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Home , Candidate gene, Quantitative genetics

Behavioural

Geetanjali Mishra

Associate Professor

Department of Zoology

University of Lucknow

Lucknow

Behavioural genetics

• Behavior, like all characteristics of animals, is shaped by a combination of and environment

• The identification of specific genes that regulate specific behaviors has been a relatively difficult thing to achieve.

• This is because behaviour is a result of activation of multiple sensory, physiological, endocrinological, neurological, muscular and many more pathways.

• Which means, that there are usually multiple genes that are activated or repressed during the display of each behaviour. Hence, in most cases it is difficult to identify a single responsible for a particular behaviour. As more refinements come to the science of behavioral genetics, it has become apparent that for most behavioral traits this is an impossible goal.

• Candidate genes (genes likely to be of importance in a particular behaviour) usually play important roles in sensory systems, channels in membranes, neurotransmitter dynamics, or muscle physiology but are rarely linked only to one behavior.

• This reflects the fact that growth and development, physiology, reproduction, and behavior in a typical organism are the products of 15,000–20,000 genes.

• However, studies have indicated that there are certain behaviours that are regulated by a single gene

• The most fruitful aspect of behavioral genetics is the ability to place genes in their evolutionary context at many different levels.

• is important in understanding whether behaviour is primarily affected by genes or can be changed and modulated by environment in which they are growing and developing. • If the effect of genetics is more then the behaviour is believed to be in the animals “nature” or instinctive/ stereotyped and if the effect of environment is more then it is considered to be shaped by “nurture” or learnt behaviour.

• What is the balance between genetics (instinct or nature) and learning (nurture) in shaping behavior is the biggest question facing the study of animal behaviour and behavioural genetics

• In the animal world, behavior can be envisioned as a continuum between learning and instinct.

• Genetic studies of behavior require much stronger problem-solving approach than other areas of behavioral science.

• This requires the choosing of the right genetic tool.

• Knowledge of genetics gives students of behavior powerful windows into the of behavior and the physiological regulation of behavior. The problem-solving abilities gained from behavioral genetics can be applied in the study of almost any type of behavior and form the scientific basis for understanding the nature–nurture debate.

Tools of Genetics and their use in Behaviour

• There are different tools involved in the study of behavioural genetics o Domestication/ Artificial Breeding/ Inbred and Outbred Lines o Phylogenetics o Quantitative and biometrical genetics o Evolutionary and o • These tools help behavioural studies through o Genetic mapping o Identifying candidate genes for behavioural regulation o Studying gene-environment interactions o Finding correlations between gene-expression and behaviour o Identifying gene regulatory mechanisms that influence behaviour o Evolution of behaviour at organismic and genomal level o Evolution of frequencies of genes responsible for behaviour in populations o Phylogeny of genes for behaviour

Domestication

• Humans have observed their surroundings and the plants and animals in it for years. • During their observations, they obviously observed the variation in individual behavioural traits amongst a species and also the inheritability of those traits, that is there were some behaviours which were being passed down from parents to their offspring. • Such inheritable behaviours that were desirable by humans were then selected for by breeding individuals having the same behaviour again and again through generations, leading to the selection of such behaviour. • The observations followed by such selections over the century are responsible for the domestications of animals and plants. • The huge numbers of dog breeds with different behaviours is a case in example of how behavioural traits can be selected for. Also what is necessary to keep in mind is that the processes of artificial selection can only be applied in cases of behaviour that is largely gene influenced or “nature” influenced or innate. • Behaviours that are largely influenced by environment of nurture cannot be artificially selected or domesticated. • Such artificial selection studies can be used in experimental biology to understand whether behaviour is genetic or environmental. • To determine if a behaviour is genetic or not we can select individuals displaying that behaviour and mate them. In the next generation we can choose their offspring which display the desired trait and mate them with each other. This process when done over generations creates an inbred line. If the trait is genetically influenced its display will be becoming more prominent and more widely expressed in higher number of offspring. It will also persist in different environments. • If the above behaviour is not genetically affected it will not be increasingly displayed through inbred lines.

Phylogeny

• Phylogeny, the history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms. • Phylogeny gives us tools to understand the evolutionary roots of behavior, to know the time frame for the evolution of innovations, and to understand evolutionary interactions at the genetic level. • Phylogenies were traditionally developed by studying similarities in among organisms, but current studies of phylogenies also rely on data gathered using molecular genetic techniques. • By studying the molecular structure of candidate genes or key genes we can trace the evolution of a particular behaviour in closely related species.

• The above phylogenetic tree or cladogram refers to the nest building behaviour of swiftlets. All the swiftlets which are part of this cladogram belong to the same genus but are different species. The nest of swiftlets is especially interesting because it is used in a traditional South Asian delicacy, the bird nest soup. This soup is supposed to have nutritional and medicinal qualities because of the salivary glue used by the swiftlets while making it. • However, in a few swiftlet species, the nest does not use glue but primarily twigs and feathers. On the other hand some species use glue as the chief construction material. • Ethologists thus first identify the various nest building behaviours in swiftlets, and then compare relevant genes in these species. This helps identify the evolution of behaviour. In this case it shows that nest building behaviour has separately evolved three times in swiftlets and not once as would have been hypothetically expected.

CLASSICAL AND MENDELIAN GENETICS

• As scientific processes gained momentum, Mendel led the charge in investigations on how phenotypes were passed from generation to generation. This led to Mendel’s laws of segregation and independent assortment and afterwards, genetic and . • Simple behavioral traits, such as affecting coordination in fruit flies, can be studied using Mendelian approaches. • A simple study using has demonstrated why some individuals of fire ants are monogynous (live in a nest with a single female) while others are polygynous (live in a nest with multiple unrelated females). • This behavioural variation of polygyny and monogyny in a single species has been identified and attributed to a single gene, Gp-9. This gene codes for a pheromone . This receptor is capable of sensing pheromones emitted by the queen. Via perception of the pheromone workers are able to identify queens as their own or as strangers and discard the latter from the nest. • Individuals without the Gp-9 gene or a defective copy of the gene have a faulty pheromone receptor, which cannot sense pheromones emitted by the queen, and are thus not able to differentiate between their own and another queen, and thus are able to tolerate multiple queens. A single gene responsible for a shift in behaviour • Another invasive ant in California, the Argentine ant (Linepithema humile), has been highly successful, possibly because it exhibits reduced intraspecific aggression. While the native population of this ant is highly aggressive • A genetic study using microsatellite DNA markers has shown that these ants experienced a population bottleneck that reduced during their introduction in California. • This is consistent with the hypothesis that reduced genetic variability may impair nestmate or colony recognition, permitting the invaders to behave like one large colony. This, in turn, allows them to rise to higher densities than they do in their native habitat • Using classical genetics studies it has been demonstrated that there are certain genes in male and female Drosophila that reduces the occurrence of courtship in males and the females’ responsiveness to them. Male behavior is inhibited by genes named Nerd and fruitless and is enhanced by Voila. Female receptiveness to mating is impaired by genes such as dissatisfaction, spinster, and chaste. • To separate the effects of nature and nurture a very important method of investigation is the use of cross fostering studies. • As the name suggests, in cross fostering studies two species of the same taxa are chosen say, two bird species A and B. Both species should show some clearly distinctive behaviour, for example unique species specific songs. Now once the individuals of these two species lay a clutch of eggs containing 4 eggs each, then two eggs from each species are placed in the nest of the other species, while two are left with their parents. Then ht songs of the offspring of A reared by parent A, offspring of B reared by parents A, Offspring of B reared by parents B and offspring of A reared by parent B are recorded and observed. If the songs have a strong genetic basis then they will be the same as the rest of their species regardless of the species of the parents who reared them. However, if they are influenced strongly by environment, then they might share a genetic foundation but the songs they produce will be modulated and influenced by the rearing environment and songs of their foster parents. • Thus cross fostering helps in identifying genetic and environmental basis of behaviour. • Adoption and twin studies are also conducted in humans. • Adoption studies are also conducted in monozygotic or identical twins. However, since it is unethical to split and foster offspring of parents, what researchers do is that they trace cases where identical twins were separated at birth and adopted by different set of parents. They then compare the behaviour of these separated identical twins and assess which behaviour is genetic and which identical. • In twin studies a comparison of behaviour of mono and dizygotic twins can help us understand which behaviours have s stronger genetic or environmental basis. • Cloned animals offer a major potential for application of the principles of twin analyses in unique ways: a clone offers the opportunity to observe the genetic equivalent of MZ twins born at different times. If a favorite pet is cloned, will the clone’s behavior resemble the original pet’s behavior in the desired ways? • An inbred line is a population in which closely related animals, such as siblings or parents and offspring, have been repeatedly mated so that nearly all is lost. This is similar in effect to cloning.

Quantitative and Biometrical Genetics

• focuses on the variability of phenotypes and the correlation between and . • Genetic variation is necessary for either natural or artificial selection to produce shifts in gene frequencies. The amount of genetic variation present in a population is key to understanding how selection will affect the population. • Measures of genetic variation also help to understand how past selection has affected a trait. • The study of genetic variation is approached through , which quantifies the portion of the phenotypic variation which is due to genetic variation. Genetic variation also links to the powerful method, quantitative trait loci (QTLs) analysis, for identifying genes that may regulate behavior through genetic mapping. • The first important term is heritability • Heritability is the proportion of phenotypic variation that is explained by genetic variation. It is very important to remember that heritability is about measuring variance and is not the same as inheritance • The second key to understanding heritability is to remember that it is a population, not an individual, measure. • The concept of heritability was developed by quantitative and has powerful applications in behavioral genetics. Neither environment nor genes determine an animal’s behavior, but both environment and genes contribute to the behavioral phenotype. • A major goal of behavioral genetics is to understand the extent of these environmental and genetic correlations with the behavioral phenotype and this is accomplished by estimating heritability • Understanding heritability and how it is used rests on the following key concepts: o Calculating heritability is done by estimating the genetic and environmental contributions to phenotypic variation at the population level. o Heritability is a population-level measure, not an individual measure; only traits in populations have heritability. o Heritability is NOT a measure of the degree of genetic control of a behavioral trait. o Heritability can differ among environments; the same population may show a different heritability for a trait if the environment is changed. o The behavior in question must vary among individuals in the population for it to have a measurable heritability; invariable behavior has no heritability. o Strong selection (natural, sexual, or artificial) on a behavioral trait reduces the heritability of that trait (because it reduces the variation).

• Consider a phenotype, such as shyness, in a population of animals, using the following notation: Vp = variation of the phenotype Ve = variation due to the environment Vg = variation due to genetic effects With these three expressions, a simple equation can be generated that describes the relationship among phenotype, environment, and genes: Vp = Vg + Ve This is a restatement of the central concept: phenotypic variation is the sum of environmental and genetic variation. • Genetic variation is a measure of the variation in phenotype that is due to all variation in genotype. • A way of looking at heritability is to explore how a behavioral trait responds to either natural or artificial selection. If selection on the trait results in change from generation to generation, this suggests that the trait is heritable because selection can act only if there is genetic variation for the trait. However, if selection has no effect on a trait, then its heritability is probably low. Another way of saying this is that a heritable trait is a selectable trait. • A quantitative trait varies continuously, in contrast to qualitative traits, which have discrete values. Another aspect of a quantitative trait is that it is determined by a number of genes acting together • Recent advances in genetics allow scientists to map the genes having the greatest influence on the trait. These genes, are called QTLs. A QTL is a gene that contributes, with other genes, to a phenotype. • Because multiple genes contribute to the phenotype, no one gene “determines” the phenotype. • QTL analysis is now a standard format for studying genetic influences on behavior • All QTL analyses rely on a linkage map and good behavioral measures. • To perform a QTL analysis, a behavioral biologist needs three critical sets of information: a linkage map, a behavioral assessment, and a breeding experiment • The following questions are usually addressed in a QTL analysis: o How many genes influence the expression of a quantitative trait o What is the level of influence of each gene on the trait? o Where are the genes located on the ? o What is the function of each gene? • • •

EVOLUTIONARY AND POPULATION GENETICS • Population genetics relies on techniques that allow measuring of the effect of selection on gene frequencies within a population. Gene frequencies in populations are at the Hardy– Weinberg equilibrium when selective forces have not acted. This equilibrium is rarely achieved in nature, because selective forces are almost always in play, but it is a useful theoretical baseline. • The extent to which gene frequencies deviate from the Hardy–Weinberg equilibrium reveals the existence and intensity of differential fitness and/or survival. • In addition to selection, other factors such as and immigration affect gene frequencies, and calculation of Hardy–Weinberg frequencies allows investigators to assess whether a population has been under various kinds of pressure, such as strong selection or immigration of animals from other locations. • Speciation is one major topic in population genetics that has a strong behavioral component. Because of behavioral choices (e.g., habitat choice, shift in activity regimes, migration, and the like), populations may become reproductively isolated, a condition that can lead to speciation. Generally speaking, new species arise when a population becomes reproductively isolated from other populations and thenevolves (gene frequencies change) so that the reproductive isolation is reinforced by mating incompatibilities. • Mating behavior is a major potential isolation mechanism among animal species, and selection can favor divergence in mating behavior between populations of newly split species.

MOLECULAR GENETICS • One major goal of many genetic studies is to map the location of genes that affect behavior on the chromosomes of an animal. • Mendelian genetics provides tools for mapping, as genes that are close to one another on a do not segregate independently. • Quantitative genetics contributes through tools such as QTL analysis, which allows an investigator to focus in on a gene’s location on a chromosome and to provide the bridge from quantitative to molecular genetics • Sequence data from molecular genetics can show the position of a gene relative to other genes, and ultimately, sequenced data can be correlated with known markers on chromosomes so that sequenced genes can be proposed as candidates for modulating the behavior being studied. Further experiments can selectively modify the expression of the to test the hypothesis that it is important in regulating the behavior. Candidate Genes • For identifying candidate genes, behaviour of closely realted species is observed and then QTL assays conducted. This helps in identifying the possible genes that differ and that may be responsible for a specific behaviour. • After identification of possible candidate genes, other expts may be carried out such as o Microarray: Are there different patterns of gene expression between cells of animals with different behaviors? This question can be answered by looking at expressed sequence tags (ESTs). An EST is a tag based on a known functional gene sequence from an animal. An EST is a genetic marker that is linked to the gene being studied. When the gene of interest is expressed, the EST “reports” that activation. This allows an investigator to see how gene activation correlates with physiological and behavioral activity. It can be amplified— reproduced many times—and then be placed as part of a microarray o RNA knockouts: This technique involves synthesizing oligonucleotides that are complementary to the RNA products of genes thought to influence a behavior. When the synthetic oligonucleotide is introduced into the organism, it binds with the RNA from the target gene, effectively inactivating (or knocking out) that gene o Epigentics: DNA methylation: The addition of methyl groups to the bases that form the backbone of DNA. Methylation silence genes, so that cells in which genes are methylated have a narrower potential range of function than cells in which genes are not methylated. Within the tissues of an animal, methylation isan important mechanism of cellular specialization. In some animals, such as honeybees and ants, methylation is associated with behavioral specialization within a social group

Reference

• Alcock, J. (2009). Animal behavior: An evolutionary approach. Sinauer Associates. • Plomin, R. (2000). Behavioural genetics in the 21st century. International Journal of Behavioral Development, 24(1), 30-34. • O'Brien, G., & Yule, W. (Eds.). (1995). Behavioural phenotypes (Vol. 138). Cambridge University Press. • Jensen, P. (2002). Behavioural genetics, evolution and domestication. The Ethology of Domestic Animals: An Introductory Text, 13-30.