The function and evolution of colour polymorphism in the tawny dragon lizard

© Dr Madeleine St Claire YewersColour polymorphic species are model systems to investigate the evolutionary processes that maintain intraspecific diversity within a population. Colour polymorphism occurs when two or more discrete, genetically inherited colour forms coexist within an interbreeding population. Almost ubiquitously, colour morphs differ in morphological, behavioural, ecological, life history and/or physiological traits in addition to colour that often form alternative strategies. Each strategy has optimal trait combinations that allow morphs to maximise fitness. Variation in the trait composition of morph-specific strategies has important implications for understanding life-history trade-offs and the maintenance of polymorphism within populations. Differences in morph composition and correlated traits between populations can also promote divergence and ultimately speciation. In this thesis, I investigate the evolutionary maintenance of colour polymorphism in the tawny dragon lizard, Ctenophorus decresii. I do so in two distinct ways; by assessing colour vision differences between genetically and geographically distinct monomorphic and polymorphic lineages, and by comparing a range of traits that could differentially affect the fitness of morphs within a colour polymorphic population. In the south of their range, male tawny dragons are monomorphic for throat coloration whereas in the north of their range, they are polymorphic. There are four discrete male colour morphs in polymorphic populations that vary in the presence/absence of yellow and orange coloration; the yellow morph, orange morph, grey morph and orange-yellow morph (a yellow background with a central orange patch). Throat colour is heritable, fixed for life, and is an important sexual signal. Males of the monomorphic southern lineage express ultraviolet (UV)-blue throat coloration, unlike males of the polymorphic northern lineage. Lineages meet at a narrow contact zone where genotypic admixture suggests potential barriers to gene flow and incipient speciation. I determined the cone photoreceptor spectral sensitivities using microspectrophotometry and opsin expression of the two lineages, to see if they differ, particularly in sensitivity to UV-blue wavelengths. I confirmed the presence of four single cone classes in both lineages and provide the first evidence of UV visual sensitivity in agamid lizards. However, whether the lineages differ in UV-blue sensitivity remains unresolved. Within a polymorphic northern population, I assessed a combination of traits associated with each colour morph. I found morph-specific alternative strategies differentiated by consistencies in behaviour and hormones. The orange morph has an aggressive strategy with high levels of androgens while the grey morph has a cautious strategy with low levels of baseline androgens. The yellow morph has a conditional strategy where its aggression depends on the colour of the intruder and exhibits a stress-induced androgen increase. The orange-yellow morph similarly shows aggression conditional on the colour of the intruder but it is the boldest with high levels of androgens. Morphs did not differ in performance (bite force) or space use, and there was no relationship between spatial arrangement and relatedness. However, genetic structure based on microsatellite markers indicated weak genetic differentiation between morphs. Therefore, there are minimal barriers to gene flow between morphs. Instead, frequency-dependent selection is likely to be maintaining polymorphism in the tawny dragon lizard

Behavioural differences across contexts may indicate morph-specific strategies in the lizard Ctenophorus decresii

Discrete colour polymorphisms are often genetically correlated with other traits under natural and sexual selection, such as behaviour, life history and physiology. Elucidating such correlations is essential to understand the adoption of alternative strategies between morphs and the role they play in the maintenance of colour polymorphisms within a population. Using field experiments, we tested the hypothesis that four visually discrete morphs (orange, yellow, yellow with a central orange patch (orange-yellow) and grey) of the tawny dragon lizard, Ctenophorus decresii, display alternative behavioural strategies. Specifically, we compared the response of colour morphs to simulated conspecific territorial intruders and predators in the wild. Although the orange-yellow morph can be objectively classified, it may behaviourally resemble the orange or yellow morph; therefore we compared statistical models in which the orange-yellow morph was considered a separate morph (four-morph model) or grouped with either pure orange or pure yellow individuals (three-morph models). For aggression, a three-morph model with orange-yellow individuals grouped as yellow morphs best fitted the data. The orange morph showed consistently high aggression to all morphs, while the grey morph showed consistently low aggression. Aggression of the yellow morph was conditional on the morph of the intruder. In addition to being the least aggressive, the grey morph was the least bold. Although the orange morph was the most aggressive, it was only the boldest under a three-morph model, which was equally likely compared to a four-morph model. Overall our results support the view that tawny dragon lizard morphs adopt different behavioural strategies, the orange and grey morphs exhibiting more aggressive and cautious strategies, respectively, and the yellow morph changing its aggression depending on its competitor's colour

Data from: Space use and genetic structure do not maintain colour polymorphism in a species with alternative behavioural strategies

Space use including territoriality and spatial arrangement within a population can reveal important information on the nature, dynamics and evolutionary maintenance of alternative strategies in colour polymorphic species. Despite the prevalence of colour polymorphic species as model systems in evolutionary biology, the interaction between space-use and genetic structuring of morphs within populations has rarely been examined. Here we assess the spatial and genetic structure of male throat colour morphs within a population of the tawny dragon lizard, Ctenophorus decresii. Male colour morphs do not differ in morphology but differ in aggressive and anti-predator behaviours as well as androgen levels. Despite these behavioural and endocrine differences, we find that colour morphs do not differ in territory size, with their spatial arrangement being essentially random with respect to each other. There were no differences in genetic diversity or relatedness between morphs; however, there was significant, albeit weak, genetic differentiation between morphs, which was unrelated to geographic distance between individuals. Our results indicate potential weak barriers to gene flow between some morphs, potentially due to non-random pre- or post-copulatory mate choice or postzygotic genetic incompatibilities. However, space use, spatial structure and non-random mating do not appear to be primary mechanisms maintaining colour polymorphism in this system, highlighting the complexity and variation in alternative strategies associated with colour polymorphism