The challenge hypothesis: Triumphs and caveats

Supergenes on steroids

At the birth of supergenes, the genomic landscape is dramatically re-organized leading to pronounced differences in phenotypes and increased intrasexual diversity. Two of the best-studied supergenes in vertebrates are arguably the inversion polymorphisms on chromosomes 2 and 11 in the white-throated sparrow (Zonotrichia albicollis) and the ruff (Calidris pugnax), respectively. In both species, regions of suppressed recombination determine plumage coloration and social behavioural phenotypes. Despite the apparent lack of gene overlap between these two supergenes, in both cases the alternative phenotypes seem to be driven largely by alterations in steroid hormone pathways. Here, we explore the interplay between genomic architecture and steroid-related genes. Due to the highly pleiotropic effects of steroid-related genes and their universal involvement in social behaviour and transcriptomic regulation, processes favouring their linkage are likely to have substantial effects on the evolution of behavioural phenotypes, individual fitness, and life-history strategies. We propose that inversion-related differentiation and regulatory changes in steroid-related genes lie at the core of phenotypic differentiation in both of these interesting species. This article is part of the theme issue 'Genetic basis of adaptation and speciation: from loci to causative mutations'.

Number of sexual partners and serum testosterone levels in a population-based sample of unpartnered heterosexual men

BACKGROUND

The relationship between men's testosterone levels and their sexual behaviors is unclear as existing studies find mixed results.

OBJECTIVES

The key objective of this study is to assess whether men's testosterone levels are affected by sexual behaviors, and to explore whether this relationship varies by age. Specifically, this study addresses the question: Are men's testosterone levels affected by the number of sexual partners they accrue?

METHODS

This analysis was based on a nationally-representative sample of 977 unpartnered heterosexual men between the ages of 20 and 65 in the United States. Serum testosterone levels were measured from blood samples using isotope dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS) and the number of sexual partners in the past 12 months was ascertained from an Audio Computer-Assisted Personal Self-Interview (ACASI). Using ordinary least squares regression, serum testosterone levels were predicted as a function of the number of sexual partners that men reported, sociodemographic characteristics, and factors known to affect endocrine regulation. An interaction term between the number of sexual partners in the past 12 months and age was included.

RESULTS

The correlation between the number of sexual partners in the past 12 months and testosterone levels is contingent upon age: The relationship is positive for younger men but negative for older men. Among 20-year-old unpartnered heterosexual men, an additional sexual partner in the past 12 months is associated with an increase of 5.55 ng/dL of testosterone (95% CI: 2.92, 8.19). This initial positive effect at age 20 is reduced by 0.25 ng/dL of testosterone (95% CI: -0.44, -0.06) with each year that men age.

CONCLUSION

Testosterone levels in men are related to reproductive behaviors, but the direction of this relationship changes as men age. This article is protected by copyright. All rights reserved.