The Neural Crest/Domestication Syndrome Hypothesis, Explained: Reply to Johnsson, Henriksen, and Wright

2 GENETICS, 2021, 219(1), iyab098 DOI: 10.1093/genetics/iyab098 Advance Access Publication Date: 19 July 2021 Letter The neural crest/domestication syndrome hypothesis, explained: reply to Johnsson, Henriksen, and Wright Adam S. Wilkins,1,* Richard Wrangham,2 and W. Tecumseh Fitch3 Downloaded from https://academic.oup.com/genetics/article/219/1/iyab098/6323656 by guest on 27 September 2021 1Institute of Theoretical Biology, Humboldt Universita¨ t zu Berlin, 10115 Berlin, Germany 2Department of Human Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA and 3Department of Behavioral and Cognitive Biology, University of Vienna, 1090 Vienna, Austria *Corresponding author: Email: [email protected] Introduction first, which eventually became genetically fixed, though early genetic changes might also have contributed. The second phase, Our 2014 hypothesis, published in GENETICS, aimed to elucidate a much longer “breed formation” stage, involved centuries to mil- a set of traits associated with mammalian domestication, a lennia of coexistence with humans, often accompanied by selec- phenomenon termed the “domestication syndrome.” Our expla- tion for various productivity properties (or later, in some species, nation focused on a special group of cells found in embryos, the various ornamental properties) plus, undoubtedly, some degree neural crest cells (NCCs) and we proposed that genetic changes of natural selection in captivity. During this second stage, large affecting their development were at the root of vertebrate domes- numbers of genetic change would have taken place, overlaying tication. We now term this idea the “neural crest/domestication the initial genetic changes. These two stages probably involved syndrome” (NCDS) hypothesis. In this issue of GENETICS, different sets of genetic change (Zeder 2015; Pendleton et al. 2018; Johnsson et al. criticize our idea, arguing that it lacked a serious Fitak et al. 2020), although the second often included further genetic foundation and claiming that, despite many citations, it changes modifying behaviors, including increased docility. has received little actual support from new findings. Indeed, what we are calling the “breed formation” stage would In this reply to their critique, we do three things. First, we note have involved multiple sub-stages for many domesticated some key facts about animal domestication that need to be rec- species. ognized in any hypothesis about its genetics. Second, we explain Our proposal concerns the early events when domestication the actual reasoning that led us to propose the NCDS hypothesis. was first established. In our hypothesis, this early stage was (Johnsson et al. give an account of its genesis rather distant to our shared by all domesticated mammals and birds while later breed thinking.) Third, we briefly discuss some of the findings, not men- formation was often highly divergent between domesticated tioned by them, that strongly support our idea and discuss how species, as when selecting for productivity traits such as wool this hypothesis can be further tested. Finally, we mention points production in sheep or high egg laying in chickens, or further di- of agreement with Johnsson et al. but also note a few incorrect versity amongst breeds of the same species, for instance for beef citations in their article. vs milk production in different cattle breeds. This distinction be- Though we disagree with their main conclusions, we were tween stages in domestication was implicit in our 2014 paper, glad to see their article. It is a serious discussion of our idea. We when we mentioned “initial selection” for increased docility, but point out here, however, that our proposal has already passed should have been stated explicitly. The basic idea, however, that one test of a worthwhile hypothesis—inspiring good research— the first events in domestication were different in character from and that it meets another, which is the ability to be falsified, as the breed-formation phase is widely recognized; for a recent we will discuss. example, see Simi c et al. (2021). Animal domestication is a multi-stage process Defining the term “domestication All traditional domesticated animals (such as dogs, cattle, sheep, syndrome” goats, pigs, camels, horses, and chickens) began the process of The term “domestication syndrome” has been applied for about domestication a long time ago, frequently millennia in the past four decades to a set of correlated changes in “domesticated” (Francis 2015). Each of these species almost certainly experienced plants, namely crop plants. We use it to refer to a suite of two semi-distinct stages in their domestication (Zeder 2015; changes in mammals and birds—but which probably occurs in Pendleton et al. 2018). The first would have involved an initial vertebrates including fishes—that distinguish many different do- state of increased habituation to human presence, yielding both mesticated animals from their wild relatives. Johnsson et al. state reduced fear and reduced reactive aggression, hence increased that the expression “domestication syndrome” was first applied docility. This stage probably involved physiological changes at to animals by E.O. Price (Price 1984, 2002). We could find no use Received: March 12, 2021. Accepted: May 29, 2021 VC The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America. All rights reserved. For permissions, please email: [email protected] 2|GENETICS, 2021, Vol. 219, No. 1 of this term, however, in either of the cited sources. Instead, Price universal.” The same claim was made by Lord et al. (2019) but referred to the “domesticated phenotype.” without the qualifier “somewhat.” However, we never believed Yet “syndrome” and “phenotype” are not strictly synonymous nor stated that the domestication syndrome involved an identical and therefore not interchangeable. Taber’s Medical Dictionary set of altered traits. Table 1 of our 2014 paper listed which defines “syndrome” (from the Greek, “running together”) as “a specific traits were associated with particular domesticated group of symptoms and signs of disordered function related to mammalian species, the clear implication being that other spe- one another by means of some anatomic, physiologic, or bio- cies did not exhibit those traits. A careful enumeration of these chemical peculiarity.” Crucially, in common medical usage a pa- domestication-trait differences amongst mammals is given in tient can suffer from a syndrome without exhibiting all of the Sa´ nchez-Villagra et al. (2016) and the general point about such associated symptoms. Two examples illustrating this point are differences in different domesticates was further discussed in Down syndrome and Ehlers-Danlos syndrome (De Paepe and Wilkins (2017, 2019) as well as Zeder (2020); we return to it in a Malfait 2012; Bull 2020). “Syndrome” is thus a generic descriptive moment. Downloaded from https://academic.oup.com/genetics/article/219/1/iyab098/6323656 by guest on 27 September 2021 term, in which variability is an intrinsic aspect. In contrast, One last relevant point: to the best of our knowledge, the term “phenotype,” when designating a mutationally altered property, “domestication syndrome” as applied to animals was first used in generally refers to a smaller number of traits without great vari- an earlier paper by one of us (RWW) and two other colleagues ability, apart from degrees of expressivity. (Hare et al. 2012), then in our paper (Wilkins et al. 2014). A search In our usage, the “domestication syndrome” refers to a set of for its frequency of usage, using the search engine “Dimensions,” unexpected physical differences that frequently show up in dif- shows a distinct inflection upwards in 2014–2015, an upward ferent domesticated mammals. The phenomenon was first de- trend that continued through 2020 (data supplied on request). scribed, though not named, by Charles Darwin in his two-volume This suggests that our paper strongly stimulated interest in this study of domesticated animals and plants, Variation of Animals phenomenon. and Plants under Domestication (Darwin 1868). Darwin’s goal in his analysis of domesticated species was to derive the general mech- anism of heredity from what had been learned about breeding Tying the domestication syndrome to the domesticated species. Although his quest to understand the basis neural crest of heredity failed, his book launched the study of the hereditary The question that motivated our paper was: “why these traits in basis of domestication (Wilkins et al. 2014). particular?” Our starting point was realizing that many of the tis- The unexpected traits accompanying domestication in mam- sues involved in the traits of the domestication syndrome derive mals that Darwin particularly focused on were: changes in coat from the neural crest. These NCC-derived tissues include major color such as white and brown patches, smaller jaws (muzzles) parts of the jaws and teeth, pigmentation cells, components of and teeth, relatively smaller brains, floppy ears, curly tails, and the external ears, and cells involved in sympathetic responses altered female sexual cycles. The striking feature of these traits and the sexual cycle (Hall 2000; Trainor 2014)(Figure 1). The was that they had turned up independently in different domesti- domestication-related traits can all be interpreted as resulting cates. In contrast to the productivity traits that had been deliber- from partial reduction in the

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