Mammalian model organisms for the study of behavioral genetics

5.5.1 From mice to men

The mouse is probably the main mammalian model organisms and a prototype of behavioral genetics (Lindzey & Thiessen, 1970). In particular, mice are highly valued for helping to identify genes and polymorphisms as well as understanding their functionality (Plomin et al., 2013). Studies on COMT and MAOA knockout mice were conducted following first evidence for a functionality from variation in these genes in humans (Cases et al., 1995; Gogos et al., 1998). Besides target knockout mice, studies on inbred strains are also common. Thousands of mice strains are available to researchers, who can choose between different databases to find strains with requested genotypes and phenotypic differences in e.g. anxiety, cognition and stress-reactivity (e.g. Mouse Phenome Database, Flint, 2011; International Mouse Strain Resource, Eppig et al., 2015).

Recent technological advances like the gene-editing tool CRISPR are promising that in the future genotype modifications will be realized in living organisms (Hess et al., 2017;

Murugan et al., 2017; Sternberg & Doudna, 2015). While the standardized studies on genotyped mice of course offer an immense research potential, their application is usually restricted to laboratory-reared, experimental conditions and invasive methods, and only partly help to understand genotypic effects on naturally occurring behaviors in the wild.

Besides mice, humans are among the best-studied vertebrate species in behavioral genetics. The human and the mouse genome are of similar size: 3500 MB, which would equal 3000 books of 500 pages each (Plomin et al., 2013). Both were sequenced around the turn of the century (2001 and 2002, Chinwalla et al., 2002; International Human Genome Sequencing Consortium, 2001; Venter et al., 2001) and around 90 % of the mouse genes have orthologues in the human genome (Gibbs et al., 2004). However, the mouse fails to mimic several aspects of primate – including human – physiology and behavior like immunity, metabolism, reproduction and complex sociality (Elsea & Lucas, 2002; Ezran et al., 2017).

Mice are investigated under laboratory conditions. Human subjects live in their present-day natural habitat, but the measures entering the statistical models are usually the results of experimental tests, ratings and questionnaires and not objective quantitative measures of how individuals really behave in their daily live. Human behavioral genetics is particularly sophisticated in research on the general heritability of traits, but there are still many unresolved issues concerning which gene variants determine phenotypic variation in which way. In humans it is difficult to get at daily face-to-face interactions and social relationships are additionally linked to emotion and culture (Fletcher et al., 2015). The ability to measure the occurrence of natural behavior in everyday life is one of the outstanding characteristics of primatology. Long-term field studies on habituated primates that can be individually identified facilitate data collection to combine phenotypic and genotypic variation. Oddly, in non-human primates it is seemingly better investigated how behavior predicts gene structure than how genetic variation predicts behavior (e.g. Altmann et al., 1996; Kalbitzer et al., 2016; Kopp et al., 2015).

As we learn more about behavioral genetics in natural animal populations, we are also learning more about shared biological processes with humans and ancestral and derived characters of human traits. Looking at the phylogenetic tree, numerous species exist between mice and humans, but have not received much attention in behavioral genetics yet. There appears a huge gap of targeted and well-studied species. Even our closest relatives, the non-human primates have been relatively understudied in this field.

With this dissertation, I started to partly close this gap by introducing the application of behavioral genetics to wild groups of macaques expressing natural behavior in their undisturbed habitat. I provide evidence for the genetic basis of natural primate behavior and hormone levels, which is essential to understand the emergence of individual phenotypic patterns.

5.5.2 The genus Macaca in the lab and the wild

Macaques share similarities in their development, immunology, neuroanatomy, behavior and genes with humans and have the potential to be superior animal models

(Geyer et al., 2000; Gibbs et al., 2007; Kalin et al., 2007). The study of non-human primates, especially macaques, is an important aspect of the broader field of behavioral genetics, but has been particularly highlighted with regard to stress-related research (Ferguson et al., 2012; Meyer & Hamel, 2014; Phillips et al., 2014; Vallender & Miller, 2013). The macaque genome includes polymorphisms orthologous to the human gene variants implicated in HPA-axis regulation, including variants in COMT (chapter 3), MAOA (Newman et al., 2005; Wendland et al., 2006), OPRM1 (Miller et al., 2004) and SLC6A4 (Bennett et al., 2002). However, most trans-species polymorphisms between humans and non-human primates have been detected in immunological genes (Cagliani et al., 2008, 2010, 2012). The number of detected trans-species polymorphisms across non-human primate species is higher, probably due to less clear taxonomic demarcations (e.g.

Macaca: Higashino et al., 2012; Li et al., 2009; Satkoski-Trask et al., 2013; Street et al., 2007; Papio: Charpentier et al., 2012; Keller et al., 2010; Zinner et al., 2013).

So far, the COMT Val¹⁵⁸Met polymorphism has not been regarded as a trans-species polymorphism, but as a unique feature of humans (Palmatier et al., 1999; Chen et al., 2004), although investigating only one to two individuals per species cannot exclude the existence of a polymorphism. This thesis now demonstrates that in fact the SNP is not unique to humans, but also present in Assamese macaques, which is suggestive of the presence of the polymorphism in further macaque species. How variations in individual genes in macaques mimic that seen in humans advance the utility of macaques as model organisms in human-related research (Vallender & Miller, 2013). Future studies investigating this locus in several other species will contribute to a broader understanding of the taxonomic distribution of this polymorphism and give insights about the evolutionary roots of this genetic variation.

Investigating more than one HPA-axis-related locus in parallel, a previous study gave important first insights about genetic predispositions of HPA-axis dysregulation by showing that certain risk genotypes explain the extreme values of adrenocorticotropic hormone suppression in response to dexamethasone administration in captive male rhesus macaques (Ferguson et al., 2012). In addition, this thesis extends the current knowledge by (i) investigating the natural variation of cortisol instead of responses to a dexamethasone suppression test, (ii) demonstrating genetic predisposition in a wild population with a natural combination of individual genotypes, (iii) including all

individuals, not just those at the extreme ends of the hormone range, (iv) adding the investigation of risk-taking, social buffering and proximity maintenance. To get at cortisol levels typically experienced by individuals when faced with everyday stressors, we collected and averaged several urinary cortisol samples. By including all individuals and not only the extremes we covered the full breadth of naturally occurring phenotypes and demonstrate that genotype contributes also to normal variation and not only abnormalities or extremes (Plomin et al., 2013). Nevertheless, compared to laboratory colonies our study has limitations due to smaller sample size and, as discussed before, the potential confounding variable of relatedness.

The investigation of wild primates, particularly in long-term field projects, holds the potential to expand the knowledge on their natural ecology and behavior and to simultaneously gain new insights into the evolutionary history of humans, current differences and similarities with non-human primates. This can help for example to elucidate the evolutionary origins of technical skills, tool use, culture, complex vocalization, stress-related drawbacks or benefits, the nature of long-lasting social relationships, the necessity of close bonds and other aspects of human social evolution (e.g. Byrne, 2007; Haunhorst, 2017; de Waal, 2009). In all probability, the here reported effects are not unique to Assamese macaques but apply to macaques in general. Going one step further, similar patterns could probably be detected in many other mammalian species living in social groups.

I expect the results in this thesis, together with continuative future work, to contribute to a reinforcement of the macaque animal model in behavioral genetics. In this respect, data collection from long-term field projects, which exist for different macaque species, will provide insights into the genetic foundations of the natural behavioral range and physiological phenomena like social buffering. Regarding the linkage between neuropsychiatric diseases and HPA-axis dysfunction in humans (e.g. alcoholism, major depressive disorder, posttraumatic stress disorder, schizophrenia, Carroll et al., 1976;

Goeders, 2003; Goodyer et al., 2009; de Kloet et al., 2005; Walker et al., 2008; Yehuda, 1997), the study to understand these traits will require standardized environmental conditions with the possibility to investigate responses to fine-tuned changes. Therefore, research on non-human primates in captivity, in order to understand the development of disorders (e.g. alcoholism in rhesus macaques, Phillips et al., 2014; Schwandt et al.,

2010), is also necessary, besides the investigation of naturally occurring, wild populations.