QUANTITATIVE GENETICS OF SEXUAL DIMORPHISM IN HUMAN BODY SIZE

Sex-biased gene regulation varies across human populations as a result of adaptive evolution

OBJECTIVES

Studies of human sexual dimorphism and gender disparities in health focus on ostensibly universal molecular sex differences, such as sex chromosomes and circulating hormone levels, while ignoring the extraordinary diversity in biology, behavior, and culture acquired by different human populations over their unique evolutionary histories.

MATERIALS AND METHODS

Using RNA-Seq data and whole genome sequences from 1000G and HGDP, we investigate variation in sex-biased gene expression across 11 human populations and test whether population-level variation in sex-biased expression may have resulted from adaptive evolution in regions containing sex-specific regulatory variants.

RESULTS

We find that sex-biased gene expression in humans is highly variable, mostly population-specific, and demonstrates between population reversals. Expression quantitative trait locus mapping reveals sex-specific regulatory regions with evidence of recent positive natural selection, suggesting that variation in sex-biased expression may have evolved as an adaptive response to ancestral environments experienced by human populations.

DISCUSSION

These results indicate that sex-biased gene expression is more flexible than previously thought and is not generally shared among human populations. Instead, molecular phenotypes associated with sex depend on complex interactions between population-specific molecular evolution and physiological responses to contemporary socioecologies.

Substantial but Misunderstood Human Sexual Dimorphism Results Mainly From Sexual Selection on Males and Natural Selection on Females

Human sexual dimorphism has been widely misunderstood. A large literature has underestimated the effect of differences in body composition and the role of male contest competition for mates. It is often assumed that sexually dimorphic traits reflect a history of sexual selection, but natural selection frequently builds different phenotypes in males and females. The relatively small sex difference in stature (∼7%) and its decrease during human evolution have been widely presumed to indicate decreased male contest competition for mates. However, females likely increased in stature relative to males in order to successfully deliver large-brained neonates through a bipedally-adapted pelvis. Despite the relatively small differences in stature and body mass (∼16%), there are marked sex differences in body composition. Across multiple samples from groups with different nutrition, males typically have 36% more lean body mass, 65% more muscle mass, and 72% more arm muscle than women, yielding parallel sex differences in strength. These sex differences in muscle and strength are comparable to those seen in primates where sexual selection, arising from aggressive male mating competition, has produced high levels of dimorphism. Body fat percentage shows a reverse pattern, with females having ∼1.6 times more than males and depositing that fat in different body regions than males. We argue that these sex differences in adipose arise mainly from natural selection on women to accumulate neurodevelopmental resources.

Patterns of recent natural selection on genetic loci associated with sexually differentiated human body size and shape phenotypes

Levels of sex differences for human body size and shape phenotypes are hypothesized to have adaptively reduced following the agricultural transition as part of an evolutionary response to relatively more equal divisions of labor and new technology adoption. In this study, we tested this hypothesis by studying genetic variants associated with five sexually differentiated human phenotypes: height, body mass, hip circumference, body fat percentage, and waist circumference. We first analyzed genome-wide association (GWAS) results for UK Biobank individuals (~194,000 females and ~167,000 males) to identify a total of 114,199 single nucleotide polymorphisms (SNPs) significantly associated with at least one of the studied phenotypes in females, males, or both sexes (P<5x10^-8). From these loci we then identified 3,016 SNPs (2.6%) with significant differences in the strength of association between the female- and male-specific GWAS results at a low false-discovery rate (FDR<0.001). Genes with known roles in sexual differentiation are significantly enriched for co-localization with one or more of these SNPs versus SNPs associated with the phenotypes generally but not with sex differences (2.73-fold enrichment; permutation test; P = 0.0041). We also confirmed that the identified variants are disproportionately associated with greater phenotype effect sizes in the sex with the stronger association value. We then used the singleton density score statistic, which quantifies recent (within the last ~3,000 years; post-agriculture adoption in Britain) changes in the frequencies of alleles underlying polygenic traits, to identify a signature of recent positive selection on alleles associated with greater body fat percentage in females (permutation test; P = 0.0038; FDR = 0.0380), directionally opposite to that predicted by the sex differences reduction hypothesis. Otherwise, we found no evidence of positive selection for sex difference-associated alleles for any other trait. Overall, our results challenge the longstanding hypothesis that sex differences adaptively decreased following subsistence transitions from hunting and gathering to agriculture.

There is uncertainty regarding the evolutionary history of human sex differences for quantitative body size and shape phenotypes. In this study we identified thousands of genetic loci that differentially impact body size and shape trait variation between females and males using a large sample of UK Biobank individuals. After confirming the biological plausibility of these loci, we used a population genomics approach to study the recent (over the past ~3,000 years) evolutionary histories of these loci in this population. We observed significant increases in the frequencies of alleles associated with greater body fat percentage in females. This result is contradictory to longstanding hypotheses that sex differences have adaptively decreased following subsistence transitions from hunting and gathering to agriculture.

Modest effects of dietary supplements during the COVID-19 pandemic: insights from 445 850 users of the COVID-19 Symptom Study app

OBJECTIVES

Dietary supplements may ameliorate SARS-CoV-2 infection, although scientific evidence to support such a role is lacking. We investigated whether users of the COVID-19 Symptom Study app who regularly took dietary supplements were less likely to test positive for SARS-CoV-2 infection.

DESIGN

App-based community survey.

SETTING

445 850 subscribers of an app that was launched to enable self-reported information related to SARS-CoV-2 infection for use in the general population in the UK (n=372 720), the USA (n=45 757) and Sweden (n=27 373).

MAIN EXPOSURE

Self-reported regular dietary supplement usage (constant use during previous 3 months) in the first waves of the pandemic up to 31 July 2020.

MAIN OUTCOME MEASURES

SARS-CoV-2 infection confirmed by viral RNA reverse transcriptase PCR test or serology test before 31 July 2020.

RESULTS

In 372 720 UK participants (175 652 supplement users and 197 068 non-users), those taking probiotics, omega-3 fatty acids, multivitamins or vitamin D had a lower risk of SARS-CoV-2 infection by 14% (95% CI (8% to 19%)), 12% (95% CI (8% to 16%)), 13% (95% CI (10% to 16%)) and 9% (95% CI (6% to 12%)), respectively, after adjusting for potential confounders. No effect was observed for those taking vitamin C, zinc or garlic supplements. On stratification by sex, age and body mass index (BMI), the protective associations in individuals taking probiotics, omega-3 fatty acids, multivitamins and vitamin D were observed in females across all ages and BMI groups, but were not seen in men. The same overall pattern of association was observed in both the US and Swedish cohorts.

CONCLUSION

In women, we observed a modest but significant association between use of probiotics, omega-3 fatty acid, multivitamin or vitamin D supplements and lower risk of testing positive for SARS-CoV-2. We found no clear benefits for men nor any effect of vitamin C, garlic or zinc. Randomised controlled trials are required to confirm these observational findings before any therapeutic recommendations can be made.

Sexual selection and physical attractiveness : Implications for mating dynamics

Sexual selection processes have received much attention in recent years, attention reflected in interest in human mate preferences. Among these mate preferences are preferences for physical attractiveness. Preferences in and of themselves, however, do not fully explain the nature of the relationships that individuals attain. A tacit negotiation process underlies relationship formation and maintenance. The notion that preferences for physical attractiveness evolved under parasite-driven "good genes" sexual selection leads to predictions about the nature of trade-offs that individuals make between mates' physical attractiveness and investment potential. These predictions and relevant data are explored, with a primary emphasis on women's preferences for men's qualities. In addition, further implications of trade-offs are examined, most notably (a) the impact of environmental variations on the nature of mating and (b) some effects of trade-offs on infidelity and male attempts to control women.

Quantitative genetics of costly neonatal sexual size dimorphism in squirrel monkeys (Saimiri boliviensis)

Offspring size is often an intimate link between the fitness of parents and offspring. Among mammals, neonate mass is also related to adult levels of dimorphism and intrasexual competitive mating. We describe the sex-specific genetic architecture of neonate mass in captive squirrel monkeys (Saimiri boliviensis), a small Neotropical primate. Best fitting quantitative genetic models show strong maternal genetic effects with little difference between sexes offering limited opportunity for neonatal dimorphism to respond to observed or hypothetical selection. Heritabilities that are approximately zero also imply it is unlikely that neonatal dimorphism can evolve as a correlated response to selection on adult size. However, male mass is also more dependent on maternal condition (age and parity) making dimorphism plastic. Finally, we hypothesize that large maternal genetic effects reflect income breeding and tightly synchronized seasonal reproduction in squirrel monkeys, both of which require strong maternal control of offspring growth and timing of birth.

Sex differences in spinal osteoarthritis in humans and rhesus monkeys (Macaca mulatta)

STUDY DESIGN

Retrospective study of male and female spinal osteoarthritis, characterized by lateral spine thoracolumbar radiographs, in humans and nonhuman primates.

OBJECTIVE

To characterize differences in prevalence and vertebral distribution of spinal osteoarthritis between men and women, between male and female macaques, and between the 2 phylogenetically related genera.

SUMMARY OF BACKGROUND DATA

Naturally occurring spinal osteoarthritis manifests similarly in humans and rhesus macaques. Other types of osteoarthritis particularly of the knee and hip have revealed sex differences in humans. In regard to spinal osteoarthritis, sex differences have been noted but without consistent results. Sex differences in macaques have not been examined.

METHODS

Radiographical evidence of disc space narrowing and osteophytosis was assessed using an atlas-scoring method. Prevalence was determined according to sex, age, body mass (for macaques only), and spinal location (human T4-L5; macaque T8-L7).

RESULTS

Average scores in macaques differed between the sexes, but they did not differ between men and women. The pattern of involvement along the spine was the same in male and female monkeys but differed between men and women: women had more thoracic involvement and men had more lumbar involvement. Overall, monkeys had a significantly higher prevalence of osteoarthritis than humans.

CONCLUSION

The appearance of sex differences in the prevalence of osteoarthritis is most likely a proxy measure for the effect of body mass. Sex differences were apparent in monkeys due to the fact that males are significantly heavier than females. No sex difference in prevalence was apparent in humans, and there is substantial overlap in body mass between men and women. Differences in the location of osteoarthritic involvement along the spine between men and women were obscured when only average scores were examined.

Evolution of female carotenoid coloration by sexual constraint in Carduelis finches

Background

Females often express the same ornaments as males to a similar or lesser degree. Female ornaments can be adaptive, but little is known regarding their origins and mode of evolution. Current utility does not imply evolutionary causation, and therefore it is possible that female ornamentation evolved due to selection on females, as a correlated response to selection on males (sexual constraint), or a combination of both. We tested these ideas simulating simple models for the evolution of male and female correlated traits, and compared their predictions against the coloration of finches in the genus Carduelis.

Results

For carotenoid-based ornamental coloration, a model of sexual constraint on females fits the Carduelis data well. The two alternative models (sexual constraint on males, and mutual constraint) were rejected as causing the similarities in carotenoid coloration between males and females. For melanin coloration, the correlation between the sexes was weaker, indicating that males and females evolved independently to a greater extent.

Conclusions

This indicates that sexual constraint on females was an important mechanism for the evolution of ornamental carotenoid coloration in females, but less so for melanin coloration. This does not mean that female carotenoid coloration is non-adaptive or maladaptive, because sexual dichromatism could evolve if it were maladaptive. It suggests, however, that most evolution of female carotenoid coloration was male-driven and, when adaptive, may not be an adaptation stricto sensu.

Genetic variation in baboon craniofacial sexual dimorphism

Sexual dimorphism is a widespread phenomenon and contributes greatly to intraspecies variation. Despite a long history of active research, the genetic basis of dimorphism for complex traits remains unknown. Understanding the sex-specific differences in genetic architecture for cranial traits in a highly dimorphic species could identify possible mechanisms through which selection acts to produce dimorphism. Using distances calculated from three-dimensional landmark data from CT scans of 402 baboon skulls from a known genealogy, we estimated genetic variance parameters in both sexes to determine the presence of gene-by-sex (G x S) interactions and X-linked heritability. We hypothesize that traits exhibiting the greatest degree of sexual dimorphism (facial traits in baboons) will demonstrate either stronger G x S interactions or X-linked effects. We found G x S interactions and X-linked effects for a few measures that span the areas connecting the face to the neurocranium but for no traits restricted to the face. This finding suggests that facial traits will have a limited response to selection for further evolution of dimorphism in this population. We discuss the implications of our results with respect to the origins of cranial sexual dimorphism in this baboon sample, and how the genetic architecture of these traits affects their potential for future evolution.

Generational effects and gender height dimorphism in contemporary Spain

We examine the influence of socio-environmental (and birth cohort specific) effects on both adult height and gender dimorphism (height gap). Our data set is from contemporary Spain, a country governed by an authoritarian regime for about 40 years. Both OLS and quantile regression approaches are used to examine these patterns. Furthermore, we then draw upon a Blinder-Oaxaca decomposition approach to explain the influence of socio-political environment in explaining gender dimorphism. Our findings point to a significant increase in adult height in the generations that benefited from the country's economic liberalization in the 1950s, and especially among those brought up after the transition to democracy in the 1970s. In contrast, individual heterogeneity suggests that only in recent generations has "height increased more among the tallest". We also find that the effects of education on height are greater among shorter individuals. Although the mean gender difference in height is 11.7cm, birth cohort and capabilities effects along with other controls explain on average roughly 4% of the gender height dimorphism, irrespective of the quantile considered.

Stature and sexual stature dimorphism in Sweden, from the 10th to the end of the 20th century

Mean stature in a population has been observed to vary with living conditions. If, and how, this affects sexual dimorphism in stature is not fully understood. We analyzed stature data from Swedish populations from the 10th to the end of the 20th century to investigate if male stature is more plastic than female stature in response to environmental changes. Further, we examined if there, as a consequence of this, exists an allometric relationship between male and female stature that is not caused by genetic factors, coupling greater stature with greater dimorphism. We found no significant change in stature from the 10th century to the 17th century, but a clear increase in both male and female stature during the 20th century, most likely because of improved living conditions. Regression analyses revealed no consistent change in sexual stature dimorphism over time for any of the time periods, including the 20th century. Further, we found no significant allometric relationship between male and female stature, and could consequently not identify any significant relationship between stature and stature dimorphism. Thus, contrary to previous suggestions, the regressions did not provide support for the assertion that male stature is more sensitive to environmental changes than female stature, nor that stature dimorphism increases with increasing stature.

Human skin-color sexual dimorphism: a test of the sexual selection hypothesis

Applied to skin color, the sexual selection hypothesis proposes that male preference for light-skinned females explains the presence of light skin in areas of low solar radiation. According to this proposal, in areas of high solar radiation, natural selection for dark skin overrides the universal preference of males for light females. But in areas in which natural selection ceases to act, sexual selection becomes more important, and causes human populations to become light-skinned, and females to be lighter than males. The sexual selection hypothesis proposes that human sexual dimorphism of skin color should be positively correlated with distance from the equator. We tested the prediction that sexual dimorphism should increase with increasing latitude, using adult-only data sets derived from measurements with standard reflectance spectrophotometric devices. Our analysis failed to support the prediction of a positive correlation between increasing distance from the equator and increased sexual dimorphism. We found no evidence in support of the sexual selection hypothesis.

Investigating the independent evolution of the size of floral organs via G-matrix estimation and artificial selection

The attractiveness of a plant to pollinators is dependent on both the number of flowers produced and the size of the petals. However, limiting resources often result in a size/number trade-off, whereby the plant can make either more flowers or larger flowers, but not both. If developmental genes underlying sepal and petal identity (some of which overlap) also influence size, then this shared genetic basis could constrain the independent evolution of floral size and attractiveness. Here, we determined whether the size of sepals and petals in the dioecious perennial, Silene latifolia, are developmentally independent by performing two experiments: a genetic variance-covariance experiment to estimate genetic correlations between calyx width, petal-limb length, flower mass, and number and a four-bout artificial-selection experiment to alter calyx width and estimate the correlated response in petal-limb length. In addition, we determined whether variation in petal-limb length is the result of cell expansion or cell proliferation. The first experiment revealed that petal-limb length is not genetically correlated with calyx width, and the second experiment confirmed this; selection on calyx width did not result in a predictable or significant change in petal-limb length. Flower number was negatively correlated with all the floral traits measured, indicating a flower size/number trade-off. Cell number, but not size, explained a significant amount of the variation in petal-limb length. We conclude that the size of the two outer floral organs can evolve independently. This species can therefore increase the number of flowers produced by decreasing investment in the calyx without simultaneously decreasing petal size and the attractiveness of each individual flower to pollinators.

Population variation in second metacarpal sexual size dimorphism

This paper contrasts levels of sexual size dimorphism in second metacarpal osteometric and geometric morphology in two bioculturally distinctive populations: 19th century Euro-Canadian settlers, and proto/historic central Canadian Inuit. Significant within-group sexual size dimorphism is found for all variables, though few show significant interpopulation differences. However, in every case the Euro-Canadian sample is more dimorphic than the Inuit sample. Notably, differences reside in geometric measures (total area, Imax) sensitive to variation in functional strain, and thus are interpretable in light of proximate causal models, i.e., activity profiles distinct from generalized mode of subsistence. Other proximate factors, such as nutritional stress acting to diminish Inuit sexual size dimorphism, may also play a role. However, models often cited to explain dimorphism, such as marriage practice (e.g., polygyny) or division of labor situated in mode of subsistence, do not. The higher sexual size dimorphism in the 19th century settler sample belies the notion that technological progress inevitably leads to reduced dimorphism.

Sexual dimorphism in stature and women's work: a phylogenetic cross-cultural analysis

The following cultural variables were tested for their association with sexual dimorphism: sexual division of labor, type of subsistence (hunting and agriculture), and polygyny. The transmission of these traits among populations was investigated. All the traits were found to be associated with phylogeny, indicating that they are inherited from mother to daughter populations. A cross-cultural comparative method was used which controls for the statistical effects of similarity due to common ancestry (Galton's problem). Cross-cultural variation in sexual dimorphism in stature is negatively associated with women's contribution to subsistence. Women are taller, relative to men, in societies where women contribute more to food production. This may be because female nutritional status is better in these societies. No relationship was found between sexual dimorphism and other aspects of subsistence or polygyny. These results are discussed in relation to other studies of sexual dimorphism in modern and archaeological populations, and in relation to cross-cultural variation in sex-biased parental investment.

A re-evaluation of subspecific variation and canine dimorphism in woolly spider monkeys (Brachyteles arachnoides)

A recent study suggests that differing populations of woolly spider monkeys exhibit a substantial degree of morphological, cytogenetic, and behavioral variation. We re-evaluate the differences between populations in the degree of canine tooth height sexual dimorphism and in the frequency of thumbs. Statistical analysis of variation in the degree of canine sexual dimorphism between these populations fails to provide strong evidence for subspecific variation: differences in the degree of canine dimorphism cannot be considered statistically significant. Differences between populations in the frequency of thumbs are, however, statistically significant. The lack of clear distinctions between populations in the degree of canine dimorphism complicates assessments of behavioral variation between these populations. We suggest that the level of geographic variation in woolly spider monkey canine dimorphism is not consistent with subspecific status.

Constraints on the evolution of attractive traits: genetic (co)variance of zebra finch bill colour

We estimated the heritability and genetic correlation between male and female bill colour in a laboratory population of zebra finches (Taeniopygia guttata) in order to examine the potential genetic constraints on the evolution of a sexually dimorphic trait. The heritability estimates of bill colour from regressions of offspring on single parents ranged from h2 = 0.34 to 0.73 and all but one of these estimates were significantly greater than zero. The restricted maximum likelihood heritability estimates for full- and half-siblings were significant for females (h2 = 0.48) but not significant for males (h2 = 0.45). The maximum likelihood estimates indicate that there is little dominance genetic variance for bill colour. The large genetic correlation between male and female bill colour (rg = 0.91) combined with opposing selection on male and female bill colour indicates that the evolution to sex-specific optima may proceed very slowly.

Quantitative genetics of sexual dimorphism in body fat measurements

A variance decomposition analysis using maximum likelihood methods was employed to examine the genetic architecture of sexual dimorphism in anthropometric traits in a large pedigreed sample of Mexican American individuals from San Antonio, Texas. For this analysis the magnitude of sexual dimorphism was viewed as arising from a special case of genotype by environment interaction (G × E), that of genotype by sex (G × S). Evidence indicates a marked G × S interaction for 9 of the 12 traits examined and 1 of the 4 indices, findings which are interpreted as indicators of a strong genetic component to the degree of sexual dimorphism expressed in these traits. Such results have important implications for the use and interpretation of these traits in an epidemiological as well as an evolutionary context. © 1993 Wiley-Liss, Inc.

Human marriage systems and sexual dimorphism in stature

Contemporary populations of Homo sapiens are sexually dimorphic on a variety of traits. In terms of stature, men are reliably between 4% and 10% taller than women in well-sampled human populations. Are cross-cultural differences in the magnitude of sexual dimorphism consistent with expectations from sexual selection theory? Prior studies have provided conflicting answers to this question in part because they failed to agree on how the force of sexual selection should or could be operationalized. Here we offer a simple and unbiased method for operationalizing sexual selection and retest two separate predictions from earlier work (Alexander et al., 1979) about its expected impact on stature dimorphism in a sample of 155 societies. Neither prediction matches the observed cross-cultural distribution of dimorphism. However, this is not the consequence of a random distribution of dimorphism across societies. Instead, the data exhibit a robust and unexpected pattern.