Affects: Cats, Dogs
Domestication syndrome refers to two sets of phenotypic traits that are common to either domesticated plants or domesticated animals.
Domesticated animals tend to be smaller and less aggressive than their wild counterparts; they may also have floppy ears, variations to coat color, a smaller brain, and a shorter muzzle. Other traits may include changes in the endocrine system and an extended breeding cycle. These animal traits have been claimed to emerge across the different species in response to selection for tameness, which was purportedly demonstrated in a famous Russian fox breeding experiment, though this claim has been disputed.
Other research suggested that pleiotropic change in neural crest cell regulating genes was the common cause of shared traits seen in many domesticated animal species. However, several recent publications have either questioned this neural crest cell explanation or cast doubt on the existence of domestication syndrome itself. One recent publication points out that shared selective regime changes following transition from wild to domestic environments are a more likely cause of any convergent traits. In addition, the sheer number, diversity, and phenotypic importance of neural crest cell-derived vertebrate features means that changes in genes associated with them are almost inevitable in response to any significant selective change.
Cause: Many similar traits – both in animals and plants – are produced by orthologs; however, whether this is true for domestication traits or merely for wild forms is less clear. Especially in the case of plant crops, doubt has been cast because some domestication traits have been found to result from unrelated loci.
In 2018, a study identified 429 genes that differed between modern dogs and modern wolves. As the differences in these genes could also be found in ancient dog fossils, these were regarded as being the result of the initial domestication and not from recent breed formation. These genes are linked to neural crest and central nervous system development. These genes affect embryogenesis and can confer tameness, smaller jaws, floppy ears, and diminished craniofacial development, which distinguish domesticated dogs from wolves and are considered to reflect domestication syndrome. The study concluded that during early dog domestication, the initial selection was for behavior. This trait is influenced by those genes which act in the neural crest, which led to the phenotypes observed in modern dogs.
The 2023 parasite-mediated domestication hypothesis suggests that endoparasites such as helminths and protozoa could have mediated the domestication of mammals. Domestication involves taming, which has an endocrine component; and parasites can modify endocrine activity and microRNAs. Genes for resistance to parasites might be linked to those for the domestication syndrome; it is predicted that domestic animals are less resistant to parasites than their wild relatives.