we find preserved in the fossil record1.
This is an example from a paper by Poongodi Geetha-Loganathan and colleagues3, which shows what happens to the limb of a chicken embryo when the expression of the developmental gene, Wnt6, is manipulated. The left images are from a normal chick embryo. The right images demonstrate what happens when the amount of Wnt6 expression is progressively increased, from A1 to C1.
So, what can evo-devo tell us about our favorite group of prehistoric animals, the dinosaurs?
In 2006, researchers from the University of Wisconsin and the University of Manchester engineered a strain of mutant chicken that grew alligator-like teeth4. The manipulated gene in these chick embryos was in a developmental gene calledShh (this is short for Sonic Hedgehog; a signaling gene named for the famous speeding videogame character). Birds today don’t have teeth, but their ancestors, the theropod dinosaurs, did. By pinpointing which genes were turned on or off to cause the teeth to develop, we can better understand how the evolutionary shift from toothed jaws to untoothed beaks may have occurred. By examining the patterning of limb development, evo-devo has also been useful in determining how bird wings developed from a dinosaur’s clawed hand. It can also hint at the developmental steps that lead to the shortening and eventually the loss of the dinosaurian tail (birds can have long tail feathers, but only very short bony tails).
Would it be possible to grow a dinosaur from a chick embryo by manipulating its developmental genes? The problem with manipulating developmental genes is that it often causes lethal mutations elsewhere in the animal. (The toothed chick embryos mentioned above never hatched for this reason). But could this issue be overcome? The jury is still out…
1 Gilbert, S F. and Epel, D. 2009. Ecological Developmental Biology: Integrating Epigenetics, Medicine and Evolution. Sinauer Associates, Inc. Sunderland, Massachusetts, USA, 480 pgs.
2 Caldwell, M. W. and Lee, M. S. 1997. A snake with legs from the marine Cretaceous of the Middle East. Nature 386, 705 – 709.
3 Geetha-Loganathan, P., Nimmagadda, S., Christ, B., Huang, R. and Scaal, M. 2010. Ectodermal Wnt6 is an early negative regulator of limb chondrogenesis in the chicken embryo. BMC Developmental Biology, 10(1), 32.
4 Harris, M. P., Hasso, S. M., Ferguson, M. W. and Fallon, J. F. 2006. The development of archosaurian first-generation teeth in a chicken mutant. Current Biology, 16(4), 371-377.