Of fingers, toes and penises

EVOLUTIONARY CO-OPTION OF AN ANCESTRAL CLOACAL REGULATORY LANDSCAPE DURING THE EMERGENCE OF DIGITS AND GENITALS

The transition from fins to limbs has been a rich source of discussion for more than a century. One open and important issue is understanding how the mechanisms that pattern digits arose during vertebrate evolution. In this context, the analysis of Hox gene expression and functions to infer evolutionary scenarios has been a productive approach to explain the changes in organ formation, particularly in limbs. In tetrapods, the transcription of Hoxd genes in developing digits depends on a well-characterized set of enhancers forming a large regulatory landscape1,2. This control system has a syntenic counterpart in zebrafish, even though they lack bona fide digits, suggestive of deep homology3 between distal fin and limb developmental mechanisms. We tested the global function of this landscape to assess ancestry and source of limb and fin variation. In contrast to results in mice, we show here that the deletion of the homologous control region in zebrafish has a limited effect on the transcription of hoxd genes during fin development. However, it fully abrogates hoxd expression within the developing cloaca, an ancestral structure related to the mammalian urogenital sinus. We show that similar to the limb, Hoxd gene function in the urogenital sinus of the mouse also depends on enhancers located in this same genomic domain. Thus, we conclude that the current regulation underlying Hoxd gene expression in distal limbs was co-opted in tetrapods from a preexisting cloacal program. The orthologous chromatin domain in fishes may illustrate a rudimentary or partial step in this evolutionary co-option.

Tgfbr1 controls developmental plasticity between the hindlimb and external genitalia by remodeling their regulatory landscape

The hindlimb and external genitalia of present-day tetrapods are thought to derive from an ancestral common primordium that evolved to generate a wide diversity of structures adapted for efficient locomotion and mating in the ecological niche occupied by the species. We show that despite long evolutionary distance from the ancestral condition, the early primordium of the mouse external genitalia preserved the capacity to take hindlimb fates. In the absence of Tgfbr1, the pericloacal mesoderm generates an extra pair of hindlimbs at the expense of the external genitalia. It has been shown that the hindlimb and the genital primordia share many of their key regulatory factors. Tgfbr1 controls the response to those factors by modulating the accessibility status of regulatory elements that control the gene regulatory networks leading to the formation of genital or hindlimb structures. Our work uncovers a remarkable tissue plasticity with potential implications in the evolution of the hindlimb/genital area of tetrapods, and identifies an additional mechanism for Tgfbr1 activity that might also contribute to the control of other physiological or pathological processes.

The role of the 5' HoxA genes in the development of the hindgut, vent, and a novel sphincter in a derived teleost (bluebanded goby, Lythrypnus dalli)

Unique expression patterns of the 5' HoxA genes are associated with the evolution and development of novel features including claspers in cartilaginous fishes, modified pectoral fins in batoids, and the yolk sac extension in Cypriniformes. Here, we demonstrate a role for HoxA11a and HoxA13a in demarcating the hindgut in fishes of the family Gobiidae, including a novel sphincter called the intestinal rectal sphincter (IRS). Disruption of 5' HoxA expression, via manipulation of retinoic acid signaling, results in failure of the IRS and/or vent to develop. Furthermore, exposure to HoxA disruptors alters 5' HoxA expression, in association with developmental phenotypes, demonstrating a functional link between 5' HoxA expression and development of a novel feature in the bluebanded goby, Lythrypnus dalli.

Direct and radiographic digit ratio (2D:4D) measurements of Tuvan children and adolescents from Southern Siberia: Sex differences and skeletal maturation

BACKGROUND

In Mongolian-origin ethnic groups digit ratio (2D:4D; a proxy for prenatal sex-steroids) is sexually dimorphic (males < females), as reported for other ethnicities. Most studies measured 2D:4D from soft tissue (directly from the digits, or indirectly from hand scans), or from radiographs. Evidence on the correspondence of 2D:4D measurements from soft tissue with measurements from radiographic images is scarce and has not been reported for a Mongolian-origin sample. In addition, previous research has not considered relationships between 2D:4D and measures of skeletal maturity.

AIM

To examine (i) associations between 2D:4D measured directly from the palms with those obtained from radiographic images of the same individuals in a sample of children and adolescents from the Tuvan population in Siberia (Russian Federation), and ii) associations between 2D:4D measurements with chronological and skeletal age.

SUBJECTS AND METHODS

Participants were Tuvan boys and girls aged 7 to 18 years. 2D:4D of the right and left hand was measured from soft tissue (directly from the palm) and compared with radiographic images (left hand only). In addition to finger length 2D:4D, we examined 2D:4D of the phalanges from measurements of radiographs. Skeletal age was assessed using the Tanner-Whitehouse method.

RESULTS

Sex differences (boys < girls) in measurements of 2D:4D from soft tissue and radiographs were found for total finger length and phalanges. In addition, 2D:4D measurements from radiographs correlated positively and significantly with those obtained from soft tissue. Sex predicted 2D:4D measurements from soft tissue and radiographs, but no effects of chronological/skeletal age and body height were detected. In girls (but not in boys), earlier skeletal maturity (relative to chronological age) was associated with higher 2D:4D in soft tissue measurements of both hands, radiographic 2D:4D, and 2D:4D of the proximal phalanges.

CONCLUSION

Consistent with reports from other ethnic groups, 2D:4D in young Tuvans was sexually dimorphic, with boys having lower 2D:4D than girls. For girls, higher 2D:4D was found for participants whose skeletal age was more advanced than chronological age. This finding was obtained from direct soft tissue and indirect radiographic measurements. Age and body height were not associated with 2D:4D, which suggests differences in hormone developmental trajectories for 2D:4D and height.

The ratio of 2nd to 4th digit length and reproductive function of infertile male patients

Purpose

To investigate the relationship between the ratio of index to ring finger lengths (2D:4D ratio), reflecting androgen exposure in utero, and reproductive function among men.

Methods

Male patients (N = 180) who consulted for fertility issues participated in the study. The palms of both hands were scanned, and the 2D:4D ratio was calculated. Data on semen volume, sperm concentration and total motility, total and motile sperm counts, and serum hormone concentrations were obtained. Spearman correlation coefficients between the 2D:4D ratio and hormone and semen quality parameters were calculated.

Results

The total sperm count was significantly negatively correlated with the 2D:4D ratio of the left hand (r = -0.154, p  = 0.039) but not with that of the right hand (r = -0.045, p  = 0.548). Testosterone showed weak negative correlations with the 2D:4D ratio in the left (r = -0.142, p  = 0.058) and right (r = - 0.149, p  = 0.046) hands. Follicle-stimulating hormone levels were negatively correlated with the 2D:4D ratios of the left (r = -0.173, p  = 0.020) and right (r = -0.164, p  = 0.027) hands. Other semen quality parameters or luteinizing hormone levels showed no significant correlation with the 2D:4D ratios.

Conclusions

No clear associations were observed between the 2D:4D ratios and reproductive function.

The Ratio of Second and Fourth Digit Length: A Biomarker for Methamphetamine Dependence?

OBJECTIVE

The ratio of 2nd and 4th digit length (2D:4D) is considered to be a sexually dimorphic trait. Low 2D:4D is implicated in alcohol dependence and heroin dependence and correlated with psychological traits such as aggression, physical aggression, and sensation. The purpose of this study is to compare the 2D:4D between methamphetamine (METH) dependence and controls and the 2D:4D ratio that is a potential biomarker for METH dependence.

METHODS

In this study, 40 patients diagnosed with METH dependence in Eulji University Gangnam Eulji Hospital and 50 healthy volunteers were all employees in the same hospital. Images of participants' hands were created using a scanning device. The images contained both the right and left hands; computer software was used to measure the 2D:4D ratio for both hands. We compared the ratios, analyzed by t test, between the METH dependence group and the control group.

RESULTS

The mean 2D:4D values were 0.941 (right hand) and 0.943 (left hand) for the patients with METH dependence; in contrast, they were 0.961 (right hand) and 0.961 (left hand) for the control group. These values were significantly smaller than the control in patients' right hands (p = 0.003) and left hands (p = 0.012).

CONCLUSION

Patients with METH dependence had smaller 2D:4D ratios than those in the control group, which is similar to the results from the previous substance use disorder studies. Thus, elevated prenatal testosterone levels during the gonadal period could be related to future METH problems. Furthermore, the 2D:4D ratio is a potential marker for the prediction of METH dependence.

2D:4D digit ratio and its relationship to BMI, sporting choices and physiological predispositions among women

The 2D:4D digit ratio has been established as a biomarker of the level of exposure to prenatal sex hormones’ balancebetween prenatal testosterone (PT) and estrogenne levels. Higher 2D:4D indicates lower PT exposure and vice versa. Data suggests that PT exposure is linked to a risk-taking attitude and physical aggressiveness, both of which are requirements in contact sport. A possible correlation between 2D:4D and human body mass index has also been identified. The aim of the study was to examine the relation between 2D:4D ratio and choice of sport. It was assumed that female soccer players who choose a contact sport would have a lower 2D:4D ratio (thus experiencing higher exposure to PT) than female volleyball players (selecting non-contact sport). The analysis was also aimed at identifying whether a correlation between prenatal testosterone level and BMI exists. The participant sample consisted of 103 women – 36 volleyball players, 33 soccer players and a control group (N=34). Measurements were collected in 2019–2020. The results suggest that 2D:4D was significantly different in women practicing various sports (contact and non-contact sports). Women engaged in contact sports had lower 2D:4D than women engaged in non-contact sports, and vice versa (p<0.05). 2D:4D correlated positively with BMI and body weight – the higher the 2D:4D ratio, the higher the BMI and body weight (and vice versa) (p<0.05). Low 2D:4D (high PT exposure) may predict the choice of more risky, aggressive contact sports, and vice versa. High 2D:4D may predict a higher BMI and body weight, and vice versa.

Anthropometric measures do not explain the 2D:4D ratio sexual dimorphism in 7-year-old children

Objectives

Digit ratio (2D:4D) might reflect prenatal testosterone exposure and has been used as a putative marker for androgen related outcomes. However, such associations might be inflicted by confounders. Application of 2D:4D in epidemiological research motivate identification of biological background determinants.

We examined sex, anthropometric measures, and maternal factors as determinants of 2D:4D in Swedish 7‐year‐old children.

Methods

The study was embedded in the Swedish Environmental, Longitudinal, Mother and Child, Asthma and Allergy (SELMA) pregnancy cohort. A total of 870 pre‐pubertal children, median 7.5 years of age, were studied.

A single assessor performed digit measurements from scanned photocopies using computer software. Child anthropometric measurements investigated were hand size, birthweight, recumbent birth length, standing height, weight, BMI, body fat percentage, and waist/hip circumference. Maternal factors included age, pregnancy length, parity, and education.

Results

We found a significant sexual dimorphism regarding digit lengths and 2D:4D, boys on average presenting a lower 2D:4D than girls also after adjustment for summed finger lengths and body fatness. In crude analyses, maternal age correlated with 2D:4D across the whole population and in females but not in adjusted models. No other study variables were associated with 2D:4D.

Conclusion

Digit ratio showed sexual dimorphism at the age of seven and seems to represent a true sex difference rather than an artifact and bias from hand size, body size or body fat content. Among the rest of our investigated variables, we found no determinants constituting important confounders in future research on 2D:4D ratio.

An Investigation of the Relationship Between the Second-to-Fourth Digit Ratio and Sagittal Synostosis

BACKGROUND

The most common presentation of nonsyndromic craniosynostosis is that of the sagittal suture. Amongst this subgroup there is a significant male preponderance. Although the etiology is largely unknown, androgen exposure in utero has been suggested as a contributing factor. The second-to-fourth digit ratio (2D:4D) is a sexually dimorphic trait, reflective of the levels of androgen and estrogen exposure in utero, with a lower 2D:4D ratio associated with higher androgen exposure.This study aimed to examine the difference in 2D:4D ratio between participants with sagittal synostosis (SS) and gender-matched controls, hypothesizing that alterations in androgen exposure would be reflected in participants' 2D:4D ratio.

METHOD

Participants with nonsyndromic SS and gender-matched controls were prospectively recruited from outpatients clinics. Photographs were taken of the right hand, and 3 independent researchers measured the length of the fingers and 2D:4D ratio, with the mean 2D:4D ratio then calculated.

RESULTS

Fifty-six participants were recruited to both groups, with 35 males and 21 females in each. The mean age of the study and control groups were 5.6 and 6.3 years, respectively. There was no difference in the 2D:4D ratio between groups overall (P = 0.126). However, males with SS had a significantly higher 2D:4D ratio in comparison to male controls (0.969 ± 0.379 versus 0.950 ± 0.354, P = 0.038).

CONCLUSIONS

Our results suggest that 1 single hormonal pathway is not responsible for suture fusion. Subsequently we consider that an imbalance between testosterone and estrogen signaling may contribute to the development of sagittal craniosynostosis.

Theories, laws, and models in evo-devo

Evolutionary developmental biology (evo-devo) is the study of the evolution of developmental mechanisms. Here, I review some of the theories, models, and laws in evo-devo, past and present. Nineteenth-century evo-devo was dominated by recapitulation theory and archetypes. It also gave us germ layer theory, the vertebral theory of the skull, floral organs as modified leaves, and the "inverted invertebrate" theory, among others. Newer theories and models include the frameshift theory, the genetic toolkit for development, the ABC model of flower development, the developmental hourglass, the zootype, Urbilateria, and the hox code. Some of these new theories show the influence of archetypes and recapitulation. Interestingly, recent studies support the old "primordial leaf," "inverted invertebrate," and "segmented head" theories. Furthermore, von Baer's first three laws may now need to be rehabilitated, and the hourglass model modified, in view of what Abzhanov has pointed out about the maternal-zygotic transition. There are many supposed "laws" of evo-devo but I argue that these are merely generalizations about trends in particular lineages. I argue that the "body plan" is an archetype, and is often used in such a way that it lacks any scientific meaning. Looking to the future, one challenge for evo-devo will be to develop new theories and models to accommodate the wealth of new data from high-throughput sequencing, including single-cell sequencing. One step in this direction is the use of sophisticated in silico analyses, as in the "transcriptomic hourglass" models.

Conserved Mechanisms, Novel Anatomies: The Developmental Basis of Fin Evolution and the Origin of Limbs

The transformation of paired fins into tetrapod limbs is one of the most intensively scrutinized events in animal evolution. Early anatomical and embryological datasets identified distinctive morphological regions within the appendage and posed hypotheses about how the loss, gain, and transformation of these regions could explain the observed patterns of both extant and fossil appendage diversity. These hypotheses have been put to the test by our growing understanding of patterning mechanisms that regulate formation of the appendage axes, comparisons of gene expression data from an array of phylogenetically informative taxa, and increasingly sophisticated and elegant experiments leveraging the latest molecular approaches. Together, these data demonstrate the remarkable conservation of developmental mechanisms, even across phylogenetically and morphologically disparate taxa, as well as raising new questions about the way we view homology, evolutionary novelty, and the often non-linear connection between morphology and gene expression. In this review, we present historical hypotheses regarding paired fin evolution and limb origins, summarize key aspects of central appendage patterning mechanisms in model and non-model species, address how modern comparative developmental data interface with our understanding of appendage anatomy, and highlight new approaches that promise to provide new insight into these well-traveled questions.

Investigating the reliability and sex differences of digit lengths, ratios, and hand measures in infants

Hands and digits tend to be sexually dimorphic and may reflect prenatal androgen exposure. In the past years, the literature introduced several hand and digit measures, but there is a lack of studies in prepubertal cohorts. The available literature reports more heterogeneous findings in prepubertal compared to postpubertal cohorts. The comparability of the available studies is further limited by the study design and different measurement techniques. The present study compared the reliability and sex differences of available hand and digit measures, namely digit lengths of 2D, 3D, 4D, 5D, digit ratios 2D:4D, 2D:5D, 3D:4D, 3D:5D, 4D:5D, relative digit lengths rel2, rel3, rel4, rel5, directional asymmetry of right and left 2D:4D (Dr-l), hand width, length, and index of 399 male and 364 female 6-month-old German infants within one study using only indirect and computer-assisted measurements. The inter-examiner reliability was excellent while the test-retest reliability of hand scans was only moderate to high. Boys exhibited longer digits as well as wider and longer hands than girls, but smaller digit ratios, with ratios comprising the fifth digit revealing the largest effect sizes. Other hand and digit ratios revealed sex differences to some extent. The findings promote the assumption of sexual dimorphic hand and digit measures. However, by comparing the results of the available literature, there remains an uncertainty regarding the underlying hypothesis. Specifically in prepubertal cohorts, i.e. before the influence of fluctuating hormones, significant effects should be expected. It seems like other factors than the influence of prenatal androgens contribute to the sexual dimorphism in hand and digit lengths.

Digit ratio (2D:4D) and handedness: A meta-analysis of the available literature

The Geschwind-Behan-Galaburda and sexual differentiation models predict an association between elevated foetal androgen exposure and left-handedness whereas the callosal hypothesis predicts the opposite. We present a meta-analysis of correlations between handedness and digit ratio (2D:4D), a putative marker of prenatal testosterone. Left-handedness predicted low (male-typical) right-hand digit ratio (R2D:4D), high (female-typical) left-hand digit ratio (L2D:4D), and low R2D:4D-L2D:4D directional asymmetry (D_[R-L]). Effect sizes were extremely small and not moderated by sex or method of measuring handedness or 2D:4D. The same general pattern was observed after excluding the very large study (110,329 males, 90,412 females) of Manning and Peters ([2009]. Digit ratio (2D:4D) and hand preference for writing in the BBC Internet Study. Laterality: Asymmetries of Body, Brain and Cognition, 14(5), 528-540. doi:10.1080/13576500802637872); however, no significant effects for R2D:4D were observed once these samples were removed. The results do not confirm any theory linking prenatal androgens with handedness, so we speculate they instead reflect the mechanical action of writing causing subtle changes in the musculature and/or fat pads of the fingers. Gripping a pen/pencil might cause an increase in 2D relative to 4D (and/or decrease in 4D relative to 2D) resulting in higher ratios on the writing-hand; furthermore, this could differ between left- and right-handers due to writing in the left-to-right direction (as in English) having asymmetrical effects depending on which hand is used.

Investigation of second to fourth finger length ratio (2D:4D) in patients with bipolar disorder

Objective:

The etiology of bipolar disorder has yet to be fully elucidated, but differences in sex hormones have been suggested to play a role in its pathogenesis. An easily assessed marker of prenatal androgen exposure is the second-to-fourth-digit ratio (2D:4D) of the hand. In this study, we aimed to compare the 2D:4D ratio of patients with bipolar disorder to that of healthy controls.

Methods:

Seventy patients with bipolar disorder and 70 healthy controls, matched for age and sex, were included in the study. Finger lengths were measured from the palmar digital crease to the tip using a digital vernier caliper.

Results:

Patients with bipolar disorder had considerably higher right-hand 2D:4D ratios compared to controls. Both the right and left 2D:4D ratios of male patients were significantly greater than those of males in the control group. Female patients showed no differences in right or left 2D:4D ratio compared to healthy controls.

Conclusion:

These findings suggest that a high 2D:4D digit ratio of right hand is associated with the presence of bipolar disorder in males. Further large-scale, prospective studies are needed to explain the validity of this marker and its relationship with bipolar disorder.

Usefulness of measurement of proximal phalanx length by using ultrasonography in newborn infants

The purpose of this study was to compare digit lengths and proximal phalanx lengths in newborn infants by using ultrasonography (USG) and to examine sex differences between the ratio of the second digit (2D) and fourth digit (4D) of digit length and that of proximal phalanx length and the associations of digit length and proximal phalanx length with birth weight and birth height. Sixty newborn infants (28 males and 32 females) were recruited. Digit lengths were measured by using a transparent ruler and proximal phalanx lengths were measured by using USG. There were significant correlations between digit length and proximal phalanx length in the left and right hands. There was no significant correlation between 2D:4D ratios of digit lengths and 2D:4D ratios of proximal phalanx lengths measured by using USG. In males, birth height was significantly associated with right fourth proximal phalanx length and with left second and fourth proximal phalanx lengths. Birth weight was significantly associated with proximal phalanx length in males. Proximal phalanx length measured by using USG was significantly associated with digit length in newborn infants. Hormonal exposure before birth may be involved in the associations of proximal phalanx length with birth weight and height in males.Impact statementWhat is already known on this subject? The ratio of the second digit (2D) and fourth digit (4D) has been used postnatally to provide a retrospective indication of the foetal hormonal environment. Digit lengths have been measured by using a direct method, photocopies, digital scans and radiographs, but the results of a study on measurement of digit lengths by using ultrasonography (USG) have not been reported.What do the results of this study add? Proximal phalanx length measured by using USG was significantly associated with digit length in newborn infants. In males, birth height was significantly associated with right fourth proximal phalanx length and with left second and fourth proximal phalanx lengths. Birth weight was significantly associated with proximal phalanx length in males.What are the implications of these findings for clinical practice and/or further research? Measurement of proximal phalanx length by using USG may be useful for a study on gender differences in foetal development and the foetal hormonal environment. Hormonal exposure before birth may be involved in the associations of proximal phalanx length with birth weight and height in males.

Depression, neuroticism and 2D:4D ratio: evidence from a large, representative sample

A body of literature reports higher rates of depression and neuroticism in female samples compared to male samples. Numerous studies have investigated the role of prenatal sex hormone exposure in this sex difference, using the ratio between the second and fourth digit of the hand (“2D:4D”) as a putative marker. However, the sample sizes of those studies were mostly small and results remained inconclusive. The aim of the present study is to test the suggested associations between depression, neuroticism and the 2D:4D ratio in a large, representative sample of over 3,000 German individuals. It was hypothesized that a higher 2D:4D (supposedly representing a more “feminine” prenatal hormone exposure) would positively predict (1) one’s history of depression as well as (2) neuroticism rates and (3) acute depressive symptom scores. Controlling for biological sex, we only found suggestive evidence for linear associations with neuroticism in the case of left hand 2D:4D ratios and the mean 2D:4D of both hands. However, additional analyses indicated that these results may have been spurious due to confounding. Our findings suggest that the 2D:4D ratio is not a relevant predictor of depression, while there was mixed evidence in the case of neuroticism.

A complex regulatory landscape involved in the development of mammalian external genitals

Developmental genes are often controlled by large regulatory landscapes matching topologically associating domains (TADs). In various contexts, the associated chromatin backbone is modified by specific enhancer-enhancer and enhancer-promoter interactions. We used a TAD flanking the mouse HoxD cluster to study how these regulatory architectures are formed and deconstructed once their function achieved. We describe this TAD as a functional unit, with several regulatory sequences acting together to elicit a transcriptional response. With one exception, deletion of these sequences didn't modify the transcriptional outcome, a result at odds with a conventional view of enhancer function. The deletion and inversion of a CTCF site located near these regulatory sequences did not affect transcription of the target gene. Slight modifications were nevertheless observed, in agreement with the loop extrusion model. We discuss these unexpected results considering both conventional and alternative explanations relying on the accumulation of poorly specific factors within the TAD backbone.

Digit ratio and length asymmetry in calves' limbs

This study considered possible sexual dimorphism in the relative lengths of the second, third and fourth digits (digit ratio), in calves. Furthermore, a different length of the bone structures of the third (3D) and of the fourth (4D) digits has been examined as an evolutionary adaptation to locomotion on soft ground. The length of the digital bones of the right fore-limb of 33 females and 15 male calves was measured in vivo using a portable X-ray machine. The vestigial structure of the second digit (2D), and 3D and 4D, from metacarpus to the third phalanx were considered in a mixed model, as well as some ratios between 2D and different parts of 3D or 4D (2D:3D and 2D:4D). A covariate for the mean finger length was considered for digit ratios to control for possible biases due to shape allometry. Shorter first phalanx and trotter were found in 3D than in 4D, and the reverse for the third phalanx. The 2D was significantly shorter in females, as well as the second phalanges of 3D and 4D. Significant sex differences in 2D:3D and 2D:4D were found for some digit parts of 3D and 4D and for the first phalanges of 3D:4D. These ratios were always shorter in females, in contrast to that found in most mammals. The asymmetry between 3D and 4D could mean a functional adaptation for locomotion. Sex differences in 2D:4D and 3D:4D were found, but with a reverse pattern than in most mammal species (males > females rather than males < females). In this regard digit ratio in calves was similar to that of Old World monkeys. This study is the first investigation of digit ratio in Ungulates, whose limbs differ from the limbs of most mammals, maintaining five digits. The reverse pattern of sex differences (digit ratios: males> females) could be due to the peculiar nature of the vestigial dewclaw of 2D and to the hormone patterns acting on this digit during development, but further research is required around this topic.

Bmp4 is an essential growth factor for the initiation of genital tubercle (GT) outgrowth

The external genitalia are appendage organs outgrowing from the posterior body trunk. Murine genital tubercle (GT), anlage of external genitalia, initiates its outgrowth from embryonic day (E) 10.5 as a bud structure. Several growth factors such as fibroblast growth factor (FGF), Wnt and Sonic hedgehog (Shh) are essential for the GT outgrowth. However, the mechanisms of initiation of GT outgrowth are poorly understood. We previously identified bone morphogenetic protein (Bmp) signaling as a negative regulator for GT outgrowth. We show here novel aspects of Bmp4 functions for GT outgrowth. We identified the Bmp4 was already expressed in cloaca region at E9.5, before GT outgrowth. To analyze the function of Bmp4 at early stage for the initiation of GT outgrowth, we utilized the Hoxa3-Cre driver and Bmp4 ^flox/flox mouse lines. Hoxa3 ^Cre/+ ; Bmp4 ^flox/flox mutant mice showed the hypoplasia of GT with reduced expression of outgrowth promoting genes such as Wnt5a, Hoxd13 and p63, whereas Shh expression was not affected. Formation of distal urethral epithelium (DUE) marked by the Fgf8 expression is essential for controlling mesenchymal genes expression in GT and subsequent its outgrowth. Furthermore, Fgf8 expression was dramatically reduced in such mutant mice indicating the defective DUE formation. Hence, current results indicate that Bmp4 is an essential growth factor for the initiation of GT outgrowth independent of Shh signaling. Thus, Bmp4 positively regulates for the formation of DUE. The current study provides new insights into the function of Bmp signaling at early stage for the initiation of GT outgrowth.

Revisiting the relationships of 2D:4D with androgen receptor (AR) gene and current testosterone levels: Replication study and meta-analyses

The relationships of digit ratio (2D:4D) with the length of AR (CAG)n, and testosterone levels from saliva and blood have been extensively debated over the years. This research including three studies further clarifies such controversies. To do so, we re-examined the relationships between the length of AR (CAG)n, 2D:4D, and current testosterone levels, through replication study and meta-analysis for each study. The results indicate: (a) the length of AR (CAG)n is not significantly associated with 2D:4D; (b) current testosterone levels are not significantly associated with the ratio; and (c) the length is not significantly associated with testosterone levels. Thus, AR (CAG)n and current testosterone levels are not significantly related to 2D:4D at individual level.

Quality of an ambulatory monitoring technique for diagnosing obstructive sleep apnea under conditions of limited resources

Objectives:

To: 1) evaluate the quality of an ambulatory monitoring technique for diagnosing Obstructive Sleep Apnea Syndrome (OSAS) while patients move through the city; and 2) identify factors that lead to data loss.

Methods:

Clinical histories were reviewed and ambulatory portable monitorings of adults with high pretest probability for OSAS were included, the signals monitored were pulse oximetry, heart rate, nasal pressure, snoring, chest band and body position. The equipment was connected from 14:00-20:00 h and then patients moved through the city turning it off and on at home. Results were analyzed visually to record all the minutes lost. A good-quality study was defined as recording time 240 min and signal loss <20%. A cost/benefit analysis was performed using Golpe et al.'s methodology.

Results:

A total of 70 recordings were analyzed. Most subjects were obese men with severe OSAS. Signal quality was determined to be good with a median signal loss of 4.9 min (0-405) that represented 1% (0-99) of total recording time. The signal lost most often was pulse oximetry at 1.8 min (0-403, p=0.0001). Of the 70 studies performed, 57 (81%) met the definition of good quality, while 13 (19%) had to be repeated. Men lost the pulse oximetry signal more often than women. This technique could represent savings of 65-75%.

Conclusions:

Placing a portable OSAS monitor during the day while patients move around the city turning it on and off at home does not affect the quality of the study results obtained and is a cost-effective method.

Associations Between the 2nd to 4th Digit Ratio and Autism Spectrum Disorder in Population-Based Samples of Boys and Girls: Findings from the Study to Explore Early Development

The ratio of the index (2nd) finger to ring (4th) finger lengths (2D:4D) is a proxy for fetal testosterone and estradiol. Studies suggesting 2D:4D is inversely associated with autism spectrum disorder (ASD) in males were limited by lack of confounder and subgroup assessments. Studies of females are sparse. We examined associations between ASD and 2D:4D among children in the Study to Explore Early Development; we considered case subgroups and numerous potential demographic and maternal-perinatal health confounders. We observed a modest inverse association between ASD and right-hand 2D:4D in males; subgroup analyses indicated associations were limited to ASD cases with birth defects/genetic syndromes or dysmorphic features. We observed a positive association between ASD and left-hand 2D:4D in females, overall and within most case subgroups.

Genome-wide association study identifies nine novel loci for 2D:4D finger ratio, a putative retrospective biomarker of testosterone exposure in utero

The ratio of the length of the index finger to that of the ring finger (2D:4D) is sexually dimorphic and is commonly used as a non-invasive biomarker of prenatal androgen exposure. Most association studies of 2D:4D ratio with a diverse range of sex-specific traits have typically involved small sample sizes and have been difficult to replicate, raising questions around the utility and precise meaning of the measure. In the largest genome-wide association meta-analysis of 2D:4D ratio to date (N = 15 661, with replication N = 75 821), we identified 11 loci (9 novel) explaining 3.8% of the variance in mean 2D:4D ratio. We also found weak evidence for association (β = 0.06; P = 0.02) between 2D:4D ratio and sensitivity to testosterone [length of the CAG microsatellite repeat in the androgen receptor (AR) gene] in females only. Furthermore, genetic variants associated with (adult) testosterone levels and/or sex hormone-binding globulin were not associated with 2D:4D ratio in our sample. Although we were unable to find strong evidence from our genetic study to support the hypothesis that 2D:4D ratio is a direct biomarker of prenatal exposure to androgens in healthy individuals, our findings do not explicitly exclude this possibility, and pathways involving testosterone may become apparent as the size of the discovery sample increases further. Our findings provide new insight into the underlying biology shaping 2D:4D variation in the general population.

Genome-wide association study identifies nine novel loci for 2D:4D finger ratio, a putative retrospective biomarker of testosterone exposure in utero

The ratio of the length of the index finger to that of the ring finger (2D:4D) is sexually dimorphic and is commonly used as a non-invasive biomarker of prenatal androgen exposure. Most association studies of 2D:4D ratio with a diverse range of sex-specific traits have typically involved small sample sizes and have been difficult to replicate, raising questions around the utility and precise meaning of the measure. In the largest genome-wide association meta-analysis of 2D:4D ratio to date (N = 15 661, with replication N = 75 821), we identified 11 loci (9 novel) explaining 3.8% of the variance in mean 2D:4D ratio. We also found weak evidence for association (β = 0.06; P = 0.02) between 2D:4D ratio and sensitivity to testosterone [length of the CAG microsatellite repeat in the androgen receptor (AR) gene] in females only. Furthermore, genetic variants associated with (adult) testosterone levels and/or sex hormone-binding globulin were not associated with 2D:4D ratio in our sample. Although we were unable to find strong evidence from our genetic study to support the hypothesis that 2D:4D ratio is a direct biomarker of prenatal exposure to androgens in healthy individuals, our findings do not explicitly exclude this possibility, and pathways involving testosterone may become apparent as the size of the discovery sample increases further. Our findings provide new insight into the underlying biology shaping 2D:4D variation in the general population.

Women and Men in the History of Western Cardiology: Some Notes on Their Position as Patients, Role as Investigational Study Subjects, and Impact as Professionals

Nowadays, it is generally appreciated that studies in the medical field should not only include sex-related aspects but also consider age. In the past, taking the era of Hippocrates as a starting point for the Western medical sciences, such aspects were less urgent and barely relevant. However, considering such details during daily life became increasingly important as the traditional roles of men and women in society and household converged. In the Western world, this fundamental transition process started recently and is advancing at an accelerated pace. Research about the role of women has also evolved, starting from plain history about the lives of women to a description of the relation between men and women, resulting in the gender concept. The present survey highlights a historical selection of observations referring to the impact of men and women on the medical sciences, as patient, study object, and professional. Whenever relevant, focus will be on the field of cardiovascular investigations as documented in the Western world. Rather than being exhaustive, we focus on a few remarkable icons, including Trota of Salerno, Hildegard von Bingen, and Miguel Serveto.

Age of pubertal onset and 2nd to 4th digit ratios: Preliminary findings

BACKGROUND

The second to fourth digit ratio (2D:4D) is used as a marker of prenatal sex hormone exposure. The aim of this study was to evaluate the possible relation between digit ratio and age of pubertal onset.

METHODS

Participants were 321 men and 202 women aged between 18 and 28 at the time of assessment who self-reported their onset of puberty (i.e., age at first conscious ejaculation or menarche). Participants' right and left digit lengths were measured from hand scans.

RESULTS

All groups of males, i.e., early, average, and late pubertal onset, had lower mean 2D:4D than the comparable female groups, regardless of the relative timing of their pubertal onset. Among the males, there was a significant difference in digit ratios between individuals who experienced early, average, and late onset of puberty. In the males, we observed a positive relationship between 2D:4D and age of pubertal onset for both right and left hands. There were no significant differences in 2D:4D among the three groups of female maturers. We observed no significant association between digit ratios and age at menarche.

CONCLUSIONS

These data lend further support for using 2D:4D as a measure of prenatal androgen exposure.

No Robust Association between Static Markers of Testosterone and Facets of Socio-Economic Decision Making

Digit ratio (2D:4D) and facial width-to-height ratio (WHR) are supposedly static indicators of testosterone exposition during prenatal and pubertal lifetime, respectively. Both measures have been linked to aggressive and assertive behavior in laboratory economic games, as well as in real world scenarios. Most of the research-often limited to male subjects-considers the associations between these behaviors, traits, and hormonal markers separately for 2D:4D and WHR. Reported associations are weak and volatile. In the present study we had independent raters assess 2D:4D and WHR in a sample of N = 175 participants who played the ultimatum game (UG). Respondent behavior in UG captures the tendency to reject unfair offers (negative reciprocity). If unfair UG offers are seen as provocations, then individuals with stronger testosterone exposition may be more prone to reject such offers. Economists argue that negative reciprocity reflects altruistic punishment, since the rejecting individual is sacrificing own resources. However, recent studies suggest that self-interest, in terms of status defense plays a substantial role in decisions to reject unfair offers. We also assessed social preferences by social value orientation and assertiveness via self-report. By applying structural equation modeling we estimated the latent level association of 2D:4D and WHR with negative reciprocity, assertiveness and prosociality in both sexes. Results revealed no robust association between any of the trait measures and hormonal markers. The measures of 2D:4D and WHR were not related with each other. Multigroup models based on sex suggested invariance of factor loadings allowing to compare hormone-behavior relationships of females and males. Only when collapsing across sex greater WHR was weakly associated with assertiveness, suggesting that individuals with wider faces tend to express greater status defense. Only the right hand 2D:4D was weakly associated with prosocial behavior, indicating that individuals with lower prenatal testosterone exposure are more cooperative. Rejection behavior in UG was not related with 2D:4D nor WHR in any of the models. There were also no curvilinear associations between 2D:4D and prosociality as theorized in the literature. Our results suggest that previous studies over-estimated the role of static markers of testosterone in accounting for aggression and competition behavior in males.

Preliminary evidence that digit length ratio (2D:4D) predicts neural response to delivery of motivational stimuli

Reduced relative length of the 2nd to 4th digits (2D:4D) is thought to partially reflect fetal testosterone (FT) exposure, a process suspected to promote relatively permanent effects on the brain and behavior via structural and functional neuroadaptations. We examined the effect of 2D:4D on neural response - assessed by P2a and feedback-related negativity (FRN) event-related potentials (ERPs) - to motivational stimuli (reward or punishment) using two counterbalanced conditions of a passive S1/S2 outcome prediction design. P2a to expected and unexpected delivered rewards or punishments ($1 or white noise burst, respectively) and FRN to withheld rewards or punishments ($0 or silence, respectively) were observed in undergraduates. Lower left 2D:4D and greater 2D:4D_R-L predicted amplified P2a to the delivery (but not FRN to the omission) of motivationally salient stimuli, regardless of valence and probability. These preliminary findings suggest that FT may organize dopamine neurons to respond more strongly to the delivery of motivational stimuli.

Appendages and gene regulatory networks: Lessons from the limbless

Among squamate reptiles, dozens of lineages have independently evolved complete or partial limb reduction. This remarkable convergence of limbless and limb-reduced phenotypes provides multiple natural replicates of different ages to explore the evolution and development of the vertebrate limb and the gene regulatory network that controls its formation. The most successful and best known of the limb-reduced squamates are snakes, which evolved a limb-reduced body form more than 100 million years ago. Recent studies have revealed the unexpected finding that many ancient limb enhancers are conserved in the genomes of snakes. Analyses in limbed animals show that many of these limb enhancers are also active during development of the phallus, suggesting that these enhancers may have been retained in snakes due their importance in regulating transcription in the external genitalia. This hypothesis is substantiated by functional tests of snake enhancers, which demonstrate that snake enhancer elements have lost limb function while retaining genital enhancer function. The large degree of overlap in the gene regulatory networks deployed during limb and phallus development may act to constrain the divergence of shared gene network components and the evolution of appendage morphology. Future studies will reveal whether limb regulatory elements have undergone similar functional changes in other lineages of limb-reduced squamates.

Digit Ratios (2D:4D and other) and Relative Thumb Length: A Test of Developmental Stability

Sex and side differences in relative thumb length of children and adolescents have been reported by prior researchers. These findings mirror those reported for the second-to-fourth digit ratio (2D:4D), a likely biomarker for the organizational (permanent) effects of prenatal androgens on the human brain and body. The present study investigated relative thumb length, in particular, its associations with all possible digit ratios (2D:3D, 2D:4D, 2D:5D, 3D:4D, 3D:5D, and 4D:5D), in a sample of young adults (60 men and 64 women). Relative thumb length was less precisely measurable than direct or image-based finger-length measurements, as used in digit ratio research. There were no significant sex or side differences in relative thumb length. Contrary to expectation, thumb size was not positively correlated with any digit ratios. Relative thumb length appears to be developmentally unstable (decreases during childhood and adolescence). Additional findings, such as the magnitude ranking of sex differences in digit ratios and the comparability of direct versus image-based finger-length measurements, are also discussed.

Exposure to PBB-153 and Digit Ratio

The ratio between the second and fourth digits is a sexually dimorphic measure, established in utero and linked to prenatal sex steroid levels. An association was found between prenatal levels of Polybrominated Biphenyls, a synthetic chemical suspected to disrupt the endocrine system function, and the digit ratio in adult women.

Assessment of the 2D:4D ratio in aggression-related injuries in children attending a paediatric emergency department

BACKGROUND

The 2D:4D ratio is a sexually dimorphic trait. A lower ratio is correlated with sporting prowess, successful financial trading, risk taking behaviour, aggression, and aggression-related injuries, such as the fifth metacarpal fractures.

AIMS

We aimed to assess if the ratio correlates to skeletally immature patient aggression.

METHODS

Radiology reports were searched for the terms "5th metacarpal" or "boxer". Of 537 X-rays, 142 had fractures. The mechanism of injury was recorded in 102 patients, differentiating aggressive and non-aggressive injury groups. Bone age and chronological age were recorded.

RESULTS

A clear male preponderance for aggression was identified with only 60% of non-aggressive injuries occurring in males compared to 95% in the aggression group (p < 0.001). Patients with aggression injuries were older chronologically (14.6 vs 12.7, p < 0.001) and by bone age (14.5 vs 13.0, p < 0.001). For male patients, there was no difference in the 2D:4D ratio (p = 0.94); however, for females, the difference was significant (p = 0.04).

CONCLUSIONS

Aggression injuries occur at an older age and are more common in males. In the female cohort, there was a distinct difference in digit ratio.

Epispadias and the associated embryopathies: genetic and developmental basis

The abnormalities in the urogenital organs are frequently observed as human developmental diseases. Among such diseases, the defects in the upper part of external genitalia are rather rare named epispadias. The cleft in the dorsal part of external genitalia often reaches to the urethra. In general, the urogenital abnormalities accompany defects in the adjacent tissues and organs. The ventral body wall and bladder can also be affected in the patients with dorsal defects of the external genitalia. Therefore, such multiple malformations are often classified as bladder exstrophy and epispadias complex (BEEC). Because of the lower frequency of such birth defects and their early embryonic development, animal models are required to analyze the pathogenic mechanisms and the functions of responsible genes. Mutant mouse analyses on various signal cascades for external genitalia and body wall development are increasingly performed. The genetic interactions between growth factors such as bone morphogenetic proteins (Bmp) and transcription factors such as Msx1/2 and Isl1 have been suggested to play roles for such organogenesis. The significance of epithelial-mesenchymal interaction (EMI) is suggested during development. In this review, we describe on such local interactions and developmental regulators. We also introduce some mutant mouse models displaying external genitalia-body wall abnormalities.

The Baculum was Gained and Lost Multiple Times during Mammalian Evolution

The rapid evolution of male genitalia is a nearly ubiquitous pattern across sexually reproducing organisms, likely driven by the evolutionary pressures of male-male competition, male-female interactions, and perhaps pleiotropic effects of selection. The penis of many mammalian species contains a baculum, a bone that displays astonishing morphological diversity. The evolution of baculum size and shape does not consistently correlate with any aspects of mating system, hindering our understanding of the evolutionary processes affecting it. One potential explanation for the lack of consistent comparative results is that the baculum is not actually a homologous structure. If the baculum of different groups evolved independently, then the assumption of homology inherent in comparative studies is violated. Here, we specifically test this hypothesis by modeling the presence/absence of bacula of 954 mammalian species across a well-established phylogeny and show that the baculum evolved a minimum of nine times, and was lost a minimum of ten times. Three different forms of bootstrapping show our results are robust to species sampling. Furthermore, groups with a baculum show evidence of higher rates of diversification. Our study offers an explanation for the inconsistent results in the literature, and provides insight into the evolution of this remarkable structure.

Prospective investigation of penile length with newborn male circumcision and second to fourth digit ratio

INTRODUCTION

We prospectively investigated the relationship between newborn male circumcision (NMC) and second to fourth digit ratio with penile length.

METHODS

As participants for our study, we identified already circumcised young patients who visited our hospital for urological treatment. The age at which the circumcision had been done was assessed. The patients' height and weight were measured. Second to fourth digit ratio was calculated by measuring the second and fourth digit lengths. The flaccid and erectile penile lengths were measured from the base of the penis to the tip of the glans in standing position.

RESULTS

A total of 248 patients were included in our study. In univariate analysis, height, second to fourth digit ratio, flaccid penile length, and age of circumcision were associated with erectile penile length. Among these variables, second to fourth digit ratio, flaccid penile length, and age of circumcision were significant predictive factors for erectile penile length in multivariate analysis. The subjects were divided into two groups, including 72 patients in the NMC group and 176 patients in the non-NMC group. No significant difference was found in height, weight, and second to fourth digit ratio between both groups. However, flaccid (p<0.001) and erectile (p=0.001) penile lengths were shorter in the NMC group than in the non-NMC group.

CONCLUSIONS

Despite the small number of subjects, this study shows that NMC was associated with shorter penile length. Second to fourth digit ratio, flaccid penile length, and age of circumcision were also significant predictive factors for erectile penile length. Further multicentre studies with larger number of subjects and biochemical analyses are needed for potential clinical applicability.

Reorganisation of Hoxd regulatory landscapes during the evolution of a snake-like body plan

Within land vertebrate species, snakes display extreme variations in their body plan, characterized by the absence of limbs and an elongated morphology. Such a particular interpretation of the basic vertebrate body architecture has often been associated with changes in the function or regulation of Hox genes. Here, we use an interspecies comparative approach to investigate different regulatory aspects at the snake HoxD locus. We report that, unlike in other vertebrates, snake mesoderm-specific enhancers are mostly located within the HoxD cluster itself rather than outside. In addition, despite both the absence of limbs and an altered Hoxd gene regulation in external genitalia, the limb-associated bimodal HoxD chromatin structure is maintained at the snake locus. Finally, we show that snake and mouse orthologous enhancer sequences can display distinct expression specificities. These results show that vertebrate morphological evolution likely involved extensive reorganisation at Hox loci, yet within a generally conserved regulatory framework.

2D:4D indicates phimosis risk: A study on digit ratio and early foreskin development

BACKGROUND

Many researchers have adopted 2D:4D (second to fourth finger length ratio) as a noninvasive retrospective biomarker for prenatal androgen exposure in recent years. It is thought to be related to diverse traits including behavioral phenotypes, disease susceptibility, and development of urogenital system.

OBJECTIVE

To examine the relationship between 2D:4D and early foreskin development.

METHODS

We analyzed the digit ratio and foreskin condition in 176 cases (range 0-6years). The boys were divided into four groups according to their ages: group 1, neonates (below 28days, n=13); group 2, infants (1-12months, n=45); group 3, toddlers (1-2years old, n=42); group 4, preschool children (3-6years old, n=76). We measured the lengths of the second and fourth digits of the left and right hands. The foreskin status was classified into 4 types. Type I (phimosis), type II (partial phimosis), type III (adhesion of prepuce), type IV (normal).

RESULTS

The phimosis rate was 92.3%, 82.2%, 45.2%, and 38.7% in group 1 to group 4. In contrast, the proportion of normal foreskin increased from 0% in neonates to 13.2% in preschool children. The percentage of higher level of foreskin development shows a downward trend with the increase of digits ratio, and as the age grows, the percentage of normal foreskin cases also increases.

CONCLUSIONS

These results suggest that a higher R2D:4D (right hand 2D:4D) is a risk factor for phimosis in the early human development. Age is also a significant influence factor of foreskin conditions. Additional research is required to identify pathophysiologic mechanisms and to determine clinical significance.

The 2D:4D-Ratio and Neuroticism Revisited: Empirical Evidence from Germany and China

The 2D:4D-Ratio, as an indirect measure of the fetal testosterone to estradiol ratio, is potentially very important for understanding and explaining different personality traits. It was the aim of the present study to replicate the findings from Fink et al. (2004) about the relation between individual differences in 2D:4D-Ratios and the Five Factor Model in different cultural groups. Therefore a sample of n = 78 Chinese and n = 370 German participants was recruited. Every participant provided hand scans of both hands, from which 2D:4D-Ratios were computed. Moreover, all participants filled in the NEO Five Factor Inventory (NEO-FFI). Significant sex differences were found for ratios of both hands in the expected direction, with females showing higher ratios than males. With respect to links between personality and the digit ratio, a positive association was observed between 2D:4D-Ratio and Neuroticism in females, as shown in the earlier study. These findings were observed in both female subsamples from China and Germany, as well as in the full sample of participants. But in contrast to the results for the whole and the German female sample, where 2D:4D-Ratio of both hands were related to Neuroticism, in the Chinese female sample only left hand 2D:4D-Ratio was significantly and positively related to Neuroticism. There were no significant correlations found in any of the male samples. Thus, prenatal exposure to sex steroids appears to influence the personality factor Neuroticism in females specifically. This finding potentially has implications for mental health, as Neuroticism has been shown to be a risk factor for various forms of psychopathology.

A role for HOX13 proteins in the regulatory switch between TADs at the HoxD locus

During vertebrate limb development, Hoxd genes are regulated following a bimodal strategy involving two topologically associating domains (TADs) located on either side of the gene cluster. These regulatory landscapes alternatively control different subsets of Hoxd targets, first into the arm and subsequently into the digits. We studied the transition between these two global regulations, a switch that correlates with the positioning of the wrist, which articulates these two main limb segments. We show that the HOX13 proteins themselves help switch off the telomeric TAD, likely through a global repressive mechanism. At the same time, they directly interact with distal enhancers to sustain the activity of the centromeric TAD, thus explaining both the sequential and exclusive operating processes of these two regulatory domains. We propose a model in which the activation of Hox13 gene expression in distal limb cells both interrupts the proximal Hox gene regulation and re-enforces the distal regulation. In the absence of HOX13 proteins, a proximal limb structure grows without any sign of wrist articulation, likely related to an ancestral fish-like condition.

The Genetic Basis of Baculum Size and Shape Variation in Mice

The rapid divergence of male genitalia is a preeminent evolutionary pattern. This rapid divergence is especially striking in the baculum, a bone that occurs in the penis of many mammalian species. Closely related species often display diverse baculum morphology where no other morphological differences can be discerned. While this fundamental pattern of evolution has been appreciated at the level of gross morphology, nearly nothing is known about the genetic basis of size and shape divergence. Quantifying the genetic basis of baculum size and shape variation has been difficult because these structures generally lack obvious landmarks, so comparing them in three dimensions is not straightforward. Here, we develop a novel morphometric approach to quantify size and shape variation from three-dimensional micro-CT scans taken from 369 bacula, representing 75 distinct strains of the BXD family of mice. We identify two quantitative trait loci (QTL) that explain ∼50% of the variance in baculum size, and a third QTL that explains more than 20% of the variance in shape. Together, our study demonstrates that baculum morphology may diverge relatively easily, with mutations at a few loci of large effect that independently modulate size and shape. Based on a combination of bioinformatic investigations and new data on RNA expression, we prioritized these QTL to 16 candidate genes, which have hypothesized roles in bone morphogenesis and may enable future genetic manipulation of baculum morphology.

Side-specific effect of yolk testosterone elevation on second-to-fourth digit ratio in a wild passerine

Second-to-fourth digit ratio is a widely investigated sexually dimorphic morphological trait in human studies and could reliably indicate the prenatal steroid environment. Conducting manipulative experiments to test this hypothesis comes up against ethical limits in humans. However, oviparous tetrapods may be excellent models to experimentally investigate the effects of prenatal steroids on offspring second-to-fourth digit ratio. In this field study, we injected collared flycatcher (Ficedula albicollis) eggs with physiological doses of testosterone. Fledglings from eggs with elevated yolk testosterone, regardless of their sex, had longer second digits on their left feet than controls, while the fourth digit did not differ between groups. Therefore, second-to-fourth digit ratio was higher in the testosterone-injected group, but only on the left foot. This is the first study which shows experimentally that early testosterone exposure can affect second-to-fourth digit ratio in a wild population of a passerine bird.

When the Snake Lost Its Limbs, What Did the Mouse and Lizard Say?

In this issue of Developmental Cell, Infante et al. (2015) compare regulatory DNA sequences in mice, lizards, and limbless snakes to reveal widespread sharing of enhancer activity in developing limbs and genitalia. Genetic deletion of a limb-genital enhancer demonstrates that common regulatory elements affect development of both appendages.

Shared Enhancer Activity in the Limbs and Phallus and Functional Divergence of a Limb-Genital cis-Regulatory Element in Snakes

The amniote phallus and limbs differ dramatically in their morphologies but share patterns of signaling and gene expression in early development. Thus far, the extent to which genital and limb transcriptional networks also share cis-regulatory elements has remained unexplored. We show that many limb enhancers are retained in snake genomes, suggesting that these elements may function in non-limb tissues. Consistent with this, our analysis of cis-regulatory activity in mice and Anolis lizards reveals that patterns of enhancer activity in embryonic limbs and genitalia overlap heavily. In mice, deletion of HLEB, an enhancer of Tbx4, produces defects in hindlimbs and genitalia, establishing the importance of this limb-genital enhancer for development of these different appendages. Further analyses demonstrate that the HLEB of snakes has lost hindlimb enhancer function while retaining genital activity. Our findings identify roles for Tbx4 in genital development and highlight deep similarities in cis-regulatory activity between limbs and genitalia.

Hadza hunter-gatherer men do not have more masculine digit ratios (2D:4D)

OBJECTIVES

The ratio between the length of the second and the length of the fourth digit (2D:4D) is sexually dimorphic such that males of many species possess a lower ratio than females, particularly in the right hand. Still, men and women often exhibit overlapping 2D:4D ranges and the ratio is highly variable between populations. In order to further explore populational variability, we chose to analyze 2D:4D in the Hadza, a population of hunter-gatherers living in Tanzania.

MATERIALS AND METHODS

Data were collected separately by two researchers over the course of three years (1998, 2001, 2006) from 152 adult participants (male: n = 76, female: n = 76). Independent samples t-tests were used to explore sex differences, paired samples t-tests were used to explore directional effects within each sex, and linear regression and one-way ANOVA were used to test possible age effects.

RESULTS

In none of the years, or pooled (n = 152), did we find evidence that adult men have a lower 2D:4D than adult women. If anything, the data suggest that women in this population have a significantly lower right hand 2D:4D than men (P < 0.001, d = 0.57). In contrast, left hand 2D:4D did not exhibit a sex difference (P = 0.862, d = 0.03).

DISCUSSION

These findings challenge the current view that lower 2D:4D in men is a uniform characteristic of our species. Cross-populational variance in 2D:4D may be related to known patterns of hormonal variation resulting from both genetic and environmental mechanisms, though this relationship merits further investigation.