Environmental levels of oestrogenic and antiandrogenic compounds feminize digit ratios in male rats and their unexposed male progeny

Abstract

Digit length ratios, especially the second-to-fourth digit ratio (2D : 4D), are associated with various pathological and behavioural conditions in many species including humans and are dependent upon prenatal androgen to oestrogen balance. It is unknown whether digit ratios are modified by environmental exposure to ubiquitous endocrine disruptors. We studied the effect on adult male Wistar rat digit ratios of a gestational exposure to the oestrogenic and antiandrogenic compounds bisphenol A (BPA), genistein and vinclozolin, in low doses, and in combination with investigating in parallel a possible sexual dimorphism of this trait. We also investigated the effects on the male progeny not exposed during gestation. X-rays were taken of the left and right forepaws, and 2D-5D proximal to distal phalanx distances were measured by a standardized procedure based on semi-automatic image analysis. We provide evidence that there is a sexual dimorphism of digit ratios in the Wistar rat, and we found that BPA alone or in combination with genistein and vinclozolin significantly feminized digit ratios in male rats. Intriguingly, significant feminization of digit ratios was also found in the unexposed male progeny of males that had been exposed to compound mixtures. In conclusion, prenatal environmental levels of endocrine-active substances permanently disrupt digit ratios. Digit ratio measurement in adults is thus a promising biomarker of prenatal exposure to low-dose endocrine disruptors in rodents, with potential implications for future studies in humans.

Keywords: 2D : 4D; bisphenol A; endocrine disruptor; endocrine-active substance; genistein; vinclozolin.

Figure 1.

Examples of X-ray pictures of the right forepaw with projected anatomical landmarks and corresponding 2D : 4D digit length ratios (calculated from duplicates of digit length measurements using the image analysis software) for a typical control male, a typical control female and a typical male exposed to BPA.

Figure 2.

Box plot displaying the 10th, 25th, 50th, 75th and 90th percentile values, the extreme 5th and 95th percentiles (circles) and the mean value (thick white line) for right forepaw 2D : 4D in (a) the F₁ males exposed to the compounds and (b) in their unexposed male offspring (F₂) carried by unexposed dams. Mann–Whitney non-parametric comparisons (#p < 0.001, ***p < 0.005, **p < 0.01, *p < 0.05) versus controls (abbreviations for both the F₁ and F₂ and doses for the F₁: BPA, bisphenol A, 5 µg kg⁻¹ d⁻¹; V, vinclozolin, 10 µg kg⁻¹ d⁻¹, ‘low’ or 1000 µg kg⁻¹ d⁻¹ ‘high’ and G, genistein, 1000 µg kg⁻¹ d⁻¹, see [15]).