Domestication syndrome

Reduction in size is regarded as a domestication syndrome trait - grey wolf skull compared with a chihuahua skull

Domestication syndrome refers to two sets of changed phenotypic traits common to many domesticated organisms: Those in domesticated animals, and those in domesticated plants.^[1] In animals these traits may include floppy ears, variations to coat colour, a smaller brain, and a shorter muzzle.^[2]

Origin[]

In ten publications on domestication syndrome in animals, no single trait is included in every one.^[3]

Charles Darwin’s study of The Variation of Animals and Plants Under Domestication in 1868 identified behavioural, morphological, and physiological traits that are shared by domestic animals, but not by their wild ancestors. These shared traits became known as the domestication syndrome.^[4] These traits include tameness, docility, floppy ears, altered tails, novel coat colours and patterns, reduced brain size, reduced body mass and smaller teeth.^[4][5] Other traits include changes in craniofacial morphology, alterations to the endocrine system, and changes to the female estrous cycles including the ability to breed all year-round.^[5]

Research indicates that neural crest cells may have been modified by domestication, which then led to those traits that are common across many domesticated animal species.^[2]

Challenge[]

The syndrome was also reported to have appeared in the domesticated silver fox that is the result of Dmitry Belyayev's breeding experiment.^[2] However, in 2015 canine researcher Raymond Coppinger found historical evidence that Belyayev's foxes originated in fox farms on Prince Edward Island and had been bred there for fur farming since the 1800s, and that the traits demonstrated by Belyayev had occurred in the foxes prior to the breeding experiment.^[6] In 2019, a further study found that the results of the "Farm fox experiment" had been overstated, based on historical records and DNA analysis. This finding questions the existence of domestication syndrome in animals and suggests that other theories need to be considered, including adaptations to a human-modified environment.^[3]

Although the soundness of the domestication syndrome, and the extent to which the Belyaev experiment could be used as evidence to support its existence, has been questioned, the hypothesis that neural crest genes underlie some of the phenotypic differences between domestic and wild horses and dogs is supported by the functional enrichment of candidate genes under selection.^[2]

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 crops, doubt has been cast because some domestication traits have been found to result from unrelated loci.^[7] 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 proposes that domestication syndrome is caused by alterations in the migration or activity of neural crest cells during their development. 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.^[8]

In animals[]

A dog's cranium is 15% smaller than an equally heavy wolf's, and the dog is less aggressive and more playful. Other species pairs show similar differences. Bonobos, like chimpanzees, are a close genetic cousin to humans, but unlike the chimpanzees, bonobos are not aggressive and do not participate in lethal inter-group aggression or kill within their own group. The most distinctive features of a bonobo are its cranium, which is 15% smaller than a chimpanzee's, and its less aggressive and more playful behavior. In other examples, the guinea pig's cranium is 13% smaller than its wild cousin the cavy, and domestic fowl show a similar reduction to their wild cousins. Possession of a smaller cranium for holding a smaller brain is a telltale sign of domestication. Bonobos appear to have domesticated themselves.^[9]: 104 In the farm fox experiment, humans selectively bred foxes against aggression, causing domestication syndrome. The foxes were not selectively bred for smaller craniums and teeth, floppy ears, or skills at using human gestures, but these traits were demonstrated in the friendly foxes. Natural selection favors those that are the most successful at reproducing, not the most aggressive. Selection against aggression made possible the ability to cooperate and communicate among foxes, dogs and bonobos. Perhaps it did the same thing for humans.^{[9]: 114 [10]} The more docile animals have been found to have less testosterone than their more aggressive counterparts, and testosterone controls aggression and brain size. One researcher has argued that in becoming more social, we humans have developed a smaller brain than those of humans 20,000 years ago.^[11]

In 2017, a researcher proposed that humans show the same domestication syndrome traits that can be found in other domesticated animals, which supports the theory of human self-domestication.^[12]

In plants[]

Syndrome traits[]

The same concept appears in the plant domestication process which produces crops, but with its own set of syndrome traits: Little to no shattering^[1]/fruit abscission,^[7] shorter height (thus decreased lodging), larger grain^[1] or fruit^[7] size, easier threshing, synchronous flowering, altered timing of flowering, increased grain weight,^[1] glutinousness (stickiness, not gluten protein content),^[7][1] increased fruit/grain number, altered color compounds, taste, and texture, daylength independence, determinate growth, lesser/no vernalization, less seed dormancy.^[7]

Genes by trait[]

Control of the syndrome traits is by:

Shattering[]

• in rice^[1][13]
• in sorghum, rice, and maize/corn^[1]
• in the rachis^[14] of rice^[13]
• in soybean^[1]
• in wheat^[13]
• in rice^[13]

Plant height[]

• //// in wheat^[1][7]
• / in rice^[1][7]

• in one Japanese cultivar^[7]

• / and barley^[1][7]
• either or in sorghum and pearl millet^[1]
• in rice^[7]
• in wheat^[7]

Grain size[]

• in maize/corn^[1]
• and in rice^[1]
• in rice^[13]
• GAD1/RAE2 (smaller) in rice^[13]
• in rice^[14]

Yield[]

• Oryza sativa SPL14/LOC4345998 in rice.^[15]
• , , in the gene family in rice^[14]

Threshability[]

• ^[1][13] and ^[1]
• (by reducing or eliminating awns) in rice^[13]
• / - small awn reduction/barbless awns^[14] - in rice^[13]
• GAD1/RAE2 - awn elimination in rice^[13]
• - naked kernels in maize^[14]

Flowering time[]

• in barley, wheat, ryegrass^[1]

Grain weight[]

• in rice,^[1][14] wheat,^[1] maize/corn^[1]
• in rice^[14]
• in rice^[14]
• in rice^[14]
• in rice^[14]
• in rice^[14]
• in rice^[14]

Glutinousness[]

• or in rice^[14] (especially glutinous rice), wheat, corn, barley, sorghum,^[1] foxtail millet^[1][16]
• in rice^[13]

Determinate growth[]

• / in Arabidopsis thaliana and orthologs^[7]
  • Specifically, four orthologs in Glycine max and eight in Phaseolus vulgaris^[7]

Standability[]

• // in rice^[13][14]
• teosinte branched1/ (apical dominance) in maize/corn^[13]

Grain/fruit number[]

• in rice^[13]
• GAD1/RAE2 in rice^[13]
• (by increasing tiller number) in rice^[13]
• in rice^[14]
• (by increasing tiller number) in rice^[14]

Panicle size[]

• in rice^[14]
• in rice^[14]

Spike number[]

• vrs1 in barley^[14]

Fragrance[]

• produces 2-Acetyl-1-pyrroline when defective in rice^[14]
  • can be artificially disrupted to produce the same compound^[14]

Delayed sprouting[]

• , , in the gene family - reduced preharvest sprouting in rice^[14]

Altered color[]

Reduced color[]

• - white pericarp in rice^[14]

Unspecified trait[]

• / in maize/corn^[13]

Many of these are mutations in regulatory genes, especially transcription factors, which is likely why they work so well in domestication: They are not new, and are relatively ready to have their magnitudes altered. In annual grains, loss of function and altered expression are by far the most common, and thus are the most interesting goals of mutation breeding, while copy number variation and chromosomal rearrangements are far less common.^[1]

See also[]

• Agricultural weed syndrome

References[]