Tuesday, September 3, 2013

Survival of the shortest (allele)

My colleague Lauren Brent and I have contributed a hypothesis & theory paper entitled "On the evolution of the serotonin transporter linked polymorphic region (5-HTTLPR) in primates" to a research topic in the open access journal Frontiers in Human Neuroscience. The paper has been accepted and should be online soon. Here is the abstract:

Some allelic variants of the serotonin transporter linked polymorphic region (5-HTTLPR) result in lower levels of expression of the serotonin transporter gene (SLC6A4). These low-expressing (LE) alleles are associated with mental-health disorders in a minority of humans that carry them. Humans are not the only primates that exhibit this polymorphism; other species, including some monkeys and apes, also have LE and high-expressing (HE) variants of 5-HTTLPR. We propose a behavioral genetic framework to explain the adaptive evolution of this polymorphism in primates, including humans. We hypothesize that both LE and HE alleles are maintained by balancing selection in species characterized by fluctuating levels of within-group competition. More specifically, we propose that LE carriers benefit from their hypervigilant tendencies during periods of elevated social competition, whereas HE homozygotes cope best when competition levels do not deviate from the norm. Thus, both alleles have long-term benefits when competition levels fluctuate substantially over time within a social group. We describe this hypothesis in detail and outline a series of predictions to test it. Some of these predictions are supported by findings in the current literature, while others remain areas of future research.

UPDATE (11/8/13): Our article is now online and freely available here.

Friday, March 29, 2013

Thanks Ben!

My new project is funded by a Franklin Research Grant from the American Philosophical Society. The title of the project is "Genetic basis of facial expression in female rhesus macaques (Cayo Santiago, Puerto Rico)." Here is a brief description of what we plan to do.
We propose to investigate the genetic basis of facial expressiveness in female rhesus macaques (Macaca mulatta). We will test the hypothesis that facial expressiveness is a genetically heritable trait by focusing on two main predictions. First, if individual differences in facial expressiveness are heritable, then closely related females will be more similar in expressiveness than distantly related females. Second, if facial expressiveness is influenced by genetics, then differences in genotype will be associated with individual differences in expressiveness. We will focus on two candidate genes associated with emotion regulation in humans, a length polymorphism within the promoter region (5-HTTLPR) of the serotonin transporter gene (SLC6A4), and a variable insertion in the gene encoding tryptophan hydroxylase (TPH2).
This project will be carried out in collaboration with Lauren Brent and Michael Platt (Duke U.).

An open mouth threat display from a female rhesus macaque

Saturday, March 2, 2013

Evolving insular cortices

I am coauthor of a forthcoming paper in the Journal of Human Evolution entitled "A volumetric comparison of the insular cortex and its subregions in primates." The lead author Amy Bauernfeind is a student in Chet Sherwood's lab. Here's the abstract:

The neuronal composition of the insula in primates displays a gradient, transitioning from granular neocortex in the posterior-dorsal insula to agranular neocortex in the anterior-ventral insula with an intermediate zone of dysgranularity. Additionally, apes and humans exhibit a distinctive subdomain in the agranular insula, the frontoinsular cortex (FI), defined by the presence of clusters of von Economo neurons (VENs). Studies in humans indicate that the ventral anterior insula, including agranular insular cortex and FI, is involved in social awareness, and that the posterodorsal insula, including granular and dysgranular cortices, produces an internal representation of the body's homeostatic state. We examined the volumes of these cytoarchitectural areas of insular cortex in 30 primate species, including the volume of FI in apes and humans. Results indicate that the whole insula scales hyperallometrically (exponent = 1.13) relative to total brain mass, and the agranular insula (including FI) scales against total brain mass with even greater positive allometry (exponent = 1.23), providing a potential neural basis for enhancement of social cognition in association with increased brain size. The relative volumes of the subdivisions of the insular cortex, after controlling for total brain volume, are not correlated with species typical social group size. Although its size is predicted by primate-wide allometric scaling patterns, we found that the absolute volume of the left and right agranular insula and left FI are among the most differentially expanded of the human cerebral cortex compared to our closest living relative, the chimpanzee.
UPDATE (3/13/13): The article is available online here (subscription required). Email me if you would like a PDF (seth dot dobson at dartmouth dot edu).

Experimental Biology 2013

I will be presenting at this year's meeting of the American Association of Anatomists in Boston as part of a symposium entitled "Comparative Perspectives on the Human Face: Biomedical Implications of Evolutionary Anatomical Research" organized by Anne Burrows. Rui Diogo will also be taking part. My talk is entitled "Neuroanatomical causes and social consequences of variation in facial motor control: what comparative primatology can teach us about autism spectrum disorders." Here's the abstract:

A limited body of evidence suggests that people with severe Autism Spectrum Disorders (ASD) are generally less expressive than mental age-matched controls. The objective of my talk is to demonstrate how a comparative primatological perspective can help to elucidate the neuroanatomical and social correlates of facial expressivity. I will review research on the evolution of facial nucleus volume in a wide range of nonhuman primates. Previous studies demonstrate that species that have relatively large facial motor nuclei tend to have larger brains, live in larger social groups, and spend more time social grooming than species with relatively small facial nuclei. These observations support the notion that the facial motor nucleus is an essential part of the network of brain regions that is impaired in ASD, i.e., the “social brain.” I will also present new results of a recent pilot study examining the correlation between emotional expressiveness and serotonin transporter genotype (5-HTTLPR) in free-ranging rhesus macaques (Macaca mulatta). Our preliminary findings suggest that female macaques with the SS genotype tend to be more expressive overall than LL individuals, even after controlling for dominance rank. This observation calls into question the hypothesis that the S allele is responsible for the relative lack of expressiveness observed in some people with ASD.

Friday, March 1, 2013

Not so universal facial attractiveness

Dartmouth alum Zaneta Thayer and I have a paper in press at PLoS ONE entitled "Geographic variation in chin shape challenges the universal facial attractiveness hypothesis." Here is the abstract:

The universal facial attractiveness (UFA) hypothesis proposes that some facial features are universally preferred because they are reliable signals of mate quality. The primary evidence for this hypothesis comes from cross-cultural studies of perceived attractiveness. However, these studies do not directly address patterns of morphological variation at the population level. An unanswered question is therefore: Are universally preferred facial phenotypes geographically invariant, as the UFA hypothesis implies? The purpose of our study is to evaluate this often overlooked aspect of the UFA hypothesis by examining patterns of geographic variation in chin shape. We collected symphyseal outlines from 180 recent human mandibles (90 male, 90 female) representing nine geographic regions. Elliptical Fourier functions analysis was used to quantify chin shape, and principle components analysis was used to compute shape descriptors. In contrast to the expectations of the UFA hypothesis, we found significant geographic differences in male and female chin shape. These findings are consistent with region-specific sexual selection and/or random genetic drift, but not universal sexual selection. We recommend that future studies of facial attractiveness take into consideration patterns of morphological variation within and between diverse human populations.
UPDATE (4/3/13): The article is now online and freely available here.