Genetics of social behavior

Social influences on gene expression

In songbirds, egr1 is a transcription factor encoding gene that is active in the auditory forebrain when hearing a song from another songbird. The strength of the expression of egr1 has much variability, dependent on the nature of the song. Songs previously unheard result in strong responses whereas familiar songs result in little or no response. Pure tones or white noise do not evoke any response. It is thought that the purpose of this genetic response to social stimuli is to update the brain's catalog of the changing natural environment. For example, it would be advantageous for a songbird to express more egr1 in a situation where there is a new song heard (a potential intruder) compared to the familiar song of a known individual. In contrast, for the species of highly social cichlid fish A. burtoni, the egr1 gene plays an indirect role in reproduction. In this species, there is an established social dominance hierarchy and an individual's position determines their access to resources for reproduction. If the alpha male is removed from the group, a previous subordinate starts exhibiting dominant behavior and egr1 is expressed in hypothalamus neurons that are responsible for producing a neuropeptide linked to sexual reproduction. Overall, the role of transcription factor egr1 in the context of social behavior clearly shows the link between genes and behavior. As a corollary to that described above, given an environmental cue egr1 will induce or suppress the transcription of other genes. Egr1 shows how social experience may trigger changes in the brain's gene networks. With higher capacity screening techniques, the expression of many genes simultaneously in response to social stimuli will provide a more complete picture.

Neurobiology of mating decisions

The neurobiology of drosophila mating behavior is an area of research that has studied in detail and has elucidated the genetic basis for behavior. The male courts a female based on pheromones from the female and previous experience courting other potential mates. The female accepts or rejects the male's courtship also based on pheromones, the acoustics of his courtship song, and her readiness to mate. The mating behavior of drosophila had been described almost a century ago, and the genetics of these behaviors have been studied for several decades. The current interest in neurobiology is trying to understand the neural circuits that provide the basis of action selection—how the brain maps sensory input, internal states, and individual experience to behavioral decisions.

The pathways that govern the mapping of pheromones to the brain of drosophila are beginning to be understood in detail. Pheromones are detected by olfactory sensory neurons (OSNs). A well-studied pheromone is cVA, which suppresses male courtship behavior when the male detects it from a female. cVA complexes with other proteins to form a ligand, which binds to the odorant receptor Or67d present in OSNs, which is specific for cVA. In turn, the OSNs grow axons that will connect to the glomerulus DA1 in the antennal lobe, which is analogous to the olfactory bulb in mammals. This glomerulus then connects to DA1 projection neurons (PNs) which relay the pheromone signal further to higher brain centers in the protocerebrum (anterior part of an arthropod brain). It is noteworthy that there is significant neuronal convergence taking place between the OSNs and PNs—about 50 OSN inputs to 4 PNs.

Another pathway that has been studied is that involving another odorant receptor called Or47b, which is connected to the VA1v glomerulus. Its respective pathway conveys pheromones from odors in both sexes, and when the genetics of the involved neurons is changed, male courtship is delayed. As a simple model, one would view the integration of the Or67d/DA1 and Or47b/VA1v pathways as a way to describe the initiation of mating behavior in drosophila. The former will tend to inhibit male courtship, whereas the latter is thought to stimulate mating.

It is thought that the difference in the fru gene causes the bulk of the distinction in sexual differentiation in neural circuits. Although the sensory and motor circuits are nearly identical in both sexes, the fru gene fine tunes these according to the needs of either sex. There is learning taking place when a male engages in courtship behavior. A male learns by experience, after courtship rejection by females that have already mated he learns not to pursue other mated females. Similarly, courtship of receptive virgin females is learned after past mating successes. The mushroom bodies are a probable site for this experience-dependent modulation of pheromones, as disrupting fru in neurons in this area reduces short term courtship behavior. Long term courtship is suppressed by the presence of certain proteins in this region of the brain.

For females, the decision whether to mate or not when courted by a male largely depends on the courtship song. Analogous to OSNs, the female's Johnston's organ neurons (JONs) distinguish the quality of the song, and a distinct mechanical stimuli projects signals to specific regions of the brain, which can lead to mating. The reluctance for a female to mate again after having mated before has a molecular basis. In the male Drosophila ejaculate, there is a sex peptide (SP) which binds to the sex peptide receptor (SRP) in the female's fru neurons, disrupting pathways that would compel the female to mate again.

Future research in this area hopes to further explain how the chemical and auditory signals are processed and mapped to behaviors in the fly's brain. Though the molecular mechanisms in the pathways vary between mammals and insects, it has been shown that the information is processed in similar ways. The relative simplicity of fly nervous systems could eventually hint at how neural circuits solve complex behavioral decision making.

Effects of oxytocin and vasopressin on animal behavior

In the past few decades, it has been discovered that oxytocin and vasopressin neuropeptides have key roles in the regulation of social cognition and behavior in mammals. Although homologs have been discovered which are pervasive across many taxa which have similar roles in social and reproductive behaviors, the specific influenced behaviors are quite diverse. For example, in snails the homolog for oxytocin/vasopressin conopressin modulates ejaculation in males and egg laying in females. On the other hand, for vertebrates there is sexual dimorphism in the neuropeptides—oxytocin induces maternal behavior in females and vasopressin induces territoriality, aggression and reproduction in males. In sheep, oxytocin has another effect on a female in addition to a generalized increase in maternal behavior—it induces mother-infant bond selectivity by priming the mother's olfactory bulb, and the mother selectively learns the offspring's scent.

An insightful study of vasopressin is its role in the stimulation of monogamous behavior. The V1a vasopressin receptor gene is a mechanism most widely studied. Different species of voles were studied, prairie voles which are monogamous in nature and montane voles, which tend towards polygamy. Using viral vector mediated gene expression, which allows the cross-species transplanting of genes, when the V1a receptor expression was increased in the reward and reinforcement neural circuitry of montane voles, they acquired a more selective preference for their mate. Their behavior became more akin to that of prairie voles. Therefore, it is alteration of vasopressin receptor gene expression patterns is thought to affect social behavior, rather than the neuropeptide itself. A proposed mechanism for the difference in these patterns is the variation of microsatellite DNA length that is upstream of the V1a receptor gene—short microsatellite DNA in the 5' flanking region of the gene has a different effect on prairie vole behavior than longer microsatellite DNA. Whether this is a mechanism that is replicable in the wild is uncertain.