From the energetic border collie to the friendly golden retriever, more than 350 dog breeds exist today, each with specific physical and behavioral traits. Although research has shown that dog genomes can be used to determine differences in body size and shape between breeds, it is difficult to identify the genetic basis for complex behaviors such as hunting, herding and guarding, pointing and drafting and many other traits.
Now, researchers at the National Human Genome Research Institute(NHGRI), have discovered that the ancestry relations of eight major dog lineages map onto distinct behaviors. The investigators were able to identify the genetic basis for these behaviors. One of the most important findings was that many genes involved with axon guidance (which determines how neurons connect in the brain) had been altered in multiple lineages. This was especially true in the sheepdog lineage. Researchers believe that the genes that affect brain development and behavior in dogs could also influence human behavior. Therefore, it is possible to identify genetic variation in the human genome that corresponds to the behavior-related regions of the dog genome. This could provide new insights into the genetic basis for human behavior, they say.
” This paper brings behavior back into the forefront when we consider where selection has acted to evolve dog breeds,” said Evan Maclean, director of Arizona Canine Cognition Center, University of Arizona. He was not involved in this study. Although it is true that we have done a lot breeding for aesthetic traits, this study shows that a lot genetic action is in pathways connected to the brain (and thus presumably connected with behavior and cognition ).
The Dog Genome Project, which began in the early 1990s, allowed researchers to map disease genes, as well as genes for traits such as body size, leg length and body shape. Elaine Ostrander, the senior author of the new study and project leader at NHGRI, says that the Dog Genome Project was originally designed to discover genes for behavior. She explains that although breeds exhibit a variety of stereotypical behaviors, it was extremely difficult to identify the genes behind these behaviors until now.
Emily Dutrow is a postdoctoral fellow at NHGRI and the lead author of the new study. She used single-cell sequencing, which is a widely used technique to sequence RNA instructions or transcripts from individual cells. This allows for an understanding of the function of different cell types and their relationship to one another. Instead of applying the method only to a set or transcripts of cells, a transcriptome; she applied it to the dog genomes to determine the ancestry relationships between dog breeds. Using genome data from more than 4,000 dogs (samples were shared with the Dog Genome Project by dog owners around the world), “I was able to draw out these really ancient relationships among all of the dog breeds that exist on Earth,” Dutrow says.
The researchers first had to identify these ancestry lines, or lineages. Then they could look for evolutionary changes along each one. Because the lineages fell into eight behavioral groups, it was possible to determine what parts of the genome made a herding dog or a guarding dog different. To do this, the researchers linked their genomic analysis to behavioral data from a survey of more than 40,000 dogs that was conducted about 15 years ago at the University of Pennsylvania School of Veterinary Medicine.
” This paper uses a new framework for thinking about the relationships among breeds,” Gitanjali Gnanadesikan (a Ph.D. candidate at Arizona Canine Cognition Center), who was not involved with the study. “In evolutionary biology, we tend not to see relationships between species or lineages in evolutionary biology as branches. While that is a valid approach, dog breeds are extremely complicated. They are not distinct species and there is a lot interbreeding, especially during breed formation. This framework can take these complex relationships and visualize and quantify them in a way that allows for new analyses .”
Bridgett VonHoldt, a geneticist at Princeton University who studies canine history, agrees. She called this new approach “incredibly elegant” as well as “impressive”. It allowed researchers to include purebred and mixed-breed dogs in their study of genetics underlying behavior. She called the work “a landmark study” in the field.
The researchers’ analysis revealed eight major trajectories of “dogdom” as vonHoldt calls it: terriers; sled dogs; sheepdogs and cattle dog breeds; sight hounds; spitzes and “primitive” dogs; scent hounds; retrievers.
The team discovered that a whole range of genes was involved in the evolution specific behaviors in different dogdom lineages. This contrasts with traits like body size and coat colour, which are controlled by a small number of genes. Ostrander says that behavior is more complex than other traits. “So it’s going be more complicated,” Ostrander explains. This means that it is important to adjust when and where the coding gene activation occurs early in life. All these genes are essential. Ostrander says that all of these genes are essential. However, you can tweak them to make nuanced behavioral changes.
Researchers examined when and where the gene variants associated to particular lineages were activated in development. They found that many were activated while the brain was still developing. The most important finding was that many of the gene variants in the sheepdog lineage were involved with axon guidance, which is the process by which neurons connect.
” “That was incredibly striking to me,” Ostrander said. “I didn’t expect that. They would be genes from different parts of the nervous system, which I assumed would allow them to do many different things. I didn’t think one pathway would be so important in executing these behavior.” Modifications could have been made to the number of cells in a particular part of the brain, or the percentages of different cell types. Dutrow says that it is really how different parts interact with each other that is most important in determining these behaviors.
The researchers discovered that motivation and learning are key factors in the differences in brain and behavior in eight dogdoms. Dutrow states that scent hounds must be self-directed learners and able to follow their noses, rather than being influenced by humans. Gun dogs are responsible for retrieving and finding game and must be in close sync with their human handlers.
The gene variants associated to these lineages were not only involved in brain and behavioral regulation. Researchers determined that most gene variants associated with sight hounds were related to the muscular system.
Another important discovery was that most of these genetic variants that were identified by the team were already found in wolves. Maclean states that this is a result of the fact that the basic ingredients for all of the variation in dogs today were present in wolves. The changes made by the wolf ancestor had different effects on the different dog lineages. Gnanadesikan states, “This is very interesting to me because it really emphasizes diversity in dogs and how evolution has progressed through both selection [random] genetic drift.”
This study provides the foundation for further research into each dog lineage and the unique characteristics of that lineage. Ostrander also hopes to examine the genetic variation in the human genome that affects behavior. She believes it’s possible that genes that affect behavior in dogs may also impact behavior in humans. Ostrander says that dog breeds exhibit stereotypical behavior and have less variation in their genetic background than humans. This makes it a “much more effective starting point for understanding the true function of some regulatory regions.” She suggests that these comparative studies could ultimately lead to new insights into human behavior, including those related to neuropsychiatric disorders.