Abstract
Domestication has been consistently accompanied by a suite of traits called the domestication syndrome. These include increased docility, changes in coat coloration, prolonged juvenile behaviors, modified function of adrenal glands and reduced craniofacial dimensions. Wilkins et al recently proposed that the mechanistic factor underlying traits that encompass the domestication syndrome was altered neural crest cell (NCC) development. NCC form the precursors to a large number of tissue types including pigment cells, adrenal glands, teeth and the bones of the face. The hypothesis that deficits in NCC development can account for the domestication syndrome was partly based on the outcomes of Dmitri Belyaev’s domestication experiments initially conducted on silver foxes. After generations of selecting for tameness, the foxes displayed phenotypes observed in domesticated species. Belyaev also had a colony of rats selected over 64 generations for either tameness or defensive aggression towards humans. Here we focus on the facial morphology of Belyaev’s tame, ‘domesticated’ rats to test whether: 1) tameness in rats causes craniofacial changes similar to those observed in the foxes; 2) facial shape, i.e. NCC-derived region, is distinct in the tame and aggressive rats. We used computed-tomography scans of rat skulls and landmark-based geometric morphometrics to quantify and analyze the facial skeleton. We found facial shape differences between the tame and aggressive rats that were independent of size and which mirrored changes seen in domesticated animals compared to their wild counterparts. However, there was no evidence of reduced sexual dimorphism in the face of the tame rats. This indicates that not all morphological changes in NCC-derived regions in the rats follow the pattern of shape change reported in domesticated animals or the silver foxes. Thus, certain phenotypic trends that are part of the domestication syndrome might not be consistently present in all experimental animal models.
Highlights
The classic selection experiments by Dmitry Belyaev have demonstrated that breeding animals for tameness can lead to a suite of physiological, cognitive and morphological changes similar to those associated with domestication [1,2]
We evaluate whether tameness and aggression causes shape changes in neural crest cell (NCC)-derived region, such as the face, and whether those changes mimic the pattern observed in the silver foxes
When the PCA was performed on multivariate regression residuals, in order to reduce the effects of allometry, the group scatters remain almost identical (Fig 2C). These results indicate that facial shape changes between these groups are independent of size-related differences
Summary
The classic selection experiments by Dmitry Belyaev have demonstrated that breeding animals for tameness can lead to a suite of physiological, cognitive and morphological changes similar to those associated with domestication [1,2]. Behavior is regulated by neurotransmitters and hormones, which when altered by strict selection for a particular trait, can cause changes in key developmental processes, thereby potentially affecting the phenotype [6]. Selection for a behavioral trait such as tameness can result in ‘off-target’/unintentional changes in integrated developmental mechanisms[9] Elaborating on this developmental perspective, Wilkins et al [10,9] recently emphasized the connection between neural crest cell (NCC) development and certain key physiological and phenotypic features that comprise the domestication syndrome. Of particular interest to this study is increased morphological variation and phenotypic novelty present in the facial structures of animals selected for tameness and aggression, and how those changes related to domesticated animals compared to their wild counterparts. The NCC gives rise to the entire facial skeleton, anterior aspects of the cranial base and frontal bones from NCC across vertebrates [11,12,13]
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