Alexey Veraksa, Ph.D.
assistant professor
University of Massachusetts

Katie, Montana, U.S.A.

What will happen if you put chicken genes into humans? Will we grow feathers? Have beaks? If you put horse DNA into a fish, will it grow hooves?

Alexey Veraksa
assistant professor,
Biology Department,
University of Massachusetts,
former HHMI predoctoral fellow

This question is simple to pose but tough to answer! What we do know about chicken, human, horse, and other vertebrates' genes? One surprising discovery in the past 10 to 20 years, made possible with the availability of fully sequenced genomes, is that vertebrate genomes are remarkably similar to one another. We humans have about 3 billion nucleotides in our genomes; chickens have about 40% of that number, or a bit more than a billion bases. 

Chickens and mammals are much more similar in their number of different genes, with chickens estimated to have between 20,000 and 23,000 genes and humans 27,000 to 37,000 (Furlong, 2005, and International Chicken Genome Sequencing Consortium, 2004). About 60% of all chicken genes have direct counterparts (similar genes) in humans. Comparisons between chicken, pufferfish, and humans revealed about 7,000 very similar genes. These genes likely compose a core gene set common to all vertebrates. They play a role in essential vertebrate cellular and developmental functions, such as intracellular protein transport and body axis development. 

What then makes vertebrates different from one another? For example, mammals appear to have lost several genes associated with egg production, in particular the avidin gene family. These genes encode egg white proteins and have homologs in invertebrates, indicating that they have been lost in mammals, probably in association with the reduction in egg size and internalization of the embryo (Furlong, 2005). In both birds and mammals the keratin gene family expanded, but in different directions. Birds use a large, avian-specific family of keratin genes to form proteins for scales, claws, and feathers. Mammals have undergone an independent expansion of a different keratin family, which is used to form hair fibers (Furlong, 2005).

So the answer to your question very much depends on which genes we choose to transplant from one species into another. If one of the conserved genes is transplanted, the recipient organism will most likely treat it as its own, and there will be no obvious change. But if a bunch of bird-specific keratin genes were put into, say, a mouse, some feathers might grow. 

One complication in this scenario is that the genes in an organism never work in isolation. In addition to having different keratins in the keratin gene family, chickens developed regulatory DNA associated with these genes that guides proper development of feathers in the right places and at the right time. Even though it may be possible to transplant these genes into a mouse, the mouse cells may not be equipped to respond to their signals and to grow feathers with the proteins they encode. This is because the accessory proteins that shape the keratins into a feather are either not present in the mouse genome or have different functions. 

Despite the complications that arise in transplanting genes between species, some spectacular scientific advances have brought us closer to growing feathers on mice (or pigs, for that matter). A notable example involves teeth in chickens. In 1980, scientists reported that transplanted mouse tissue induced formation of teethlike structures in chickens (Kollar and Fisher, 1980). This discovery stirred some controversy, because contamination by small amounts of mouse tooth tissues could not be excluded. A more careful genetic study done much later demonstrated that chickens possess the genes required for making teeth but that these genes are turned off in birds (Chen et al., 2000). Finally, a dramatic recent discovery showed that certain mutations activated these inactive genes, which then stimulated the formation of real teeth in chickens (Harris et al., 2003). Remarkably, the mutant teeth resembled alligator teeth! These experiments show that the capacity to grow teeth was lost in birds but is in fact dormant and can be reactivated by mutation or by transplantation of foreign DNA.

It would be impossible to transplant a whole genome because of its size. To obviate this problem, scientists sometimes create hybrid organisms made from genomes of two different species. These hybrids usually do not survive long because of chromosomal incompatibilities, but a few viable ones exist; one example is the mule, a hybrid of a horse and a donkey, with genes from each. Mules and other such hybrids are almost always sterile because of the different numbers of chromosomes in the two parental species. 

To sum up, many genes are common in all vertebrates and have not changed their function during evolution. Daring gene induction experiments such as causing chickens to grow teeth are becoming possible but are difficult to perform because many genes have evolved in concert and are all required to form organs in specific species. We may see more interesting hybrids as we learn more about the structure, function, and evolution of vertebrate genomes (see, for example, a recent paper on the platypus genome, Warren, 2008).

References 

Chen, Y., Zhang, Y., Jiang, T.X., Barlow, A.J., St. Amand, T.R., Hu, Y., Heaney, S., Francis-West, P., Chuong, C.M., and Maas, R. 2000. Conservation of early odontogenic signaling pathways in Aves. Proceedings of the National Academy of Sciences of the United States of America 97:10044–10049.  

Furlong, R.F. 2005. Insights into vertebrate evolution from the chicken genome sequence. Genome Biology 6:207. 

Harris, M.P., Hasso, S.M., Ferguson, M.W.J., and Fallon, J.F. 2006. The development of archosaurian first-generation teeth in a chicken mutant.
Current Biology 16:371–377.

International Chicken Genome Sequencing Consortium. 2004. Sequencing and comparative analysis of the chicken genome provide unique perspectives on vertebrate evolution. Nature 432:695–716.

Kollar, E.J., and Fisher, C. 1980. Tooth induction in chick epithelium:
expression of quiescent genes for enamel synthesis. Science 207:993–995. 

Warren, W.C., Hillier, L.W., Marshall Graves, J.A., Birney E., Ponting, C.P., Grützner, F., et al. 2008. Genome analysis of the platypus reveals unique signatures of evolution. Nature 453:175–183.

08/19/09 20:23