Testosterone-Induced Expression of Male Colour Morphs in Females of the Polymorphic Tawny Dragon Lizard, Ctenophorus decresii.

Katrina Rankin,Devi Stuart-Fox,Alexandre Roulin

doi:10.1371/journal.pone.0140458

Abstract

Many colour polymorphisms are present only in one sex, usually males, but proximate mechanisms controlling the expression of sex-limited colour polymorphisms have received little attention. Here, we test the hypothesis that artificial elevation of testosterone in females of the colour polymorphic tawny dragon lizard, Ctenophorus decresii, can induce them to express the same colour morphs, in similar frequencies, to those found in males. Male C. decresii, express four discrete throat colour morphs (orange, yellow, grey and an orange central patch surrounded by yellow). We used silastic implants to experimentally elevate testosterone levels in mature females to induce colour expression. Testosterone elevation resulted in a substantial increase in the proportion and intensity of orange but not yellow colouration, which was present in a subset of females prior to treatment. Consequently, females exhibited the same set of colour morphs as males, and we confirmed that these morphs are objectively classifiable, by using digital image analyses and spectral reflectance measurements, and occur in similar frequencies as in males. These results indicate that the influence of testosterone differs for different colours, suggesting that their expression may be governed by different proximate hormonal mechanisms. Thus, caution must be exercised when using artificial testosterone manipulation to induce female expression of sex-limited colour polymorphisms. Nevertheless, the ability to express sex-limited colours (in this case orange) to reveal the same, objectively classifiable morphs in similar frequencies to males suggests autosomal rather than sex-linked inheritance, and can facilitate further research on the genetic basis of colour polymorphism, including estimating heritability and selection on colour morphs from pedigree data.

Highlights

The study of colour polymorphism, that is, the presence of multiple, discrete, genetically determined colour forms within one sex and within an interbreeding population [1], has provided fundamental insights into evolutionary processes and phenomena such as frequency dependent selection, speciation, mimicry and sexual selection [2,3,4,5,6,7,8]
Testing whether artificial elevation of testosterone induces sex-limited colour polymorphism in females provides important insight into the degree to which testosterone has an activational influence on colour expression and whether this differs for different colours
After testosterone-induced colour expression, females were visually assigned to one of the four male colour morph categories, with ten females assigned to orange, seven to yellow, seven to grey and eleven to orange+yellow morphs

Summary

Introduction

The study of colour polymorphism, that is, the presence of multiple, discrete, genetically determined colour forms within one sex and within an interbreeding population [1], has provided fundamental insights into evolutionary processes and phenomena such as frequency dependent selection, speciation, mimicry and sexual selection [2,3,4,5,6,7,8]. Testosterone Induced Colour Morphs in Female Lizards are present only in one sex (sex-limited), usually males, presumably because the polymorphic colour signal is sexually selected. Artificial elevation of testosterone in females induces expression of the male colour morphs in colour polymorphic birds [21] and lizards [14]. Testing whether artificial elevation of testosterone induces sex-limited colour polymorphism in females provides important insight into the degree to which testosterone has an activational influence on colour expression and whether this differs for different colours. To reliably score the underlying genotype, requires that testosterone consistently induces females to express the same set of discrete colour morphs that are present in males, in similar frequencies. It is important to quantify changes in female expression of each colour component in response to testosterone and compare objective classification of observed colour morphs in males and females

Methods

Results

Conclusion